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Optical properties of self-assembled quantum wires for application in infra-red detection

a rX iv:c ond-ma t/1183v2[c ond-m at.m trl-sci ]9J a n201Optical properties of self-assembled quantum wires for application in infra-red detection Liang-Xin Li,Sophia Sun,and Yia-Chung Chang Department of Physics and Materials Research Laboratory University of Illinois at Urbana-Champaign,Urbana,Illinois 61801(February 1,2008)Abstract We present theoretical studies of optical properties of Ga 1−x In x As self-assembled quantum-wires (QWR’s)made of short-period superlattices with strain-induced lat-eral ordering.Valence-band anisotropy,band mixing,and eﬀects due to local strain distribution at the atomistic level are all taken into ing realistic ma-terial parameters which are experimentally feasible,we perform simulations of the absorption spectra for both inter-subband and inter-band transitions (including the excitonic eﬀect)of this material.It is shown that the self-assembled QWR’s have favorable optical properties for application in infra-red detection with normal inci-dence.The wavelength of detection ranges from 10µm to 20µm with the length of QWR period varying from 150˚A to 300˚A .I.INTRODUCTIONQuantum-well infra-red photodetectors(QWIP’s)have been extensively studied in recent years. The main mechamism used in QWIPs is the inter-subband optical transition,because the wavelengths for these transitions in typical III-V quantum wells can be tailored to match the desired operating wavelength(1-20µm)for infra-red(IR)detection.Due to its narrow band absorption,QWIP’s are complementary to the traditional HgCdTe detectors,which utilize the inter-band absorption, and therfeore are applicable only for broad-band absorption.The main drawback of QWIP’s is the lack of normal-incidence capability,unless some processing is made to create diﬀraction gartings on the surface,which tends to reduce the responsivity of the material to the incident radiation. Because electrons in quantum wells have translational invariance(within the eﬀective-mass model) in the plane normal to the growth axis,the electron inter-subband transitions for normal-incident radiation is zero(or very small even if the coupling with other bands is considered).One way to break the translational invariance is to introduce the surface diﬀraction grating as commonly adopted in many QWIP’s fabricated today.A better(and less expensive)way to break the in-plane translational invariance is to utilize the strain-induced lateral modulation provided in self-assembled nano-structure materials.These nano-structures inculde quantum dots and quantum wires.Because the lateral modelation is formed via self-assembly,the fabrication of this type of materials will be much more eﬃcient once the optimized growth parameters are known.Hence,it will be cost eﬀective to use them for device fabrications.Self-assembled III-V QWR’s via the strain-induced lateral-layer ordering(SILO)process have attracted a great deal of attention recently.[21−23]The self-assembly process occurs during the growth of short-period superlattices(SPS)[e.g.(GaAs)2/(InAs)2.25]along the[001]direction on InP substrate.The excess fractional InAs layer leads to stripe-like islands during the initial MBE growth.[4]The presence of stripes combined with strain leads to natural phase separation as additional layers of GaAs or InAs are deposited and the structure becomes laterally modulated in terms of In/Ga composition.A self-assembled QWR heterostructure can then be created by sandwiching the laterally modulated layer between barrier materials such as Al0.24Ga0.24In0.52As(quarternary),Al0.48In0.52As (ternary),or InP(binary).[4-6]It was found that diﬀerent barrier materials can lead to diﬀerent degree of lateral composition modulation,and the period of lateral modulation ranges from100˚A to 300˚A depending on the growth time and temperature.In this paper,we explore the usefulness of InGaAs quantum wires(QWR’s)grown by the strain-induced lateral ordering(SILO)process for IR detection.Our theoretical modeling inculdes the eﬀects of realistic band structures and microscopic strain distributions by combining the eﬀective bond-orbital model(EBOM)with the valence-force-ﬁeld(VFF)model.One of the major parameters for the IR detectors is the absorption quantum eﬃciency which is directly related to the absorption coeﬃcient byη=1−e−αl whereαis the absorption coeﬃcient and l is the sample length.Thus,to have a realistic accessment of the materials for device application,we need to perform detailed calculations of the absorption coeﬃcient,taking into account the excitonic and band structure eﬀects.Both inter-subband and inter-band transitions are examined systematically for a number of structure parameters (within the experimentally feasible range)chosen to give the desired eﬀect for IR detection.It is found that the wavelengths for the inter-subband transitions of InGaAs self-assembled QWR’s range from10to20µm,while the inter-band transitions are around1.5µm.Thus,the material provides simultaneneous IR detection at two contrasting wavelengths,something desirable for appli-cation in multi-colored IR vedio camera.Several structure models with varying degrees of alloy mixing for lateral modulation are con-sidered.For the inter-band absorption,the excitonic eﬀect is important,since it gives rise a large shift in transition energy and substantial enhancement of the absorption spectrum.To study the excitonic eﬀect on the absorption spectrum for both discrete and contunuum states,we use a large set of basis functions with aﬁnite-mesh sampling in the k-space and diaginalize the exciton Hamilto-nian directly.Emphasis is put on the analysis of line shapes of various peak structures arising from discrete excitonic states of one pair of subbands coupled with the excitonic(discrete and continuum) states associated with other pairs of subbands.Weﬁnd that the excitonic eﬀect enhances theﬁrst absorption peak around1.5times and shifts the peak position by20-30meV.II.THEORETICAL MODELThe QWR structures considered here consist of8pairs of(GaAs)2(InAs)2.25short-period super-lattices(SPS)sandwiched between Al0.24Ga0.24In0.52As barriers.The SPS structure prior to strain induced lateral ordering(SILO)is depicted in Fig.1.With lateral ordering,the structure is modeled by a periodic modulation of alloy composition in layers with fractional monolayer of(In or Ga)in the SPS structure.In layers7and9(starting from the bottom as layer1),we havex In=x m[1−sin(πy′/2b)]/2for y′<b0for b<y′<L/2−bx m{1+sin[π(y′−L/2)/2b]}/2for L/2−b<y′<L/2+bx m for L/2+b<y′<L−bx m{1−sin[π(y′−L)/2b]}/2for y′>L−b,(1)where x m is the maximum In composition in the layer,2b denotes the width of lateral composition grading,and L is the period of the lateral modulation in the[110]direction.The experimental feasible range of L is between100˚A and300˚A.The length of L is controled by the growth time and temperature.In layers3and13,we havex In=0for0<y′<5L/8−bx m{1+sin[π(y′−5L/8)/2b]}/2for5L/8−b<y′<5L/8+b,x m for5L/8+b<y′<7L/8−bx m{1−sin[π(y′−7L/8)/2b]}/2for7L/8−b<y′<7L/8+b0for7L/8+b<y′<L.(2)Similar equation for x Ga in layers5and11can be deduced from the above.By varying the parameters x m and b,we can get diﬀerent degrees of lateral alloy mixing.Typically x m is between0.6and1, and b is between zero and15a[110]≈62˚A.A VFF model[13-15]is used toﬁnd the equilibrium atomic positions in the self-assembled QWR structure by minimizing the lattice energy.The strain tensor at each atomic(In or Ga)site is then obtained by calculating the local distortion of chemical bonds.Once the microscopic strain distribution in the model structure is determined,the energy levels and wave-functions of self-assembled quantum wires are then calculated within the eﬀective bond-orbital model(EBOM).Detailed description of this method can be found in Refs.24,25−26.EBOM used here is a tight-binding-like model in which two s-like conduction bands(including spin)and four valence bands with total angular momentum J=3/2(due to spin-orbit coupling of p-like orbitals with the spinor).Thus,the present model is comparable to the six-band k·p model as adpoted in Ref.?To minimize the computing eﬀort,we express the electron and hole states for the quantum wire structures in terms of eigen-states of a quantum well structure with diﬀerent in-plane wave vectors.The quantum well consists of8pairs of(GaAs)2(InAs)2short-period superlattice(SPS)plus two InAs monolayers(one inserted after the second pair of SPS and the other after the sixth pair of SPS),so the the total In/Ga composition ratio is consistent with the(GaAs)2(InAs)2.25SPS.The whole stack of SPS’s is then sandwiched between two slabs of Al0.24Ga0.24In0.52barriers.Let us denote the quantum well eigen-states as|n,k1,k2 QW where n labels the subband,k1denotes the wave vector along thewire([1¯10])direction and k2labels the wave vector in the[110]direction,which is perpendicular to the wire and the growth axis.Expanding the quantum well states in terms of bond-orbitals,we have |n,k1,k2 QW=1L α,R f n,k1,k2(α,R z)exp(ik2R2)exp(ik1R1)|uα(R) ,where L is the sample length along the wire axis,f n,k1,k2(α,R z)is the eigen-vector for the quantum well Hamiltonian and uα(R)denotes anα-like bond orbital state at site R(α=1,···,6for two s-like conduction-band and four J=3/2valence-band orbitals).Here R runs over all lattice sites within the SPS layer(well region)and AlGaInAs layer(barrier region).We then diagonalize the hamiltonian for the quantum wire(QWR)within a basis which consists of the quantum well states with k2’s separated by reciprocal lattice vectors g m=m(2π/a[110]);m= ly,|i,k1,k2 = n,m C i,k1(n,k2+g m)|n,k1,k2+g m QWwhere C i,k1(n,k2+g m)is the eigen-vector for the quantum-wire hamiltonian matrix for the i−th QWR subband at wave vector(k1,k2).In terms of the bond orbitals,we can rewrite the QWR states as|i,k1,k2 = α,R F i,k1,k2(α,R)|uα(R)whereF i,k1,k2(α,R)=1Ln,mC i,k1(n,k2+g m)f n,k1,k2+g m(α,R z)exp[i(k2+g m)R2]exp(ik1R1)is the QWR envelope function.For the laterally conﬁned states,the dispersion of bands versus k2is negligible;thus,the k2dependence can be ignored.The absorption coeﬃceient for inter-subband transitions between subbands i and j is given by αij(¯hω)=4π2e2¯hwhere n r is the refractive index of the QWR,V is the volume of the QWR sample restricted within the SPS region,f i(f j)is the Fermi-Dirac distribution function for subbnad i(j).The optical matrix elements between QWR subband states are related to those between bond orbitals byi,k1,k2|ˆǫ·p|j,k1,k2 = α,α′,τF∗i,k1,k2(α,R)F j,k1,k2(α′,R) uα(R)|ˆǫ·p|uα′(R+ τ) ,where τruns over on-site or the12nearest-neighbor sites in the fcc lattice.The optical matrix elements between bond orbitals are related to the band parameters by requiring the optical matrix elements between bulk states near the zone ceneter to be identical to those obtained in the k·p theory28.We obtain27langleu s(R)|pαuα′(R) =2a)(E p/Eg−m0/m∗e)τα;α=x,y,z,whereταis theα-th of the lattice vectorτin units of a/2,E p is the inter-band optical matrix element as deﬁned in Ref.28,and m∗e is the electron eﬀective mass.Next,we study the inter-band transitions.For this case,the excitonic eﬀect is important.Here we are only interested in the absorption spectrum near the band edge due to laterally conﬁned states.Thus,the dispersion in the k2direction can be ignored.The exciton states with zero center-of-mass momentum can then be written as linear combinations of products of electron and holes states associated with the same k1(wave vector along the wire direction).We write the electron-hole product state for the i-th conduction subband and j-th valence subband as|i,j;k1 ex=|i,k1 |j,k1≡α,β,R e,R h F i,k1(α,R e)G j,k1(β,R h)|u(α,R e)>|u(β,R h)>.The matrix elements of the exciton Hamiltonian within this basis is given byi,j,k1|H ex|i′,j′,k′1 =[E i(k1)δi,i′−E j(k1)δj,j′]− R e,R h F∗ii′(R e)v(R e−R h)G jj′(R2),(4) where v(R e,R h)=4πe2F ii′(R e)= αF∗i,k1(α,R e)F i′,k1(α,R e)describes the charge density matrix for the electrons.Similarly,G jj′(R h)= βG∗j,k1(β,R h)G j′,k1(β,R h)describes the charge density matrix for the holes.In Eq.(x),we have adopted the approximation u(α,R e)| u(β,R h)|v|u(α′,R′e) |u(β′,R′h) ≈v(R e−R h)δα,α′δβ,β′δR e,R′eδR h,R′h,since the Coulomb potential is a smooth function over the distance of a lattice copnstant,except at the origin,and the bond orbitals are orthonormal to each other.At the origin(R e=R h),the potential is singular,and we replace it by an empirical constant which is adjusted so as to give the same exciton binding energy as obtained in the eﬀective-mass theory for a bulk system.The results are actually insensitive to the on-site Coulomb potential parameter,since the Bohr radius of the exciton is much larger than the lattice constant.After the diagonalization,we obtain the excitonic states as linear combinations of the electron-hole product states,and the inter-band absorption coeﬃcient is computed according to4π2e2¯hαex(¯hω)=m0E p/2is needed,In order to obtain a smooth absorption spectrum,we replace theδfunction in Eq.(1)by a Lorentzian function with a half-widthΓ,δ(E i−E)≈Γ/{π[(E i−E)2+Γ2]}(7)Γis energy width due to imhomogeneous broadening,which is taken to be0.01eV(??).III.RESULTS AND DISCUSSIONSWe have performed calculations of inter-subband and inter-band absorption spectra for the QWR structure depicted in Fig.1with varaying degree of alloy mixing and diﬀerent lengths of period (L)in lateral modulation.Weﬁnd that the inter-subband absorption spectra are sensitive to the length of period(L),but rather insensitive to the degree of of alloying mixing.Thus,we only present results for the case with moderate alloy mixing,which are characterized by parameters b=33˚A and x m=1.0.In all the calculations,the bottom layer atoms of QWR’s are bounded by the InP substrate,while the upper layer atoms and GaAS capping layer atoms are allowed to move freely. This strucure is corresponding to the unclamped struture as indicated in reference10.For diﬀerent period length L of QWR’s,the strain distribtion proﬁles are qualitatively similar as shown in reference10.As L decreases,the hydrostatic strain in rich In region(i.e.right half zone of QWR’s unit)increase,while it decreases in rich Ga region.The bi-axial strain has the opposite change with L.The variation of hydrostatic and bi-axial strains with deducing QWR’s period reﬂects in the potential proﬁles as the diﬀerence of CB and VB band eage increases,which can be seen in Figure2.It can be easily understood that the shear strains increase when L is decuded.The potential proﬁles due to strain-induced lateral ordering seen by an electron in two QWR structures considered here(L=50a[110]and L=40a[110])are shown in Fig.2.more discussions...The conduction subband structures for the self-assembled QWRs with alloying mixing(x m=1.0 and b=8a[110])for(L=50a[110]and L=40a[110])are shown in Fig.3.All subband are grouped in pairs with a weak spin splitting(not resolved on the scale shown).For L=50a[110],the lowest three pairs of subbands are nearly dispersionless along the k2direction,indicating the eﬀect of strong lateral conﬁnement.The inter-subband transition between theﬁrst two pairs give rise to the dominant IR response at photon energy around60meV.For L=40a[110],only the lowest pair of subbands(CB1) is laterally conﬁned(with a weak k2dispersion).The higher subbands corresponding to laterally unconﬁned states(but remain conﬁned along the growth axis)and they have large dispersion versus k2.Weﬁnd three pairs of subbands(CB2-CB4)are closely spaced in energy(within5meV?).State orgion of degeneracry??The valence subband structures for the self-assembled QWRs with alloying mixing(x m=1.0and b=8a[110])for(L=50a[110]and L=40a[110])are shown in Fig.4.more discussions??A.Inter-subband absorptionInter-subband absorption spectrum is the most relavent quantity in determining the usefulness of self-assembled QWR’s for application in IR detection.Fig.5shows the calculated inter-subband absorption spectra of the self-assembled QWR structure(as depicted in Fig.1)for three diﬀerent lengths of period:L=72,50,and40a[110](approximatley300˚A,200˚A,and160˚A,respectively).In the cacluation,we assume that these QWR structures are n-type doped with linear carrier density around1.65×106cm−1(which corresponds to a Fermi level around25meV above the condunction band minimum).For comparison purposes,we show results for polarization vector along both the[110](solid curves)and[001]directions(dashed curves).The results for[1¯10]polarization are zero due to the strict translational invariance imposed in our model calculation.The peak positions for the inter-subband transition with normal incidence(with[110]polarization) are around65meV,75meV,and110meV for the three cases considered here.All these are within the desirable range of IR detection.As expected,the transition energy increases as the length of period decreases due to the increased degree of lateral conﬁnement.However,the transition energy will saturate at around110meV as we further reduce the length of period,since the bound-to-continuum transition is already reached at L=40a[110].The absorption strengths for theﬁrst two cases(L=72a[110]and L=50a[110])are reasonably strong(around400cm−1and200cm−1,respectively).They both correspond to the bound-to-bound transitions.In contrast,the absorption strength for the third case is somewhat weaker(around50 cm−1),since it corresponds to the bound-to-continuum transition.For comparison,the absorption strength for typical III-V QWIPs is around??The inter-subband absorption for the[001]polarization is peaked around??meV.The excited state involved in this transition is a quantum conﬁned state due to the Al0.24Ga0.24In0.52barriers. Thus,it has the same physical origin as the inter-subband transition used in typical QWIP structure. Although this peak is not useful for IR detection with normal incidence,it can be used as the second-color detection if one puts a diﬀraction grating on the surface as typically done in the fabrication of QWIPs.B.Inter-band absorptionThe inter-band optical transitions are important for the characterization of self-assembled QWR’s, since they are readily observable via the Photoluminescence(PL)or optical transimission experiment. For IR-detector application,they oﬀer another absorption peak at mid IR wavelengths,which can be used together with the inter-subband transitions occured at far IR wavelengths for multi-colored detection.Thus,to understand the full capability of the self-assembled QWR material,we also need to analyze the inter-band absorption.Fig.6shows the squared optical matrix elements versus k2for two self-assembled QWR’s con-sidered in the previous section(with L=50and40a[110]).For the case with L=50a[110],the optical matrix elements for both[110]and[1¯10]polarizations are strong with a polarization ratio P[1¯10]/P[110] around2.This is similar to the case with L=72a[110]as reported in Ref.xx.For the case with L=40a[110],the optical matrix elements for both[110]and[1¯10]polarizations are weak.This is due to the fact that the electrons and hole are laterally conﬁned in diﬀerent regions in the QWR,as already indicated in the potential proﬁle as shown in Fig.2(b).Thus,the inter-band absorption for this case will be uninteresting.Fig.7shows the inter-band absorption spectra for SILO QWR’s with L=72and50a[110], including the excitonic eﬀects.The PL properties of the L=72a[110]structure with alloying mixing characterized by x m=0.1and b=8a[110]has been studied in our previous paper.The QWR structure has a gap around0.74eV with a PL polarization ratio(P[1¯10]/P[110])around3.1.The absorption coeﬃcient for this structure has a peak strength around250cm−1.The binding energy for the ground state exciton labeled1-1(derived primarily from the top valence subband and the lowest conduction subband)is around20meV.Thus,the peak position in the absorption spectrum shifts from0.76meV(without the excitonic eﬀect)to0.74meV(with the excitonic eﬀect).The exctionic eﬀect also enhances the peak strength from200cm−1to250cm−1.The other peak structures(labeled 2-2,2-3,...etc.)are derived primarily from the transitions between the lower valence subbands to the higher conduction subbands).For the QWR structure with L=50a[110],we obtain similar absorption spectrum with a peak strength around400cm−1(??).The exciton binding is around40??meV,and the excitonic en-hancement factor of theﬁrst peak is around1.15(??),higer than the case with L=72a[110].This indicates that the case with L=50a[110]has stronger lateral conﬁnement for electrons and holes, which leads larger exciton binding energy and stronger excition oscillator strength(due to the largerprobability that the electron and hole appear at the same position).The secondary peaks due to excitonic states derived from higher subbands are also subtantially stronger than their counterparts in the L=72a[110]case.IV.SUMMARY AND DISCUSSIONSWe have studied the inter-subband and inter-band absorption spectra for self-assembled InGaAs quantum wires for consideration in IR-detector application.Detailed band structures,microscopic strain distributions,and excitonic eﬀects all have been taken into account.A number of realistic structures grown via strain-induced lateral ordering process are examined.Weﬁnd that the self-assembled InGaAs quantum wires are good candidate for multi-colored IR detector materials.They oﬀer two groups of strong IR absorption peaks:one in the far-IR range with wavelengths covering10 -20µm(via the inter-subband transition),the other in the mid-IR range with wavelengths centered around1.5µm(via the inter-band transition).Due the strain induced lateral modulation,the inter-subband transition is strong for normal incident light with polarization along the direction of lateral modulation([110]).This gives the self-assembled InGaAs quantum wires a distinct advantage over the quantum well systems for application in IR detection.The inter-subband absorption is found to be sensitive to the length of period(L)of laterial modulation with the aborption peak position varying from60meV to110meV as the length of period is reduced from300˚A to160˚A.However,further reduction in the length of period does not shift the absorption peak very much,as the excited states become laterally unconﬁned.For the inter-band transition,weﬁnd that the excitonsic eﬀect enhances the absorption peak strength by about10-20%,and shift the peak position by about20-40meV for the structures considered.The reduction in the period length(L)leads to stronger lateral conﬁnement,hence larger exciton binding and stronger absorprtion strength.As conclusion,this paper should give the experiment the realistic guidance in the growth of the IR detector and present the interesting physical thoughts for the theoretists and experimentists.In conclusion,we successfully demonstrated that self-assembled quantum wires are promising IR-detector materials and we provided theoretical modeling for the optical characteristics for realistic QWR structures,which can be used to guide future fabrication of quantum wire infrared detectors.REFERENCES1A.R.Adams,Electron.Lett.22,249(1986).2A.C.Gossard,P.M.Petroﬀ,W.Weigman,R.Dingle,and 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In this structure,four pairs of(2/2.25)SPS(or17diatomic layers)form a period,and the period is repeated twice in the unit cell.Filled and open circles indicate Ga and In rows(each row extends inﬁnitely along the[1¯10]direction).Fig.2.Conduction band and valence band edges for self-assembled QWR structure depicted in Fig. 1for(a)L=50a[110]and(b)L=40a[110].Dashed:without alloy mixing.Solid:with alloy mixing described by x m=1.0and b=8a[110].Fig.3.Conduction subband structure of self-assembled QWR for(a)L=50a[110]and(b)L=40a[110] with x m=1.0and b=8a[110].Fig.4.Valence subband structure of self-assembled QWR for(a)L=50a[110]and(b)L=40a[110] with x m=1.0and b=8a[110].Fig.5.Inter-subband absorption spectra of self-assembled QWR for(a)L=72a[110],(b)L=50a[110], and(c)L=40a[110]with x m=1.0and b=8a[110].Solid:[110]polarization,dashed:[001] polarization.Fig. 6.Inter-band optical matrix elements squared versus k1of self-assembled QWR’s for(a) L=50a[110]and(b)L=40a[110]with x m=1.0and b=8a[110].Fig.7.Inter-band absorption spectra of self-assembled QWR’s for(a)L=72a[110]and(b)L= 50a[110]with x m=1.0and b=8a[110].Solid:[110]polarization with excitonic eﬀect.Dotted:[1¯10] polarization with excitonic eﬀect.Dashed:[110]polarization without excitonic eﬀect.。

The Structure of Solids and Their Properties

The Structure of Solids and TheirPropertiesIntroduction:Solids are one of the three basic states of matter, along with liquids and gases. In a solid, the particles are closely packed together and held in place by strong, attractive forces. This results in a rigid and definite shape, which is one of the fundamental properties of solids. However, the structure of solids and their properties go much deeper than just their shape. In this article, we will explore the various aspects of solids, from their crystal structures to their unique mechanical, electrical, and thermal properties.Crystalline Structure:Most solids are crystalline, meaning that their atoms are arranged in a highly ordered and repeating pattern. This arrangement gives rise to a characteristic crystal structure, which can be determined by X-ray diffraction or other methods. There are many different types of crystal structures, each with its own set of properties and behaviors.One of the most common types of crystal structures is the cubic structure, in which the atoms are arranged in a simple repeating cube. This structure is seen in metals like copper and aluminum, as well as in ionic compounds like sodium chloride. Other common crystal structures include the tetragonal, orthorhombic, and hexagonal structures, which have varying degrees of symmetry.The crystal structure of a solid plays a significant role in determining its mechanical and physical properties. For example, metals with a cubic crystal structure tend to be ductile and malleable, while those with a hexagonal or tetragonal structure are often harder and more brittle. The crystal structure can also affect the electrical conductivity and thermal conductivity of a solid.Mechanical Properties:The mechanical properties of solids refer to how they respond to external stresses and strains. These properties can be described by various measures, such as elasticity, plasticity, hardness, and toughness. Elasticity refers to a material's ability to deform and return to its original shape when the stress is removed. Plasticity refers to a material's ability to be permanently deformed without breaking. Hardness refers to a material's resistance to indentation or scratching, while toughness refers to its ability to absorb energy and resist fracture.The mechanical properties of a solid depend on several factors, including the crystal structure, composition, and processing. For example, metals with a high degree of crystalline order tend to be more ductile and less prone to fracture than those with a more random structure. Similarly, alloys that contain elements with similar atomic radii tend to have higher strength and hardness than those with large mismatches.Electrical Properties:The electrical properties of solids play a crucial role in various technological applications, from electronics to energy generation and storage. Conductors are materials that allow electric charges to flow freely through them, while insulators are materials that resist the flow of charges. Semiconductors are materials that have intermediate conductivity, allowing them to be used in electronic devices like transistors and diodes.The electrical conductivity of a solid depends on several factors, including the crystal structure, composition, and defects. For example, metals with a high degree of crystalline order tend to be good conductors, while insulators often have a highly ordered structure with few defects. Semiconductors have a specific band structure that allows them to be easily manipulated for use in electronic devices.Thermal Properties:The thermal properties of solids refer to how they respond to changes in temperature, including their ability to conduct, store, and transfer heat. Heat capacity is a measure of how much heat a material can absorb or release without undergoing a change intemperature, while thermal conductivity is a measure of how rapidly heat can be conducted through a material.The thermal properties of a solid depend on several factors, including the crystal structure, composition, and defects. For example, metals with a high degree of disorder tend to have lower heat capacity than those with a more ordered structure. Similarly, materials with high thermal conductivity often have a highly ordered structure with few defects, while those with low thermal conductivity often have a more random structure.Conclusion:Solids are a fundamental part of our daily lives, from the metals in our cars to the silicon chips in our electronics. Understanding the structure of solids and their properties is essential for designing and improving the materials we use. From the mechanical properties that determine a material's strength and toughness to the electrical and thermal properties that govern its behavior under different conditions, the intricate nature of solids continues to fascinate and inspire scientists and engineers around the world.。

TCO膜之氧化锌

The effect of front ZnO:Al surface texture and optical transparency on efﬁcient light trapping in silicon thin-ﬁlm solar cellsMichael Berginski,a͒Jürgen Hüpkes,Melanie Schulte,Gunnar Schöpe,Helmut Stiebig,and Bernd Rech b͒Institute of Photovoltaics(IPV),Forschungszentrum Jülich GmbH,D-52425Jülich,GermanyMatthias WuttigInstitute of Physics(IA),RWTH Aachen University,D-52056Aachen,Germany͑Received18June2006;accepted27January2007;published online9April2007͒This study addresses the material properties of magnetron-sputtered aluminum-doped zinc oxide͑ZnO:Al͒ﬁlms and their application as front contacts in silicon thin-ﬁlm solar cells.Optimizedﬁlmsexhibit high conductivity and transparency,as well as a surface topography with adaptedlight-scattering properties to induce efﬁcient light trapping in silicon thin-ﬁlm solar cells.Weinvestigated the inﬂuence on the ZnO:Al properties of the amount of alumina in the target as wellas the substrate temperature during sputter deposition.The alumina content in the target inﬂuencesthe carrier concentration leading to different conductivity and free carrier absorption in the nearinfrared.Additionally,a distinct inﬂuence on theﬁlm growth of the ZnO:Al layer was found.Thelatter affects the surface topography which develops during wet-chemical etching in dilutedhydrochloric acid.Depending on alumina content in the target and heater temperature,threedifferent regimes of etching behavior have been identiﬁed.Low amounts of target doping and lowheater temperatures result in small and irregular features in the postetching surface topography,which does not scatter the light efﬁciently.At higher substrate temperatures and target doping levels,more regularly distributed craters evolve with mean opening angles between120°and135°andlateral sizes of1–3␮m.These layers are very effective in light scattering.In the third regime—atvery high substrate temperatures and high doping levels—the postetching surface is ratherﬂat andalmost no light scattering is observed.We applied the ZnO:Alﬁlms as front contacts in thin-ﬁlmsilicon solar cells to study their light-trapping ability.While high transparency is a prerequisite,lighttrapping was improved by using front contacts with a surface topography consisting of relativelyuniformly dispersed craters.We have identiﬁed a low amount of target doping͑0.5–1wt%͒andrelatively high substrate temperatures͑about350–450°C as sputter parameters enablingshort-circuit current densities as high as26.8mA/cm2in␮c-Si:H pin cells with an i-layer thicknessof 1.9␮m.Limitations on further improvements of light-trapping ability are discussed incomparison with the theoretical limitations and Monte Carlo simulations presented in the literature.©2007American Institute of Physics.͓DOI:10.1063/1.2715554͔I.INTRODUCTIONVarious solar cell concepts can be used to utilize the photovoltaic effect for energy production.1–3At present,the photovoltaic world market is dominated by crystalline silicon cells which accounted for nearly95%of world photovoltaic cell and module production in2004.4The remaining share of the world photovoltaic market mainly relies on thin-ﬁlm so-lar cell concepts.It can be assumed that several thin-ﬁlm module companies already operate proﬁtably.5Even though thin-ﬁlm solar cell production based on copper-indium dis-elenide͑CIS͒,cadmium telluride͑CdTe͒,and amorphous silicon had comparably small market shares in2004of about 0.25%,1.1%,and3.9%,respectively,these concepts are be-lieved to be candidates for a signiﬁcant production volume in the future.5–7Thin-ﬁlm solar cells might be among the very few photovoltaic techniques to reach the very low cost target ofϽUS$1/watt.Such low costs are required for photovol-taics to compete with retail electricity prices but,in the in-termediate term,this can only be achieved by increasing the production capacity signiﬁcantly.5,6,8However,in the case of CIS and CdTe thin-ﬁlm technologies,the use of nonabundant materials like In,Se,and Te may mean that these materials will become a dominant price factor in mass production.6 Silicon is an abundant material.Thus,silicon thin-ﬁlm solar cells,due to their low costs,have a good chance of gaining a signiﬁcant market share once mass production has started.There are a variety of different technical realizations for silicon thin-ﬁlm solar cells.For example,silicon thin-ﬁlm solar cells can be made by using one single amorphous sili-con͑a-Si:H͒junction.However,these cells are degraded to some extent upon light soaking due to the light-induced cre-ation of metastable defects,known as the Staebler-Wronski effect.9,10This problem can be reduced by using a very thin a-Si:H layer or a stack of thin layers,because in this latter case the photogenerated carriers need to move only a short distance before reaching the electrodes.11,12A signiﬁcant in-a͒FAX:ϩ492461613735;electronic mail:m.berginski@fz-juelich.deb͒Current address:Department of Silicon Photovoltaics͑SE1͒,Hahn-Meitner-Institut Berlin GmbH,D-12489Berlin,Germany.JOURNAL OF APPLIED PHYSICS101,074903͑2007͒0021-8979/2007/101͑7͒/074903/11/$23.00©2007American Institute of Physics101,074903-1crease in efﬁciency is possible if the long wavelength re-sponse is also enhanced.Thus,another approach is to use astack of amorphous silicon and amorphous silicon germa-nium͑a-SiGe:H͒.13A further approach is to employ micro-crystalline silicon␮c-Si:H.14,15Microcrystalline silicon so-lar cells are stable against light soaking or show only a verylittle degradation as compared to amorphous silicon solarcells.16,17Thus,a stack consisting of a thin amorphous sili-con top cell and a microcrystalline bottom cell in a so-calledmicromorph tandem structure offers the potential for fairlyhigh efﬁciency at low cost in mass production.18–21A stabi-lized efﬁciency of12.0%has recently been published for atandem structure module22͑for comparison,solar modules with sliced single crystalline and polycrystalline silicon havebeen reported to show efﬁciencies of up to22.7%and15.3%,respectively23͒.Even though silicon thin-ﬁlm mod-ules still show rather low efﬁciencies compared to the other approaches,from the application point of view the annual energy production and especially the cost per unit of annual energy production is much more relevant.Focusing on these ﬁgures,thin-ﬁlm amorphous silicon performs better than crystalline silicon due to a lower temperature coefﬁcient for power loss.5Thus,the aim of future research and develop-ment in the area of thin-ﬁlm silicon solar cells is low cost annual energy production rather than merely high nominal efﬁciency.Even though microcrystalline silicon solar cells or mi-cromorph tandem cells can utilize a broader spectral rangethan amorphous silicon solar cells,an efﬁcient light-trappingscheme is still essential to enhance the intrinsically low ab-sorbance of microcrystalline silicon in the long wavelengthrange͑wavelength␭of aboveϷ850nm͒.24,25Light trappingis achieved by combining textured transparent conductiveoxide͑TCO͒ﬁlms as front contacts and highly reﬂectiveback contacts.However,due to the multiple passage of scat-tered light within the solar cell device,parasitic light absorp-tion in the photovoltaic nonactive layers͑highly doped n-and p-type silicon layers,front TCO,and back reﬂector͒isincreased as well.In the p-i-n conﬁguration the cell is illuminated throughTCO coated glass.The TCO has to have counteracting prop-erties:high conductivity to obtain low series resistance andlow carrier concentration to avoid absorption losses in thered and near-infrared͑NIR͒wavelength range.It should benoted that in the NIR the absorbance of a single passagethrough a1␮m thick layer of␮c-Si:H is lower than that of typical TCOs.In earlier work,Agashe et al.studied the in-ﬂuence of the target doping concentration on the electrical, structural,and optical properties of sputter-deposited and as-deposited smooth ZnO:Alﬁlms.26They found much lower parasitic absorptions with similar conductivity values using sputter targets with a low amount of alumina.Besides the transparency,it is crucial to develop a TCO with a suitable surface texture which scatters the light very efﬁciently in order to extend the effective path length within the active silicon layers.27,28Thus,a special design of the TCO is nec-essary to fulﬁll all these requirements.For initially smooth sputter-deposited ZnO:Alﬁlms a surface texture is realized by postdeposition wet-chemical etching.29,30Kluth et al.re-lated the inﬂuence of pressure and substrate temperature dur-ing radio frequency͑rf͒sputter deposition of ZnO:Al at a ﬁxed alumina content of2wt%in the sputter target to struc-tural properties and postetching surface topography in a modiﬁed Thornton model.30They showed that,depending on sputter parameters,craterlike surface topography with typical lateral length scales of1–2␮m and depths of about 200–400nm develops in a self-organized fashion.This etch-ing behavior is affected very sensitively by structural ZnO:Al properties.Theﬁlm structure can be controlled by the depo-sition parameters,but there are a number of hidden param-eters that are not yet fully understood.31For example,Hüp-kes et al.and Kluth et al.showed that the amount of oxygen added to the process gas strongly inﬂuencesﬁlm properties.32,33Another of these unknown inﬂuence factors might be the target alumina concentration͑TAC͒.In the present study,we extend the modiﬁed Thornton model by introducing the alumina content in the target as an inﬂuential parameter for varying the etching behavior and thus surface topography for light scattering.This knowledge is essential for developing surface-textured ZnO:Alﬁlms for efﬁcient light trapping based on the improved optical and electrical properties of as-deposited smooth thinﬁlms found by Agashe et al.We brieﬂy show structural,optical,and electrical properties supporting the results of Agashe et al. with additional data.Aiming to design an optimized front TCO for thin-ﬁlm solar cells,we study the growth of the ZnO:Alﬁlms and characterize the surface texture of theﬁlms after etching in HCl.Finally,optimizedﬁlms are applied in solar cells and calculations of an effective light pass en-hancement factor are presented.We focus on single junction microcrystalline silicon thin-ﬁlm solar cells in order to study the light-trapping ability of the front contacts,particularly in the long wavelength range.Optimized ZnO:Alﬁlms can later be applied in stacked multijunction cells with amorphous and microcrystalline silicon absorber layers.Finally,in order to quantify the success of the experimental achievements,we compare the light-trapping ability in our cells with simula-tions of nearly idealized conditions and statistical mechani-cally derived general limitations.II.EXPERIMENTAL DETAILSZnO:Alﬁlms were prepared on Corning1737glass by rf magnetron sputtering from ceramic targets in static or dy-namic modes.The amount of doping was varied by using ZnO:Al2O3targets with0.2,0.5,1,or2wt%of Al2O3.The concentration of Al2O3in the target is henceforth referred to as TAC.At constant deposition pressure of0.3Pa͑in the case of0.2,0.5,and2wt%TAC͒and0.1Pa͑in the case of 1wt%TAC͒the substrate temperature was varied in a range of60–490°C.Substrate temperature was calibrated by a thermal sensor and was controlled by the heater temperature. Film characterization was done by a four-point probe,room temperature Hall effect measurements at room temperature and by x-ray diffraction analysis.Optical transmittance and reﬂectance of smoothﬁlms,as well as the total and diffuse transmission on texture-etchedﬁlms͑in order to calculate the haze͒,were measured in air with a dual beam spectrometer.For textured ZnO:Alﬁlms,CH2I2was applied in transmit-tance and reﬂectance measurements as an index-matching ﬂuid to avoid systematic measurement errors due to light scattering of the roughﬁlms.34By wet-chemical etching in diluted hydrochloric acid͑0.5%HCl͒,the approximately 800nm thick initially smoothﬁlms͑rms roughness of about 15nm͒were transformed into surface-texturedﬁlms with a typical rms roughness of more than100nm.The surface topography and characteristic feature sizes were studied by scanning electron microscopy͑SEM͒and atomic force mi-croscopy͑AFM͒.The surface topographies determined by atomic force microscopy were statistically analyzed.How-ever,the most effective way to characterize the light-trapping ability of a speciﬁc TCOﬁlm is by applying it in solar cells. For this purpose,we deposited␮c-Si:H single junction solar cells by plasma-enhanced chemical vapor deposition ͑PECVD͒at13.56MHz excitation frequency in a30ϫ30cm2reactor.35Double layers of sputter-deposited ZnO:Al͑80nm͒and thermally evaporated silver͑700nm͒served as the back reﬂector and rear side contact.Details ofcell preparation are given elsewhere.28Solar cell character-istics were measured using a sun simulator͑Wacom-WXS-140S-Super͒under standard test conditions͑AM 1.5,100mW/cm2at25°C͒.The external quantum efﬁciency ͑QE͒of some solar cells was measured by differential spec-tral response at zero bias.From the QE the short-circuit cur-rent density,which corresponds to the maximum current den-sity generated by the solar cell,was calculated employing theAM 1.5solar spectrum with an illumination density of100mW/cm2with the corresponding photonﬂux.In the fol-lowing,the short-circuit current density calculated in thisway is referred to as the cell current density.III.RESULTSA.Structural,electrical,and optical ZnO:Alﬁlm propertiesAs is typical of magnetron-sputtered ZnO:Alﬁlms,36alltheﬁlms in this study exhibit a strong texture with the c axispredominantly orientated parallel to the substrate normal.Therefore,the x-ray diffraction spectra in Bragg-Brentanogeometry exhibit strong peaks corresponding to the͓001͔direction.All other peak intensities are at least a factor of onehundred lower than that of the͑002͒peak.Figure1showsthe peak position and full width at half maximum͑FWHM͒of the͑002͒peak as function of substrate temperature T.While the peak position shifts towards higher angles withincreasing substrate temperature in the case of low TAC,thetrend is opposite for the target with2wt%TAC.A shift ofpeak position towards lower angles usually indicates com-pressive stress,which is attributed to high-energy ion bom-bardment during growth by sputter deposition.The FWHMof the͑002͒peak decreases with substrate temperature forlow doping concentration and,once again,only in case of2wt%TAC does the FWHM strongly increase at high sub-strate temperatures.This means that the number of coher-ently scattering crystal planes decreases.This either corre-sponds to crystallite size or is limited by microstress within asingle crystallite.The TAC inﬂuences the amount of Al doping and in this way determines the number of free carriers in the ZnO:Al ﬁlms.37Figure2shows the carrier concentration N and mo-bility␮as a function of substrate temperature T.While in the case of0.5and1wt%TAC,the carrier concentration tends to be maximal at substrate temperatures of300–350°C;ﬁlms sputtered with a TAC of0.2and2wt%gradually change their carrier concentration.Increasing the substrate temperature during sputtering leads to a higher N in case of 0.2wt%TAC and to a lower N in case of2wt%TAC.The carrier mobility seems to be maximal at a certain substrate temperature which increases when TAC is reduced.Carrier mobility was further improved by optimized deposition con-ditions compared to the data of Agashe et al.26Figure3shows transmission and absorption spectra of ZnO:Alﬁlms sputtered at aﬁxed substrate temperature of 375°C using0.2,0.5,and2wt%TAC.The absorption was calculated using transmission and reﬂection spectra.Increas-ing the TAC mainly increases the absorption of NIR light and the carrier concentration N͑see inset,values are given in 1020cm−3͒in theﬁlms.Simultaneously,the resistivity␳͑see FIG.1.X-ray diffraction spectra in Bragg-Brentano geometry were evalu-ated with respect to the position͑upper graph͒and the full width at half maximum͑lower graph͒of the dominant͑002͒peak.The data points at,e.g., 300°C and1wt%TAC were measured for nominally identical ZnO:Al ﬁlms.Scattering of data points is caused by statistical experimentalerrors.FIG.2.Carrier concentration N͑top graph͒and carrier mobility␮͑bottom graph͒measured using Hall measurements in van der Pauw geometry.inset,values are given in 10−3⍀cm ͒decreases.In addition,the fundamental absorption band edge is shifted to smaller wavelengths ͑Burstein-Moss shift 38,39͒.Note that this effect is partially masked by the index-matching ﬂuid applied dur-ing measurements.For high doping levels,the parasitic free carrier absorption mechanism signiﬁcantly reduces the trans-mission in the active range of the solar cells ͑up to about 1100nm ͒.A compromise has to be found between transmis-sion and conductivity in order to prepare optimized ZnO:Al front contacts for solar cells.However,even though high transmission and low resistance are prerequisites,earlier work has shown that surface roughness,feature size,and shape are key parameters for highly efﬁcient light trapping.27,40Thus,we concentrate in the following section on ZnO:Al surface structure observed after etching.B.Postetching surface topographyAfter deposition the initially smooth ﬁlms were surface textured by wet-chemical etching in diluted hydrochloric acid.The etching time was adapted in such a way that ap-proximately 150nm of the ﬁlm thickness was removed,leading to a ﬁnal thickness of about 650nm.Depending on the TAC and substrate temperature,three clearly different postetching surface structures appear.One specimen of each type is shown in Fig.4͑a ͒.The ﬁrst type ͑denoted by I in Fig.4͒typically comprises a very rough surface with lateral fea-ture sizes of about 300nm and with rather steep edges.For the second type ͑II ͒,the surface is almost uniformly covered by large craters with diameters ranging from 1to 3␮m and depths of about 150–400nm.The third type ͑III ͒develops a comparatively ﬂat surface with plenty of shallow craters with depths of up to about 100nm and a few large craters with lateral diameters of up to 3␮m and depths of up to 700nm ͑limited by ﬁlm thickness ͒in the case of the large craters.The postetching topography types I,II,and III are found in different growth regimes,as illustrated in Fig.4͑b ͒.With increasing T the growth changes from types I and II to III.The transition temperature for changing the regimes shifts to higher values if TAC is reduced.At the same time the Trange for type II ﬁlms becomes narrow.The transition be-tween the regimes is not sharp.In order to incorporate these ﬁndings into the modiﬁed Thornton model introduced by Kluth et al.,30the substrate temperature axis has to be res-caled taking TAC into account since increasing substrate temperature and increasing TAC both have similar effects on changing growth conditions.Four examples of type II topographies are illustrated by SEM micrographs in Fig.5.Over a broad range of TAC and substrate temperature a postetching surface topographycanFIG. 3.Transmission ͑left axis ͒and absorption ͑right axis ͒spectra of surface-textured front contacts sputtered at substrate temperature of 375°C using ZnO:Al 2O 3targets with 0.2,0.5,and 2wt %TAC,respectively.The optical data can only be considered for ␭Ͼ400nm,as CH 2I 2is strongly absorbing for smaller wavelengths.The discontinuity in the data around ␭=900nm is due to an artifact of the measurementsystem.FIG.4.͑a ͒AFM 10ϫ10␮m 2measurements ͑top ͒and ͑b ͒schematic distri-bution of etching behavior types in a matrix of parameter substrate tempera-tures and TAC ͑crosses:type I,squares:type II,and circles:type III ͒.The parameter space where we expect to ﬁnd type II ﬁlms ishatched.FIG.5.SEM micrographs of texture-etched ZnO:Al ﬁlms of surface topog-raphy type II.be achieved that is uniformly covered by similarly sized cra-ters.Changing the TAC has only a minor inﬂuence on the crater size distribution if the substrate temperature is ad-justed.AFM measurements were used to determine the rms roughness values that are given as a function of substrate temperature in Fig.6.In the case of 0.2,0.5,and 1wt %TAC,the rms roughness increases steeply beyond a certain substrate temperature.This transition temperature increases with decreasing TAC.Films sputtered from 2wt %TAC show decreasing rms roughness with increasing T .Correlat-ing this behavior to the postetching surface topography types indicates that surface topographies of regime II typically have the highest rms roughness of up to 160nm.In general,with surface topography changing from type I or III to type II the rms values increase signiﬁcantly.Besides calculating rms roughness,we analyzed the AFM data with a computer program developed by Stiebig and Schulte ͑for details,see Ref.41͒.The computer program deﬁnes—by means of each set of three neighboring AFM data points—a corresponding plane.The inclination of these planes was calculated with respect to the ﬁlm normal.The angles can be expressed in terms of crater opening angles ␥,which correspond to twice the inclination angle to the sub-strate normal.Figure 7shows a histogram of opening angles for two examples of types I and II surface topography.Bymeans of a Gaussian ﬁt,the mean crater opening angles and corresponding FWHM values were obtained ͑see Fig.8͒.While the histogram of the ﬁlm with surface topography type I ͑sputtered at heater temperature of 65°C ͒is centered around angles of approximately 90°with a broad variation,the frequency distribution of the ﬁlm of type II ͑sputtered at 460°C ͒is centered around signiﬁcantly higher angles and is comparatively paring all the data ͑see Figs.4and 8͒,we ﬁnd a mean ␥in the range of 120°–135°for type II topographies.The FWHM of the crater opening angle distri-butions is between 20°and 45°.Type I topographies ͑TAC of 0.2and 1wt %͒show a considerably larger FWHM,indicat-ing that in this case there is much more deviation in surface topography feature dimensions.C.Light scattering and solar cell applicationIn order to further study the light scattering of the texture-etched ﬁlms,the haze of these layers was determined ͑Fig.9shows haze values at a ﬁxed wavelength of ␭=1␮m ͒.While the analysis of the AFM topography has shown a continuous shift in crater opening angle,the haze value increases perceptibly when substrate temperature ex-ceeds a critical value.This temperature coincides with the change in topography type from I to II.Again,thelayersFIG.6.Calculated rms roughness data points using AFM measurements of texture-etched ZnO:Al ﬁlms with different TACs.Guide-to-the-eye lines are additionallydrawn.FIG.7.Relative frequency distribution of crater opening angles ͑data points ͒and Gaussian ﬁt ͑lines ͒of two ﬁlms sputtered using a TAC of 0.2wt %and different substrate temperatures.The center of the Gaussian ﬁt and its FWHM areindicated.FIG.8.Mean values ͑top ͒and FWHM ͑bottom ͒of the crater opening angle ␥distributions determined by Gaussian ﬁt of the frequency distribution functions of the AFM data ͑compare Fig.7͒.FIG.9.Haze values at wavelength ␭=1␮m.Type II topographies tend to have the highest haze values.Films sputtered using 2wt %TAC in general have lower haze values.sputtered using a TAC of 2wt %deviate in their behavior:the haze decreased signiﬁcantly for high substrate tempera-tures.In the investigated temperature range,we cover a to-pography type change from II to III for TAC of 2wt %cor-responding to a decrease in haze values.In order to study the light-trapping ability in thin-ﬁlm solar cells,we applied the ZnO:Al ﬁlms in ␮c -Si:H single junction cells with intrinsic ␮c -Si:H layer absorber thick-ness of about 1␮m.Figure 10shows the QE data and cell reﬂections of solar cells prepared on ZnO:Al ﬁlms with dif-ferent surface topography types.In the case of type I topog-raphy ͑TAC of 0.2wt %,T =290°C ͒,a certain degree of light trapping is already apparent because of the roughness of these layers.Due to the poor electrical quality of the ZnO:Al ﬁlm,the quantum efﬁciency of the corresponding cell had to be measured with applied negative bias in order to collect all the carriers generated.The cell current density of this cell was 22.1mA/cm 2.The electrical performance and light-trapping ability was improved by using type II front contacts ͑TAC of 0.2wt %,T =460°C ͒,consisting of relatively uni-formly dispersed craters.In this case,the cell current density reached 22.9mA/cm 2due to an improved QE in the red and NIR wavelength ranges.The gain in efﬁciency is consider-able due to the much better electrical properties of the ZnO:Al leading to a high ﬁll factor and open circuit voltage.A front TCO with type III topography ͑TAC of 2wt %,T =375°C ͒is noticeably less successful in trapping the light within the cell.It mainly consists of ﬂat regions without signiﬁcant light scattering,leading to a low QE with a cell current density of only 17.2mA/cm 2and interference fringes similar to cells prepared on smooth front contacts.28Figure 11shows a summary of cell current densities as a function of TAC and substrate temperature during sputtering.These results can be partly explained by employing the rms roughness and haze data ͑see Fig.6͒and the assumption that higher roughness implies better light trapping.It should benoted that this model is too simple to explain the light-trapping ability in all details.A detailed discussion of the relation of surface topography and light trapping can be found in the literature.40,41In the case of 2wt %TAC,the topography type changes from regime II to regime III by increasing the substrate temperature from 300to 400°C.Due to the poor light-trapping ability of type III ﬁlms,the cell current density drops steeply towards the level of cells fabricated on nontextured front contacts ͑cf.data of Rech et al.:15.6mA/cm 2͒.28In the case of 0.2,0.5,and 1wt %TAC,we mainly studied topography types I and II.There is a rather steep increase in roughness changing the surface topography re-gime.The cell current densities observed for type II front contacts are signiﬁcantly higher than 21mA/cm 2.Neverthe-less,the highest currents are found for type II ﬁlms and are found at high substrate temperatures with a rather low TAC.To further study the necessity of TCO transparency for optical cell efﬁciency we deposited a series of ␮c -Si:H solar cells with varying silicon thicknesses of 0.5,1,and 1.9␮m on type II front contacts with TAC of 0.5wt %͑T =375°C ͒and TAC of 1wt %͑T =300°C ͒.Figure 12shows the quantum efﬁciencies of these cells as well as the cell reﬂections of the two cells with a 1.9␮m ␮c -Si:H layer.Reducing the parasitic absorption in the front TCO increases the quantum efﬁciency and the cell reﬂection and thus short-ens the gap between quantum efﬁciency and 1−R .With in-creasing thickness of the intrinsic silicon layer the inﬂuence of parasitic losses in the front TCO,in the doped silicon layers,and at the back reﬂector is reduced since the absor-bance of the intrinsic silicon layer increases relative to the parasitic absorption.A cell current density of up to 26.8mA/cm 2has been achieved experimentally in the case of an intrinsic silicon layer thickness of 1.9␮m ͑see quan-tum efﬁciency in Fig.12͒.The QE data shown in Fig.12were used to estimate an effective light pass enhancement factor f .In order to con-sider the different TCO absorptions A TCO during the ﬁrst pass before entering he ␮c -Si:H,QE is normalized to:QE rescaled =QE/͑1−A TCO ͒.The absorption A TCO was calculated from the transmission and reﬂection ͑see Fig.3͒.Using an ab-sorber layer thickness d and absorption coefﬁcient ␣ofFIG.10.Quantum efﬁciency ͑left axis ͒and cell reﬂection ͑right axis ͒of a series of ␮c -Si:H solar cells with intrinsic silicon layer thickness of about 1␮m.Front TCOs of surface topography types I,II ͑both with 0.2wt %TAC ͒,and III ͑2wt %TAC ͒are compared.Cell current densities were cal-culated using the solar AM 1.5spectrum ͑see inset,values are given in mA/cm 2͒.The cell with front contact of type I was measured with an ap-plied bias of −0.5V due to poor electrical properties of the corresponding front TCO.The discontinuity in the data of the cell reﬂection around ␭=900nm is due to an artifact of the measurement system.Note the different direction of the right axis.In this case,the reﬂection curves can be inter-preted as total cellabsorption.FIG.11.Current densities calculated using measured quantum efﬁciencies of ␮c -Si:H pin cells ͑i -layer thickness of 1␮m ͒.In the case of very high-Ohmic front contacts the cells were measured with a negative bias of up to −0.5V in order to be able to collect all the carriers generated.。

钙钛矿配体连接方式

钙钛矿配体连接方式英文回答：The ligand connectivity in perovskite structures can vary depending on the specific compound and its composition. Perovskite materials are typically composed of a metal cation, such as calcium (Ca), and a ligand, such astitanium (Ti), which forms a coordination complex with the metal cation. The ligands can be connected to the metal cation in different ways, resulting in various coordination geometries.One common ligand connectivity in perovskite structures is the octahedral coordination, where the ligands surround the metal cation in a symmetrical manner. In this arrangement, six ligands are connected to the metal cation, forming an octahedron. Each ligand is connected to themetal cation through a single bond. This octahedral coordination geometry is often observed in perovskite compounds with a general formula of ABX3, where A is themetal cation, B is another metal cation, and X is the ligand.Another ligand connectivity in perovskite structures is the tetrahedral coordination, where the ligands are connected to the metal cation in a tetrahedral arrangement. In this case, four ligands are connected to the metal cation, forming a tetrahedron. Each ligand is connected to the metal cation through a single bond. This tetrahedral coordination geometry is less common in perovskite compounds, but can still be found in certain materials.In addition to these two common coordination geometries, there can be variations and combinations of ligand connectivity in perovskite structures. For example, some perovskite compounds may exhibit a combination ofoctahedral and tetrahedral coordination, where both typesof ligand connectivity are present in the structure. This can result in a more complex arrangement of ligands around the metal cation.It is important to note that the specific ligandconnectivity in perovskite structures can greatly influence the properties and behavior of the material. The arrangement of ligands can affect factors such as the stability, electronic structure, and optical properties of the perovskite compound. Therefore, understanding theligand connectivity is crucial for designing andengineering perovskite materials with desired properties.中文回答：钙钛矿结构中的配体连接方式可以根据具体的化合物和其组成而有所不同。

拓扑物态总理报告英语

拓扑物态总理报告英语## Topological Quantum Matter: A Prime Ministerial Report.Topological quantum matter is a new state of matterthat has emerged as a major focus of research in condensed matter physics. These materials have unusual electronic properties that are protected by topology, a branch of mathematics that deals with the properties of shapes and spaces.One of the most striking features of topological quantum matter is its ability to conduct electricity without any loss of energy. This is in contrast to ordinary metals, which lose energy due to the scattering of electrons by impurities and other defects. The ability of topological materials to conduct electricity without loss of energy is due to the topological protection of their electronic states.Topological quantum matter has been found to occur in a variety of materials, including semiconductors, insulators, and superconductors. Some of the most well-known examples of topological materials include topological insulators, topological superconductors, and Weyl semimetals.Topological insulators are materials that areinsulating in the bulk but conducting on the surface. This is due to the fact that the surface of a topological insulator has a different topology than the bulk. The surface of a topological insulator is home to a type of electron called a Dirac fermion, which is a massless particle that behaves like a relativistic electron.Topological superconductors are materials that are superconducting in the bulk but insulating on the surface. This is due to the fact that the surface of a topological superconductor has a different topology than the bulk. The surface of a topological superconductor is home to a type of electron called a Majorana fermion, which is a particle that is its own antiparticle.Weyl semimetals are materials that are semimetals, meaning that they have a non-zero density of electrons and holes. Weyl semimetals are characterized by the presence of Weyl fermions, which are massless particles that behavelike relativistic electrons.Topological quantum matter has a wide range ofpotential applications, including in the development of new electronic devices, such as topological insulators for spintronics and topological superconductors for quantum computing. Topological quantum matter also has thepotential to lead to new discoveries in physics, such as the discovery of new particles and the development of new theories of quantum matter.Recommendations.In light of the great potential of topological quantum matter, I recommend that the government take the following steps to support research in this area:Increase funding for research in topological quantummatter.Establish a national center for topological quantum matter research.Create a fellowship program to support graduate students and postdoctoral researchers working intopological quantum matter.Develop educational programs to train the next generation of scientists in topological quantum matter.By taking these steps, the government can help to ensure that the United States remains a leader in the field of topological quantum matter and that this exciting new area of research continues to yield new discoveries and applications.Conclusion.Topological quantum matter is a new and exciting state of matter with the potential to revolutionize ourunderstanding of physics and lead to the development of new technologies. The government should take steps to support research in this area in order to ensure that the United States remains a leader in this field.。

The Role of Mould Fluxes in Continuous Casting-So Simple Yet So Comples

Fig.1.Schematic drawing of the various slag layers formed in the mould.©2003ISIJof the ﬂuxes have property values which are consistent with those derived from empirical rules. Thus there is simplicity in the way that mould ﬂuxes perform the required func-The complexity arises from the huge range of “in mould conditions” that the ﬂux has to deal with. The large number of variables in the continuous casting process and their ef-fect on both the surface quality of the product and on process control are shown in Fig. 2. The mould ﬂux is ex-pected to compensate for many of the variations in casting conditions by being “ﬂexible” and “forgiving”. Given the large differences in the casting conditions at different plants, it is not surprising that ﬂuxes known to work suc-cessfully on one plant, frequently do not perform so well on another plant.The principal factors affecting ﬂux performance are:Casting conditions (casting speed, V c , oscillation charac-teristics).Steel grade and mould dimensions.Mould level control (which can lead to depressions etc .).Metal ﬂow since turbulent ﬂow can lead to several prob-lems eg gas and slag entrapment.In this study we will examine the effects of mould ﬂux on lubrication and heat transfer and how ﬂux performance is affected by both casting conditions and steel grade being cast. Then we will look at how the mould ﬂux is expected to deal with problems caused by turbulent metal ﬂow in the mould and how recent developments affecting metal ﬂow control may help to simplify the tasks carried out by mould Mould Flux LubricationThe liquid mould ﬂux lubricates the steel strand. It is im-portant that there is liquid lubrication throughout the strand since problems (such as star cracking) can occur if the ﬂux crystallises completely in the lower half of the mould and liquid lubrication is lost.4)The liquid friction (F l ) is given by Eq. (1) where V m is the velocity of the mould. It can be seen that the friction decreases as the viscosity (h ) decreas-es and the liquid ﬂux ﬁlm thickness (d l ) increases.F l ϭh (V m ϪV m )/d l (1)Powder consumption provides a measure of the lubrication supplied and it is very dependent upon mould size since the friction increases as the distance from the corner increases.Thus frictional forces are much larger in slabs Ͼblooms billets. Powder consumption (Q t ) is usually measured as kg ﬂux (tonne steel)Ϫ1. However, Q t can be converted to with units of kg ﬂux m Ϫ2(of mould) using Eq. (2),Q s ϭf *.Q t .7.6/R ϭd l r (2)where f * is the fraction of powder producing slag, density of the liquid slag and R is the surface area to vol-ume of the mould and is given by 2(w ϩt )/wt where are the thickness of the mould. The effect of mould dimen-sions on powder consumption, Q s , can be clearly seen in Fi g. 3since R has values of Ͻ10 for slabs, 10–15 for blooms, Ͼ20 for billets and Ͼ30 for thin slabs.2003ISIJ1480Fig.2.Schematic diagram showing the cause of various defects and operational problems.Fig.3.Powder consumption, Q s as a function of the parameter,R ϭ(surface area/volume).ϭ2/(RϪ5) (3)Empirical rules for the selection of ﬂuxes for “optimum casting” in terms of casting speed and ﬂux viscosity haveWolf subsequently converted these into empirical rules to derive the required powder consump-However, powder consumption is also dependent upon other casting parameters such as the oscillation char-acteristics, solidiﬁcation or break temperature etc. The vari-reported for powder consumptionTable 1. Itoyama17)reported a model contained contributions from (i) ﬂow emanating from the molten pool, (ii) ﬂow between parallel plates (mould and strand), (iii) the oscillation of the mould and(iv) slag trapped in oscillation marks (Qom ).In order to test the validity of these various relationships we have collected plant data from steel-plants all round the world when casting slabs, blooms, billets and thin slabs. Database 1 was collected from plant data from single tri-als for billet-bloom-slab and thin slab-casting in Ͼ30 steel-works. The following data were included: powder con-, mould dimensions, casting speed, steel composition and ﬂux chemical composition, viscosity and. The variation in powder consump-tion values for runs carried out under similar casting condi-%.19)Database 2 contained all the information listed for Database 1 plus the oscillation characteristics, frequency). Mean powder consumption values were derived in trials with identical casting conditions but% variation in the casting speed. The varia-tion of averaged powder consumption values is less than The performances of the various relationships werechecked by comparing the calculated Qs against the mea-; the results are given in Fig. 4.It can be seen from Fig. 4 and Table 1 thatthe Tsutsumi, Maeda and Kwon relations provide thesfollowed by modiﬁed Wolf andthe required viscosity (at 1573K) of the ﬂux can now be calculated for the speciﬁc casting conditions and mould dimensions. (The Tsutsumi relation16)has been adopted when the oscillation characteristics areavailable and the modiﬁed Wolf relationthese data are not available.)(iii)there are deviations from these relations for billet-casting where frequently high-viscosity ﬂuxes areused to minimise problems related to turbulent metalﬂow (e.g.slag entrapment) because the lubrication re-quirements for billets are low.It should also be noted that low powder consumption can occur when casting steels containing Ti, due to the copious formation of TiN which prevents the inﬁltration of liquid slag into the mould/strand gap.It is our belief that these high viscosity ﬂuxes operate ona different principal to conventional ﬂuxes. Most conven-tional ﬂuxes show a marked increase in viscosity when crystalline solids are formed at the break temperature (on cooling (Fig. 5(a)). However, high-viscosity slags form super-cooled liquids on cooling (Fig. 5(b)) which persist down to their glass transition temperatures,(dPa s)ϭ1013.4). No break temperature would be recorded with this type of slag. Furthermore, a super-cooled liquid will move with the strand despite having a high viscosity value.There are also a small number of ﬂuxes (about 10which work well enough in practice but do not ﬁt the re-quirements derived from the empirical rules.Thus, in summary, the only ﬂux properties which are im-portant for lubrication are the viscosity and the break tem-perature.3.Mould Flux and Heat TransferHeat transfer in the mould can be conveniently classiﬁed into vertical and horizontal heat transfer. Decreased vertical heat transfer has been reported20)of pinholes and (ii) the depth of oscillation marks by reduc-ing the length of steel meniscus. However, it is the horizon-tal heat transfer between steel shell and mould which is the more important since it has such a signiﬁcant effect on the surface quality of the steel.Horizontal heat transfer is complex involving two mech-anisms, namely, lattice or phonon conductivity (diation conductivity (kR). Radiation conductivity involves absorption and re-emission of radiated energy and can be the dominant mechanism in glassy materials at high tem-1481peratures.21–23)However, k R can be signiﬁcantly decreased by the presence in the slag ﬁlm of (i) crystallites which scatter the radiation and (ii) transition metal oxides (e.g.FeO) which absorb the radiation. It has been estimated that ϭ10–30% k c 21–23)for heat transfer across slag ﬁlms formed during slab casting. However, it may be much more signiﬁcant in glassy slag ﬁlms formed using high-viscosity ﬂuxes for billet casting. The overall resistance to thermal transfer (R *total ) between shell and mould can be regarded as a series of resistances as shown in Fig. 6and Eq. (4).R *total ϭR *Cu/sl ϩ(d /k )l ϩ(d /k )gl ϩ(d /k )cry (4)where R *Cu/sl is the interfacial resistance and d and k are thethickness and thermal conductivity of the layers in the slag ﬁlm and subscripts l , gl and cry denote the liquid, glass and crystalline layers, respectively.Y amauchi 23)introduced the contribution from radiation conduction as a parallel resistance. The most signiﬁcant terms affecting (R total ) are (i) R Cu/sl and (ii) the thickness of the solid slag ﬁlm i.e.d solid ϭd gl ϩd cry . The interfacial resis-tance R Cu/sl was found to increase with (i) increasing solid slag thickness, d solid and (ii) increasing crystallinity. This is best understood as an increase in R Cu/sl results from an in-crease in the thickness of an air gap, formed as glass trans-forms into the more-dense, crystalline phase (r cry Ͼr gl Thus the two key parameters are (i) the thickness of the solid slag ﬁlm (d solid ) and (ii) the % crystalline phase devel-oped in the slag ﬁlm.Longitudinal cracking in medium carbon (MC) steels re-sults from the 4% mismatch in the thermal shrinkage coef-2003ISIJ1482Fig.5.Arrhenius plots showing log 10viscosity (dPa s) versus reciprocal temperature (K Ϫ1) for (a) conventional ﬂux and (b) high-viscosity ﬂux for billet-casting.Fig.6.Schematic diagram showing the thermal resistances be-tween shell and mould.Fig.4.Measured versus calculated powder consumption values using (a) Wolf equation, (b) modiﬁed Wolf equation, (c)Ogibayashi equation, (d) Jenkins equation, (e) Modiﬁed Jenkins equation, (f) Tsutsumi equation, (g) Maeda equa-tion and (h) Kwon equation.Fig.7.Heat ﬂux as function of liquidus and solidus temperatures of ﬂux.23)Fig.8.Break temperature as a function of ﬂux viscosity.Fig.9.Flow diagram for model to calculate required powder consumption, viscosity and break temperature.19) Problems and Defects4.1.Control of Metal FlowThe complexity in mould ﬂux performance arises when we try to use the ﬂux to combat problems other than those related to lubrication and heat transfer. One example is using the mould ﬂux to deal with problems arising from turbulent metal ﬂow (e.g.slag and gas entrapment and SEN erosion). One way of reducing these problems is to use a high-viscosity ﬂux but this has the disadvantage that it re-duces powder consumption.©2003ISIJ10.Schematic drawing showing double roll ﬂow.ϭ1393K–%MO: b T solϭ1515K–x(MO) where xϭmole3.The effect of different ﬂux components on relevant properties. (a Tbrfraction and MOϭoxide and c refers to %F:)ISIJ1484There is a basic simplicity in the way ﬂuxes work since there are only three properties determining the opti-mum lubrication and heat transfer for the given casting con-ditions, mould dimensions and steel grade, namely, the vis-cosity, break temperature and % crystallinity in the slag When the mould ﬂux is used to combat other prob-slag entrapment due to turbulent metal ﬂow) this frequently leads to the use of ﬂuxes which do not provide optimal lubrication and heat transfer.Several new devices could help to reduce turbu-lence in the metal ﬂow and if these were successfully im-plemented they would allow the mould ﬂux to concentrate on providing optimum lubrication and horizontal heat trans-fer.AcknowledegementsThe authors would like to thank Dr. Adrian Normanton (Corus Teesside Technology Centre), Mr. Tim Mallaband (Metallurgica UK), Ms. Carolina Bezerra (Carboox, Brazil) and Dr. Shuji Takeuchi (Kawasaki Steel) for valuable dis-cussions and the provision of information.Nomenclaturedl:Thickness (m)f:Frequency of oscillation (Hz)f*:Fraction of powder containing slagQs:Powder consumption (kg1485。

APF Series STPI IHPI-LED 跑道和出发区域灯光说明书

PRO APF Series STPI/IHPI-LED, ICAO Stopbar/ Intermediate Holding Position Light Compliances:ICAO Annex 14 Vol. 1 and Part 4NATO: STANAG 3316FAA: AC 150/5345-46: Current EditionFAA: EB67 Current EditionApplicationsThe STP fixtures are used for runway incursion protection and traffic control on taxiways just before active runways. They are required for SMGCS operations. This unidirectional LED light is designed for use in all weather conditions.The IHP is an intermediate holding position light used to mark a holding position that has been established to protect a priority route. The IHP is a unidirectional yellow light with 12W power consumption.FeaturesWavetrac™ technology reduces power consumption up to60%compared to incandescentLong life LED technology with rated projected life greater than50,000h at 6.6ADust and water tight to IP 68Modular design makes fixtures easy to maintain with fewer partsto stockEngineering Brief 67 current edition compliant dimming curveHigh frequency 400Hz PWM helps eliminate flicker perceptionCompatible with all knownCCR types*3.5 Crest factor ensurescompatibility with Series CCRs10kV/5kA surge protection-40C to +85C operatingtemperature ensures safemovement of aircraft in themost challenging locationsFailure monitoring of fixturesIEC Style 3, 6-13mm low protusion housingLow Profile, < .25˝ (6.35mm) –FAA Style 3Optional arctic kitFixture T ype:STPI = StopbarIHPI = Intermediate HoldPositionSeries Version:AP1= ProAPF1Color:R/N = Red/No Light Source (STPI Only)Y/N = Y ellow/No light Source(IHPI Only)ICAO Figure:12 = Wide Beam, Cat III, ICAO fig A2-12 (IHPI Only)13 = Narrow Beam, Cat III, ICAO fig A2-1314 = Curved Sections, Cat III, ICAO fig A2-1415 = Wide Beam , Cat II, CAO fig A2-1516 = Curved Sections, Cat II, ICAO fig A2-16Fixture Dimension:08S = IEC compliant 203mm/8inch straight08R = IEC compliant 203mm/8inch right-toe08L = IEC compliant 203mm/8inch left-toeR and L options for ICAO figures 2-14 and 2-16 only (no straight option) Plug Options:P1 = One plugHeater Options:0 = No options1=ArctickitGasket Options:0 = No options1 = Gasket 1 (C.H. Canister #22537)Contact factory for other gasket sizingOrdering Information* O perates with Thyristor (Series) CCRs across 6 or 8 steps for ICAO/airportlightingOutline Drawings2-12IHPI Y 10.110W/15W 0.992-13IHPI Y 6.910W/15W 0.992-13STPI R 7.110W/15W 0.992-14IHPI Y 9.410W/15W 0.992-14STPI R 9.810W/15W 0.992-15IHPI Y 7.910W/15W 0.992-15STPI R 7.510W/15W 0.992-16IHPI Y 7.910W/15W 0.992-16STPIR8.110W/15W0.99* Use 30W/45W with the Arctic Kit Option.Figure 2-14Figure 2-13Figure 2-12Figure 2-15Dimensions:Inches (mm)Instruction Manual: 1136Shipping Weight: 5.4 lbs. 2.44kg.Shipping Volume:.17 cu. ft. .004 m3*Order base separatelyAccessories & Renewal PartsItem DescriptionCatalog Number Item DescriptionCatalog Number LED Module Replacements –Fig A2-12/Fig A2-15 Yellow Module21664-F212-F215-Y LED Module Replacements – Fig A2-15 Red Module 21664-F215-R LED Module Replacements – Fig A2-13 Yellow Module 21664-F213-Y LED Module Replacements – Fig A2-13 Red Module 21664-F213-R LED Module Replacements –Fig A2-14/Fig A2-16 Yellow Module 21664-F214-F216-Y LED Module Replacements – Fig A2-14/Fig A2-16 Red Module21664-F214-F216-R Lens Replacements – Fig A2-12, A2-13, A2-15 21642-C Lens Replacements – Fig A2-14, A2-1621643Lens Replacements, Arctic Kit Option Fig A2-12, A2-13, A2-1521644-1Lens Replacements, Arctic Kit Option Fig A2-14, A2-16 21644-2Power Supply Replacement Kit21645-1Power Supply Replacement Kit Arctic Kit Support 21645-2Feed through Assembly 21122O-Ring, Optical Housing10035-0062External Seal Gasket, 165mm bore 21612External Seal Gasket, 167.9mm bore 21613External Seal Gasket, 168.7mm bore 216142 Piece Lock Washer Kit, Qty 221647-2Home Ofﬁce: United States – +1 860-683-4300International Ofﬁces: Canada • China • Dubai • Mexico • Brazil。

toppaste

toppasteTopPaste: A Powerful Tool for Organizing and Managing Clipboard DataIntroduction:In today's digital world, Copy and Paste is an indispensable function used by millions of people daily. However, managing and organizing copied content can be a cumbersome task. That's where TopPaste comes in. TopPaste is a robust tool that enhances the copy and paste experience, providing users with a seamless way to organize, edit, and access their clipboard data efficiently. In this document, we will explore the features, benefits, and usage of TopPaste.Section 1: Getting Started with TopPaste1.1 Installation:To get started with TopPaste, users can visit the official website and download the application. TopPaste supports various operating systems, including Windows, macOS, and Linux, offering a cross-platform experience. Once downloaded, installing the application is straightforward, with user-friendly prompts guiding the process.1.2 User Interface:TopPaste boasts an intuitive user interface that is easy to navigate. The main window of the application displays a list of clipboard items, along with preview text or snippets. The toolbar provides access to various features, such as searching, sorting, and organizing clipboard data.Section 2: Key Features of TopPaste2.1 Clipboard History:One of the standout features of TopPaste is its ability to keep track of clipboard history. It saves all the text, images, and files copied to the clipboard, allowing users to access previously copied content effortlessly. With a simple click, users can view and retrieve any item from their clipboard history, even if the content was copied hours or days ago.2.2 Organization and Categorization:TopPaste allows users to organize clipboard items into categories for easy retrieval. Users can create custom categories and assign clipboard items to them. This feature is particularly useful for users who frequently copy and pastedifferent types of content, such as code snippets, website URLs, or project notes.2.3 Quick Edit:Editing clipboard content has never been easier. With TopPaste's quick edit feature, users can make changes to the copied text directly within the application. Users can delete unwanted parts, modify text, or format it according to their requirements, eliminating the need to open a separate application to make edits.2.4 Search and Filter:Finding specific clipboard items from a long list can be time-consuming. TopPaste provides a powerful search and filter functionality, enabling users to quickly locate specific content. Users can search for keywords or apply filters based on categories or time stamps, significantly improving the efficiency of managing clipboard data.2.5 Sync and Backup:TopPaste offers synchronization and backup options to ensure that clipboard data is accessible across multiple devices. Users can sync clipboard history between theircomputers or create backup files to restore data in case of system failures or device replacements.Section 3: Benefits of TopPaste3.1 Improved Workflow Efficiency:TopPaste streamlines the copy and paste process, saving users time and effort. With its advanced clipboard management features, users can quickly access previously copied content, edit it if needed, and organize it into categories. This eliminates the need to search for information repeatedly, leading to improved productivity.3.2 Enhanced Collaboration:For teams working on collaborative projects, TopPaste simplifies the sharing of information. It allows team members to share clipboard items effortlessly, ensuring that everyone has access to the same information and reducing the chances of miscommunication.3.3 Increased Data Security:TopPaste takes data security seriously. Clipboard data is securely stored on the local device, and users have control over the synchronization and backup settings. This ensuresthat sensitive information remains protected and not accessible to unauthorized individuals.Section 4: ConclusionIn conclusion, TopPaste revolutionizes the copy and paste experience, making clipboard management effortless. With its features like clipboard history, organization, quick edit, search, and synchronization, TopPaste empowers users to manage and utilize their clipboard data more efficiently. Whether you are a student, professional, or team member, TopPaste is a valuable tool that will enhance your workflow and boost productivity. Download TopPaste now and take control of your clipboard data management.。

A Steelpan Band Gig

**A Steelpan Band Gig**In the vibrant realm of musical performances, a steelpan band gig emerges as a soul-stirring and electrifying experience that leaves an indelible mark on both the heart and the senses.The prelude to a steelpan band gig is an atmosphere crackling with energy and anticipation. The venue, alive with the buzz of eager conversations and the hum of excitement, awaits the moment when the first beats of the steelpan will resound. As the stage lights up and the musicians take their positions, a palpable sense of expectancy fills the air.The opening notes of the steelpan band explode like a burst of fireworks, igniting the atmosphere with a contagious rhythm. The initial moments are a visceral invitation to surrender to the pulsating beats and let the music carry you away.The middle of the gig is a symphony of syncopation and melody. The steelpans, each with its distinct tone and pitch, come together in a harmonious blend that is both intoxicating and mesmerizing. I recall a particular performance where the band played a medley of Caribbean classics. The way the pans interwove, creating a rich tapestry of sound that evoked images of sandy beaches and tropical breezes, was truly magical.One of the most captivating aspects of a steelpan band gig is the sheer exuberance and joy that emanates from the musicians and permeates the audience. The infectious energy is contagious, making it impossible to resist moving to the rhythm.Steelpan music has deep roots in Caribbean culture, carrying with it the spirit of celebration, community, and resilience.As Duke Ellington said, "Music is a powerful force. It can make you laugh, it can make you cry, it can make you dance, it can make you think." A steelpan band gig embodies this sentiment, as it has the power to evoke a wide range of emotions and unite people in a shared experience.In conclusion, a steelpan band gig is not just a musical event; it is a celebration of life, a testament to the power of rhythm, and a bridge that connects people from different walks of life through the universal language of music.It is a moment where time stands still, and the boundaries of worry and stress dissolve in the face of the infectious beats. The magic of a steelpan band gig lies in its ability to create memories that will be cherished for a lifetime.。

光材料学英文文献(1)

Mid-Infrared Fiber LasersMarkus Pollnau1and Stuart D.Jackson21Advanced Photonics Laboratory,Institute for Biomedical Imaging,Optics and Engineering,Swiss Federal Institute of Technology1015Lausanne,Switzerlandmarkus.pollnau@epfl.ch2Optical Fibre Technology Centre,Australian Photonics CRC.The University of Sydney206National Innovation Centre,Australian Technology ParkEveleigh NSW1430,Australias.jackson@.auAbstract.The current state of the art in mid-infraredﬁber lasers is reviewed in this chapter.The relevantﬁber-host materials such as silicates,ﬂuorides,chalco-genides,and ceramics,theﬁber,pump,and resonator geometries,and the spectro-scopic properties of rare-earth ions are sers at transitions ranging from1.9to4µm occurring in the rare-earth ions Tm3+,Ho3+,and Er3+and their population mechanisms are discussed on the basis of the fundamental spectroscopic properties of these ions.Continuous-wave,fundamental-mode power levels ranging from a few mW near4µm up to≈10W near2µm have been demonstrated in recent years.Power-scaling methods and their limitations,the possibilities to op-timize the population mechanisms and increase the eﬃciencies of these lasers,as well as the prospects of future mid-infraredﬁber lasers in a number of rare-earth ions at transitions in the wavelength range beyond3µm and extending to5µm are described.1IntroductionSince the introduction of the double-cladﬁber more than a decade ago and with the recent technological advances in theﬁelds ofﬁber fabrication and beam-shaped high-power diode lasers,the performance of diode-pumpedﬁber lasers has steadily improved.Today,ﬁber lasers can compete with their cor-responding bulk crystalline systems in certain applications,especially when transverse-fundamental-mode,continuous-wave(CW)laser operation at out-put powers in the milliwatt to multiwatt range is required.The increased recent interest inﬁber lasers emitting at mid-infrared wavelengths between 2and3µm primarily relates to the high potential of these wavelengths for applications in laser microsurgery.Due to the high absorption of water in the spectral region at2.7–3.0µm,high-quality laser cutting or ablation has been demonstrated in biological tissues.In addition,laser wavelengths near 2µm could be suitable for tissue welding.A number of other potential laser applications in the mid-infrared spectral region,e.g.environmental trace-gas I.T.Sorokina,K.L.Vodopyanov(Eds.):Solid-State Mid-Infrared Laser Sources,Topics Appl.Phys.89,219–255(2003)c Springer-Verlag Berlin Heidelberg2003220Markus Pollnau and Stuart D.Jacksondetection,are currently becoming increasingly important.In all these appli-cations ﬁber lasers may ﬁnd their niches.The high development costs of fabricating ﬁbers with suﬃciently low losses in the mid-infrared spectral region has impeded the necessary research eﬀorts in the ﬁeld of mid-infrared ﬁber lasers.The currently available ﬁber materials that are suitable as host materials for speciﬁc rare-earth-doped ﬁber lasers in the spectral region 2–5µm will be introduced in Sect.2.More than any other idea,the invention of the double-clad ﬁber geometry has accelerated the output-power scaling and hence the success of ﬁber lasers.The various aspects of the ﬁber,pump,and resonator geometries will be de-scribed in Sect.3.A signiﬁcant number of spectroscopic investigations has led to a better understanding of the population mechanisms of rare-earth-doped laser systems.The fundamental spectroscopic properties of rare-earth ions in solid-state host materials will be reviewed in Sect.4.Equipped with this general information,the performance of the most important mid-infrared ﬁber laser transitions in the wavelength range 2–3µm can be understood in detail.Sect.5will be devoted to the Tm 3+ﬁber lasers at 1.9and 2.3µm,whereas the Ho 3+ﬁber lasers at 2.1and 2.9µm will be discussed in Sect.6.An impressive example of the variety of population mechanisms and operational regimes in a single system is the Er 3+2.7µm ﬁber laser transition that will be investigated in Sect.7.At wavelengths beyond 3µm,it becomes increasingly diﬃcult to ﬁnd suitable host materials for actively doped laser systems.This statement holds true for glass ﬁbers in the same way as for crystalline materials.The prospects of future mid-infrared ﬁber lasers in this wavelength range will be discussed in Sect.8.Besides general introductions to the diﬀerent topics of lasers [1,2]that include many aspects relevant also to mid-infrared ﬁber lasers,a comprehen-sive introduction to the ﬁeld of rare-earth-doped ﬁber lasers can be found in [3].2Fiber MaterialsThe choice of the ﬁber material involves a number of considerations:the maximum phonon energy,the environmental durability,the draw ability,the rare-earth solubility,and the purity of the starting materials.The maximum phonon energy of the glass sets the overall infrared transparency range of the ﬁber and the multiphonon relaxation rates which inﬂuence the quantum eﬃciency.The multiphonon relaxation rates for the common ﬁber glasses as a function of the energy gap between energy levels are shown in Fig.1.The optical transparency range relates to both the size of the band gap and also the infrared absorption cut-oﬀ,hence to the vibrational frequency νof the anion–cation bonds of the glass.For an ordered structure,ν=(1/2π) k/M ,(1)Mid-Infrared Fiber Lasers221101010M u l t i p h o n o n R e l a x a t i o n R a t e (s -1)Energy Gap (cm )Fig.1.Calculated and measured multiphonon relaxation rates as a function of the energy gap between energy levels for glasses with diﬀerent maximum phonon energies.(Data taken from [4,5])where M =m 1m 2/(m 1+m 2)is the reduced mass for two bodies m 1,m 2vibrating with an elastic restoring force k .While for disordered structures like glass,this is not an accurate expression,nevertheless,it does highlight the important contributions to the glass transparency.The relative cation–anion bond strength is intimated by the ﬁeld strength Z /r 2,where Z is the valence state of the cation or anion and r is the ionic radius.Generally,glasses composed of large anions and cations with low ﬁeld strengths display high transparency in the mid-infrared spectral region.The important physical properties of the popular glasses used for optical ﬁbers are shown in Table 1.Table 1.Properties of popular ﬁber materialsFibermaterialMax.phonon energy (cm −1)Infrared transparency (µm)Propagation losses (λat minimum)(dB/km)Thermal conductivity (W/K m)Silica1100[4]<2.50.2(1.55µm) 1.38[6]ZBLAN550[7]<6.00.05(2.55µm)0.7–0.8[8]GLS 425[5]<8.00.5(3.50µm)0.43–0.5[9]2.1SilicatesThis glass is perhaps the most important material used for optical ﬁber pro-duction [3,10],however,the maximum phonon energy is high (≈1100cm −1)and has so far limited the emission wavelength of mid-infrared ﬁber lasers us-ing this material to ≈2.2µm [11].Silica is robust and involves the very eﬀec-222Markus Pollnau and Stuart D.Jacksontive modiﬁed chemical vapor deposition(MCVD)technique forﬁber fabrica-tion.Reducing the OH−content in the glass,which has two main absorption peaks in the range1.3–2.0µm[12],improves the near-to-mid-infrared utility. Rare-earth ions such as Nd3+and Er3+which have highﬁeld strengths have low solubility in silicate glass which can lead to clustering and micro-scale phase separation.2.2FluoridesThe use ofﬂuoride glasses,especially the heavy-metalﬂuorides[13,14],as host materials for mid-infraredﬁber lasers has found wide acceptance.The most common form of heavy-metalﬂuoride glass is theﬂuorozirconate(ZrF4) composition and the most widespreadﬂuorideﬁber material is ZBLAN[15], a mixture of53mol.%ZrF4,20mol.%BaF2,4mol.%LaF3,3mol.%AlF3, and20mol.%NaF.Since it can be readily drawn into single-mode optical ﬁber[16]it is particularly important to mid-infraredﬁber lasers[17].The large atomic weight of the zirconium atom combined with relatively weak bonding provides a maximum phonon energy for ZBLAN of≈550cm−1and allows for high infrared transparency up to≈6µm.Multiphonon relaxation, however,becomes signiﬁcant for transitions at wavelengths longer than≈3µpared to silica,ZBLAN has a lower damage threshold and a lower level of inhomogeneous spectral-line broadening(Sect.4.1)because the rare-earth ion is placed in sites of a less perturbed network.The crystal-ﬁeld strength is also comparatively weaker[18].An overview of the spectroscopic properties of rare-earth ions doped into ZBLAN has been given in[7].2.3ChalcogenidesChalcogenides are composed of the chalcogen elements S,Se and Te[19,20,21]. They are environmentally durable,have a low toxicity and have reasonably large glass forming regions.When the rare-earth ions are doped into these glasses[22],the radiative transition probabilities and,therefore,the absorp-tion and emission cross-sections are high as a result of the high refractive in-dex(≈2.6)of the glass and the high degree of covalency of the rare-earth ion with the surrounding medium.Maximum phonon energies of300–450cm−1 produce low rates of multiphonon relaxation,see Fig.1,and therefore high quantum eﬃciencies.The low thermal conductivity,see Table1,is however an important factor to be considered in the design of chalcogenide-based lasers. Of the large number of rare-earth chalcogenides studied for luminescent emis-sion,the most important glasses are the sulﬁde glasses GaLaS(GLS)[23]and GeGaS[24]because of the reasonably high rare-earth solubility.2.4CeramicsStudies into the use of ceramics as host materials for the rare earths have recently made a lot of progress[25].These ceramics are composed of nano-Mid-Infrared Fiber Lasers223 crystallites of materials such as Y3Al5O12(YAG)and can be produced ina simple and cost-eﬃcient process at relatively low temperatures.This allows the fabrication of materials with very high melting points[26]that are diﬃcultto grow by other techniques such as the Czochralski method[27].This class ofmaterials is also available inﬁber geometry[28].Ceramicﬁbers combine the characteristics of crystalline materials such as high absorption and emissioncross-sections,large thermal conductivity,and even the possibility of doping with transition-metal ions[28]with the convenience of guiding the pump andsignal light in aﬁber.Currently,the losses of theseﬁbers are comparativelyhigh,but further improvement can be expected.3Fiber,Pump,and Resonator GeometriesThe light oscillating in aﬁber-laser resonator can be either free running or deliberately modulated depending on whether CW or pulsed output,re-spectively,is desired.Consequently,a large number of techniques for pulsedoperation including Q-switching and mode locking ofﬁber lasers have been explored.These techniques have been investigated intensively for the commonlaser transitions at1µm in Nd3+and Yb3+and at1.5µm in Er3+,and are usually described in combination with these lasers.The smallﬁber size limitsthe peak power through the damage-threshold intensity(propagating powerper core area)and,hence,crystalline lasers in bulk geometries or optical parametric processes are often preferred when high-energy short pulses areneeded.This argument accounts especially for mid-infrared ZBLAN-basedﬁber lasers,because theseﬁbers possess a lower damage threshold compared to silicaﬁbers.The description of mid-infraredﬁber lasers is,therefore,con-ﬁned to CW operation and speciﬁc techniques for pulsed operation ofﬁber lasers are not discussed in this chapter.In an analogous way to the optical excitation of bulk gain media,dopedopticalﬁbers can be either end pumped(core pumped)or side pumped (cladding pumped).The former method is less scalable since it relies onthe use of expensive high-beam-quality pump sources because core areas areusually<100µm2.On the other hand,the larger cladding area(>104µm2) allows for high-power diode-array pumping[29,30,31,32,33].The obvious sim-plicity of the core-pumping method negates further explanation and we will concentrate on the cladding-pumping technique:one of the most important developments inﬁber-laser technology.3.1Fiber Designs for Cladding PumpingIn the design ofﬁbers for cladding pumping,the core of theﬁber is gener-ally made to guide a single-transverse LP01mode.The shape of the mul-timode pump cladding,see Fig.2,however,remains somewhatﬂexible and can be shaped with a number of considerations in mind.The pump cladding,224Markus Pollnau and Stuart D.Jackson (a)(b)(d)CorePump cladding Outer cladding Jacket Fig.2.Principal double-clad ﬁber geometries which include (a )circular shaped pump cladding with axially positioned core,(b )circular shaped pump cladding with oﬀ-axially positioned core,(c )rectangular shaped pump cladding and (d )D-shaped pump claddingwhich in turn is surrounded by a low-refractive-index transparent polymer or glass,provides a high numerical aperture (NA)of 0.3–0.55for the pump cladding.There are three main double-clad-ﬁber layouts:circular,circular with oﬀset core,and rectangular as shown schematically in Fig.2.Maxi-mum pump-light absorption sees the core near the outer edge of the circular pump cladding [34]because a portion of the launched light is skew to the ﬁber axis and produces an inner caustic and never crosses the central re-gion of the pump cladding.Scrambling these skew rays by bending [35]or by using a graded and slightly elliptical pump cladding [36]increases the pump-absorption eﬃciency as does spatially varying refractive-index ﬂuctuations in inhomogeneous pump claddings [37].Inner caustics can be avoided by rectilinearly shaping the pump cladding [38]which has the ancillary advantage of matching the shape of diode-array output.The overall absorption coeﬃcient of the ﬁber is reduced by the ratio of the core area to the area of the pump cladding [34].The propagation losses for the rectangular-shaped pump cladding are higher and the eﬀective numerical aperture lower as compared to the circular shape [39];however,in certain cases higher dopant concentrations can provide shorter ﬁber lengths that also lead to reduced nonlinear eﬀects.A D-shaped or trun-cated circular pump cladding [40],see Fig.2d,is also eﬀective while be-ing easier to make than rectangular preforms.The circular-multimode pump cladding may also have the gain medium distributed in a ring around the edge of the pump cladding either discretely or continuously in multi-core [41]and M-proﬁle [42]arrangements,respectively.The eﬀective absorption coef-ﬁcient is now further increased while maintaining high-beam-quality output.A large-mode-area core [43]can also increase the eﬀective absorption coeﬃ-cient of the ﬁber.Recently,double-clad pump schemes have been demonstrated also with holey ﬁbers [44].These structures oﬀer the additional advantage of single-mode guiding over a broad spectral range [45].Mid-Infrared Fiber Lasers 2253.2Fiber-Laser ResonatorsTypical free-running ﬁber-laser resonators are shown schematically in Fig.3.In the simplest resonator,see Fig.3a,the pump light passes through a dichroic mirror that is highly reﬂective for the oscillating laser light.Fresnel reﬂection at the cleaved output end facet of the ﬁber can provide suﬃcient feedback for laser oscillation;however,with an output-coupler mirror –and pump retro-reﬂector –placed at the output end of the ﬁber the optical eﬃciency can be maximized.In an alternative arrangement,the pump light can be launched into the output end of the ﬁber,see Fig.3b.A dichroic mirror oriented at 45◦to the ﬁber axis extracts the laser output and a broadband highly reﬂecting mirror is placed at the rear ﬁber end.To scale the output power,each end of the ﬁber can be pumped,see Fig.3c.Periodic V-grooves [46]or prism coupling [47]along the ﬁber to distribute the pump access allow one to further scale the output power and are useful for pumping ﬁber ring resonators.Spectrally combining the output from a number of separate ﬁber lasers is also a promising power-scaling technique [48,49,50].The highest reported ﬁber-laser output powers of 110W in a singly Yb 3+-doped ﬁber [51]and 150W in a Nd 3+,Yb 3+-codoped ﬁber [52]have been obtained using arrangements as shown schematically in Fig.3c.Bragg gratings can substitute the ﬁber-butted mirror if spectrally well-deﬁned output is required.PumpPump Pump Pump Output Output OutputMM M MMFiberFiberFiber(a)(b)(c)Fig.3.Schematic diagram of resonators used for free-running ﬁber lasers with (a )a single-end co-propagating pump,(b )a single-end counter-propagating pump and (c )dual end pumps.M represents the mirror226Markus Pollnau and Stuart D.Jackson3.3Thermal IssuesAs higher pump powers become available from laser-diode systems,it is gen-erally recognized that thermal and thermo-optical issues set limitations to the power scalability of end-pumped bulk-laser systems.Owing to the unfavor-able temperature dependence of thermal and thermo-optical parameters[53], the large heat load in the crystal leads,ﬁrstly,to a signiﬁcant temperature increase in the rod,secondly,to strong thermal lensing with pronounced spherical aberrations,and ultimately,to rod fracture in a high-average-power end-pumped system.Due to its geometry,theﬁber provides potentially high pump-and signal-beam intensities without the drawbacks of signiﬁcant thermal and thermo-optical eﬀects.Its large surface-area-to-volume ratio means that the heat generated from multiphonon relaxation in the core is dissipated eﬀectively by radiation and convection from the outer surface of theﬁber.This is es-pecially true for single-clad,core-pumped single-modeﬁbers where this ratio is highest[54].Double-cladﬁbers have a relatively smaller surface-area-to-volume ratio and thermal issues need to be taken into account[6,55,56]. Thermal management will be required when very high output powers are desired.In particular,for high-power mid-infrared operation,thermal man-agement may be very important because of the decreased quantum eﬃciency and the consequently higher amount of heat dissipation.4Spectroscopic and Laser Propertiesof Rare-Earth IonsThe structure of a glass is less well deﬁned as compared to a crystalline mate-rial.The local variation of the chemical environment of active ions in a glass has a number of consequences.Most important,the active ions may undergo chemical reactions during the fabrication process and be incorporated in the host in several oxidation states with diﬀerent spectroscopic properties.Oxi-dation states other than the desired one may act as impurities that introduce undesired optical eﬀects such as parasitic pump absorption,the reabsorption of oscillating laser light,the lifetime quenching of the laser ion,and the trap-ping of the excitation energy.A stable oxidation state of the optically active ion is thus highly desirable.The necessity of a stable oxidation state excludes a number of transition-metal ions from the list of suitable dopants in glass environments.This is one of the possible reasons why examples of transition-metal-ion-doped lasers in glass hosts are rare.On the other hand,most of the rare-earth ions prefer to stabilize in the trivalent oxidation state and are, therefore,suitable candidates as glass andﬁber dopants.This chapter will, therefore,concentrate on the rare-earth ions as active dopants ofﬁber lasers.Mid-Infrared Fiber Lasers227 4.1Spectra of Rare-Earth Ions in GlassesThe optical transitions of lanthanide(rare-earth)ions in the visible and in-frared spectral region occur within the4f subshell.This subshell is shielded by the outer5s and5p subshells and the inﬂuence of the host material isrelatively small compared to,e.g.,the3d transitions in transition-metal ions.The electronic structure of trivalent rare-earth ions derives from the perturba-tion of the4f energy level in the central-ﬁeld approximation by the noncen-trosymmetric electron–electron interaction,the spin–orbit interaction,andthe crystal-ﬁeld splitting(Stark eﬀect);see the example of the energy-level scheme of Er3+in Fig.4.The spin–orbit multiplets are commonly denotedby their2S+1L J terms in Russell–Saunders coupling,although the4f elec-trons of lanthanide ions exhibit intermediate coupling and the total angularmomenta J of the spin–orbit multiplets are linear combinations of the totalorbital angular momenta L and total spins S.Single crystal-ﬁeld(Stark)tran-sitions between two spin–orbit multiplets cannot be distinguished in glasses at ambient temperature,because inhomogeneous spectral-line broadening oc-curs due to the local variation of the ligand electricﬁeld.Also homogeneous (lifetime)broadening mechanisms are relevant in a number of glasses.This spectral-line broadening makes glasses the preferred hosts when broadband,Fig.4.Energy-level scheme of triva-lent erbium indicating the splitting ofthe4f11conﬁguration in the central-ﬁeld approximation by the noncen-trosymmetric electron–electron inter-action,the spin–orbit interaction,andthe Stark splitting by the local elec-tricﬁeld of the host material(indi-cated only for selected spin–orbit mul-tiplets)228Markus Pollnau and Stuart D.Jacksoncontinuous tunability of lasers is desired.On the other hand,the spectral-line broadening leads to lower absorption and emission cross-sections for the same transition in glasses compared to single-crystalline hosts.The reducedcross-sections lead to generally higher pump threshold of laser transitions inglasses,a fact that is compensated inﬁber geometry because a high pump conﬁnement is achieved over the wholeﬁber length.4.2Intraionic ProcessesGenerally,the probability of an allowed electric-dipole transition is seven or-ders of magnitude larger than that of an allowed magnetic-dipole transition.Since electric-dipole transitions within the4f subshell are parity forbidden,the intensities of radiative transitions in rare-earth ions are weak and the radiative lifetimes of the emitting states are long,typically in the ms range.Mixing of the4f states with higher-lying(typically5d)electronic states of opposite parity at ion sites without inversion symmetry,however,means thatelectric-dipole transitions become partially allowed and are usually the dom-inant transitions between4f electronic states.The oscillator strengths f and integrated absorption and emission cross-sectionsσof these spin–orbit mul-tiplet-to-multiplet transitions can be calculated with the help of the semi-empirical Judd–Ofelt theory[57,58].If the degree of inhomogeneous spectral-line broadening is relatively small and the absorption and emission spectraremain structured,as is the case for ZBLAN,the cross-sectionsσ(λ)at in-dividual wavelengths that are relevant to pump absorption and stimulatedemission of narrow laser lines must be determined experimentally.Besides ground-state absorption(GSA),excited-state absorption(ESA) of pump photons,see Fig.5a,can play a signiﬁcant role inﬁber lasers,specif-ically in the case of high-intensity core pumping.An experimental examplewill be given later in Sect.7.1.Since the absorption increases exponentially with the absorption coeﬃcientα(λP)=Nσ(λP),ESA becomes relevant forthe population dynamics of a laser when(a)the ESA and GSA cross-sectionsσ(λP)are comparable at the pump wavelengthλP and(b)the population density N of the excited state in which the second pump-absorption steporiginates becomes a signiﬁcant fraction of the density of ions in the ground state,i.e.,a large degree of ground-state bleaching must be present for ESA to play a signiﬁcant role.A radiative transition from an excited state i to a lower-lying state j is characterized by the radiative rate constant A ij.If the decay occurs to sev-eral lower-lying states,the overall radiative rate constant A i is the sum of all individual rate constants.The branching ratio of each radiative transition is deﬁned asβij=A ij/A i.Radiative decay of excited states is in competition with nonradiative decay by interaction with vibrations of the host material, called multiphonon relaxation.The rate constant of a multiphonon relaxation process decreases exponentially with the energy gap to the next lower-lying state and with the order of the process,i.e.,the number of phonons required(a)Ion (b)12Donor Ion Acceptor IonAcceptor Ion (c)Sensitizing Ion ALaser Ion B(d)1Laser Ion AQuenching Ion B (e)Donor Ion Acceptor Ion(f)1Donor Ion Acceptor Ion Fig.5.Intra-and interionic processes inﬁber lasers:(a)excited-state absorption (ESA);(b)energy migration;(c)sensitization and(d)quenching of a laser ion by an ion of a diﬀerent type;(e)cross-relaxation and(f)energy-transfer upconversionto bridge the energy gap[59,60].This fact is illustrated in Fig.1for diﬀer-ent glasses.The rate constant of multiphonon relaxation increases with host temperature.The measurable luminescence lifetimeτi of an excited state i is the inverse of the sum of the overall radiative rate constant A i and the rate constant of multiphonon relaxation,W i.The radiative quantum eﬃciency is deﬁned asη=A i/(A i+W i).The inﬂuence of multiphonon relaxations is stronger in oxides as com-pared toﬂuorides because of the smaller atomic mass m2of the anion and the larger elastic restoring force k,see(1),due to stronger covalent bonds in oxides[3],both resulting in larger maximum phonon energies in oxides.A brief example:The luminescence lifetime of the4I11/2upper laser level of the erbium3µm laser(Sect.7)is partly quenched by multiphonon relaxation. Typically,nonradiative decay becomes dominant ifﬁve or less phonons are required to bridge the energy gap.With an energy gap between the4I11/2and the next lower lying4I13/2levels of≈3400–3500cm−1,radiative decay pre-vails for phonon energies below≈600cm−1,roughly the maximum phonon energy of ZBLAN,see Table1.Fluorides are,therefore,preferred over oxides as host materials for most of the mid-infrared laser transitions.Like absorption,the strength of a stimulated-emission process is char-acterized by the emission cross-sectionσ(λL)of the laser transition.From a simple analysis,for one resonator round-trip of oscillating laser photons, the productτσ(λL)withτthe luminescence lifetime of the upper laser level, is identiﬁed as a“ﬁgure of merit”for a possible laser transition.The larger this product,the lower is the expected pump threshold of the laser transition. This“ﬁgure of merit”,however,does not take into account the numerous par-asitic eﬀects that can occur in the population dynamics of a laser system,such as pump ESA,reabsorption of laser photons,and energy-transfer processes. It is often these parasitic processes that lead to surprising performance char-acteristics–as likely in the negative as in the positive sense–and make the interpretation of rare-earth-doped solid-state lasers challenging.Examples will be discussed in Sects.5–7.4.3Interionic ProcessesIn addition to intraionic excitation and decay mechanisms,radiative en-ergy transfer due to reabsorption of emitted photons by other active ions in the sample and nonradiative energy-transfer processes due to multipole–multipole or exchange interactions between neighboring active ions can occur. Radiative energy transfer leads to an increase in the luminescence lifetime. Among the nonradiative energy-transfer processes,most common is the elec-tric dipole–dipole interaction,which can occur as a direct[61]or phonon-assisted[62]energy transfer.A direct energy transfer requires spectral reso-nance between the involved emission and absorption transitions whereas an indirect transfer can also be nonresonant,i.e.,an existing energy gap between the emission and absorption transitions involved in the transfer is bridged by。

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A toppled structure with sliding in the Siwalik Hills,midwestern NepalNaresh Kazi Tamrakar a ,Shuichiro Yokota b,*,Osamu Osaka caCentral Department of Geology,Tribhuvan University,NepalbDepartment of Geoscience,Faculty of Science and Engineering,Shimane University,1060Nishikawatsu,Matsue 690-8504,JapancNihonkai Engineering Consultants Co.,JapanReceived 8September 2000;accepted 4September 2001AbstractNot only the high mountainous regions but also the southern hilly regions of Nepal frequently suffer from landslides and debris flows.An interesting toppled structure with sliding was distinguished on an excavated slope facing the major highway in one such region,the Siwalik Hills.These hills are mostly composed of alternating beds of sandstones and mudstones of the Siwalik Group.A distinct contrast in the direction of dip of the strata was recognized on the excavated slope;the strata in the upper portion of the slope dipped southward,whereas those of the foot slope dipped northward.This indicated that toppling of strata had occurred.The northward direction of toppling and the angle and axis of rotation of the block were determined by examining the distribution of the attitudes of the strata.Steeply dipping strata influenced by southward thrusting along the major tectonic line,contrasting rigidity between sandstone and mudstone,and rapid dissection by a river were the major causes of the toppling.The bottom surface of the toppled part dipped gently not only northward but also partially westward,showing that the toppled block slid westward after toppling.Evidence for tension cracking and scarplets on the top slope indicated that the block was still actively sliding.Because steeply dipping sandstone and mudstone strata are widely distributed in the Siwalik Hills,similar structures are inferred to exist in many places.Therefore,the investigation of such slope structures is important,and their instability must be evaluated before road construction and excavation projects are carried out in Nepal.D 2002Elsevier Science B.V .All rights reserved.Keywords:Landslide;Topple;Slide;Nepal;Siwalik Group;Siwalik Hills1.IntroductionNepal is a mountainous country where landslides are likely to occur along steep slopes.Although land-slides with debris flows are well known in the high mountainous regions (for example,Sharma,1981;Heuberger et al.,1984;Yagi et al.,1990;Dhital et al.,1993),they also frequently occur in the southern hilly regions,including the Siwalik Hills in midwest-ern Nepal (Sharma,1981;Dangol,1998).Individual landslides occurring in the Siwalik Hills are small in scale,but total damage due to them is large,because the Siwalik Hills include population-dense areas and important infrastructure such as the East–West High-0013-7952/02/$-see front matter D 2002Elsevier Science B.V .All rights reserved.PII:S 0013-7952(01)00095-3*Corresponding author.Fax:+81-852-32-6469.E-mail addresses:mama@.np (N.K.Tamrakar),yokota@riko.shimane-u.ac.jp (S.Yokota),nityousa@plum.ocn.ne.jp (O.Osaka)./locate/enggeoEngineering Geology 64(2002)339–350way,the only transportation link between eastern and far western Nepal.An interesting structure related to the occurrence of landslides in the Siwalik Hills was distinguished on an excavated slope facing the East–West Highway in the Chor Khola area(Fig.1).The slope is mainly com-posed of sandstones and mudstones of the late Mio-cene Siwalik Group(Appel and Rosler,1994).Strata in the area generally dip steeply northward due to southward thrusting along the major tectonic line,the Fig.1.Chor Khola landslide area along the Surai Khola River,midwestern Nepal,and outline of the lithological distribution of the Siwalik Group.Boundaries between lithologic units are based on Dhital et al.(1995).A topographical and geological profile along X–X V is shown in Fig.2.N.K.Tamrakar et al./Engineering Geology64(2002)339–350340Main Frontal Thrust.However,a portion of the structure mentioned above shows a characteristic southward dip.Because slopes along the highway are steep and face a major river that deeply dissects the Siwalik Hills,most of these slopes may be relatively unstable. Although the above-mentioned structure is not a new landslide,it is regarded as an important slope structure that may influence future landsliding.Therefore,it is necessary to clarify its structural details for the eval-uation of future slope instability along the highway. We also investigated and analyzed the structure as an example of one type of landslide in Nepal,to under-stand the formative processes,mechanisms and causes and to evaluate the probability of future landsliding.2.Steeply dipping strata of the Siwalik Group and rapid dissection by the Surai Khola River The Chor Khola area of the Siwalik Hills is located in a tectonic zone bounded by the Main Frontal Thrust (MFT)on the south and the Rangsing Thrust(RT)on the north(Fig.1).The strata of the Siwalik Group in the region constitute a homoclinal structure with a roughly E–W strike and a northward dip,as shown in Fig.2.The dip angle,however,changes from place to place and reaches a maximum value in the Chor Khola area(Fig.2).The hills in the Chor Khola area range from500to 1000m in elevation and are deeply dissected by the Surai Khola River,which flows southward(Fig.1). The Siwalik Group is divided into several formations in this tectonic zone on the basis of lithofacies:from lower to upper are the Bankas,Chor Khola,Surai Khola,Dobata,and Dhan Khola formations(Dhital et al.,1995).Taken as a whole,these formations attain a thickness of more than5km(Tamrakar et al.,1999). The strata in the Chor Khola area belong to the Chor Khola Formation,which is estimated to be younger than9Ma(Appel and Rosler,1994).The formation is subdivided into two members,the Jungli Khola (lower)and Shivgarhi(upper)members.The former is composed mainly of fine-to medium-grained cal-careous sandstones and variegated mudstones, whereas the latter is mainly coarse-grained sandstone. However,the lithofacies of the Shivgarhi Member change from place to place and sometimes include thick mudstone layers.Medium-to coarse-grained sandstones generally have a‘‘salt-and-pepper’’texture (spotted appearance of dark minerals on the ground-mass of light minerals).Although the mechanical properties of sandstones of the Siwalik Group depend on texture,grain size, and calcium carbonate content,the uniaxial compres-sive strength of sandstones of the Chor Khola area ranges from50to100MPa according to laboratory measurements(Tamrakar et al.,1999).In contrast,the strength of the mudstones is far less than that of the sandstones,and may be lower than10MPa,based on the results of field measurements for penetrative hard-ness using a portable hardness tester(Murata Seisa-kusho,Japan).The highest river terraces along the Surai Khola River range in altitude from310to420m above sea level(Tamrakar,1999).However,these terraces grad-ually increase in relative height from80to120m above the riverbed,respectively,from north to south, and is95m high in the Chor Khola area.This means that this area is an uplifted zone that has suffered simultaneously from dissection by a large river.The Surai Khola River flowing through this upliftedzone Fig.2.Steeply dipping strata of the Siwalik Group.The southern rim is bounded by the Main Frontal Thrust.Location of X–X V is shown in Fig.1.N.K.Tamrakar et al./Engineering Geology64(2002)339–350341forms,therefore,an antecedent valley.Steep slopes facing the river indicate that rapid dissection contin-ues.The Siwalik Hills are generally believed to have uplifted after1.7Ma,and turned out to hills around 0.7Ma(Kizaki,1994).Because most of the strata dip steeply,the uplifting might have continued as the strata were tilted northward,and both uplift and northward dipping might have been brought about by southward thrusting along the MFT(Nakata,1982, 1988).Because the rate of displacement along the thrust is estimated to be1.8mm/year since the Neo-gene(Nakata,1988),the tilting of the northward dipping strata may also have been rapid,and they are probably still actively moving.Not only southward dipping strata but also many landslides and rock falls have been observed on slopes in and around the Chor Khola area(Tamrakar,1999). Those phenomena may relate closely to the conditions of the steep slopes facing the river.3.Distribution of overturned strata and geologic structures of the slopeFig.3shows a southward dipping structure observed on an excavated slope near the Chor Khola Bridge in the Chor Khola area.As shown in this figure,alternating strata of sandstones and mudstones dip southward in the upper portion of the slope, whereas they dip northward in the lower portion. The strata of adjacent slopes also dip northward.The upward-fining sequence in each stratum indicates that the southward dipping strata are reversed.There-fore,this structure shows that upper portion of the slope has toppled northward and consequently has overturned,covering the underlying normal strata,at least after sedimentation and consolidation.The boun-dary between normal and reversed strata,which exhibits a smooth but asymmetric curve,was traced for more than70m,and is indicated by the broken line in Fig.3.Fig.4is a detailed topographic and geologic map that we constructed from surveys on and around the hill slope.The exposure with overturned strata shown in Fig.3is the west-facing slope to the east of the highway shown on the map.The northern to north-western foot slope faces the Chor Khola River,a tributary of the Surai Khola River.The overturned strata were traced eastward,as indicated by the broken line in Fig.4.They extended over17000m2and covered an ellipsoidal-shaped area,with the longer axis extending E–W,which is concordant with the strike of the strata.The western margin of the ellipsoidal block is the left bank of the Chor Khola River,whereas the eastern one was not confirmed.The western margin was also partially cut by steep slopes,as shown in Fig.3.The steep slope facing the East–West Highway was arti-ficially excavated,but it may roughly reflect the original natural slope formed by river erosion.This block comprises a narrow ridge trending N–S and a gentle,northward-concave slope on its east(Fig.4).The boundary between the normal andreversed Fig.3.A sketch showing strata with normal or reversed facing exposed in the Chor Khola landslide area along the East–West Highway.The boundary between normal and reversed strata is shown as a broken line.N.K.Tamrakar et al./Engineering Geology64(2002)339–350342strata was confirmed in some exposures around the ridge and slope.From these exposures,the boundary plane was estimated to be 270to 280m above sea level on the northern margin and to dip gently south-ward.In contrast,the southern margin of the block was estimated to be at an elevation of between 340and 350m,and the boundary plane slopes steeply northward (Fig.3).A gently sloping surface was recognized on the narrow ridge at 290to 315m elevation (Fig.4).Although no river terrace deposits were recognized here,the altitude of this surface correlates with that of terrace surfaces distributed along the Surai Khola River (Tamrakar,1999).These gentle slopes are thus probable terraces.As already described,the strata composing the block were mainly alternating beds of sandstones and mudstones belonging to the Chor Khola Forma-tion.Although lithofacies of the formation as a whole were sandstone-rich,the strata in the vicinity of the block had a higher proportion of mudstones.The sandstones were medium-to coarse-grained and each bed was 1to 2m thick.The mudstones were thick-bedded and massive to laminated.Both overturned and normal strata on the gentle north-facing slope were covered with thin debris flow deposits consisting of loose,unconsolidated materials.The fundamental geological structures of this area are shown in Fig.5:(a)of Fig.5is profile A–A V trending NW–SE,and (b)is profile B–B V ,whichFig.5.Toppled structure in geological profiles A –A V (a)and B –B V (b).Profile locations are shown in Fig.4.N.K.Tamrakar et al./Engineering Geology 64(2002)339–350344trends N–S.The locations of both profiles are shown in Fig.4.A mass of overturned strata overlies normal strata.The thickness of the mass is 20to 30m in the A–A V profile,but less than 10m in the B–B V profile (Fig.5).Most of the overturned strata along the B–B V profile may have been eroded away (Fig.5b).The normal strata struck N55j –75j E,and dipped 50j to 70j N (Fig.4).On the other hand,the reversed strata stroke N70j –80j E and dipped 20j to 50j S.Although the dip angles between the normal and reversed strata differed,their strikes were almost the same.From these structural characteristics,we in-ferred that the overturned strata might have formed by a forward rotational movement,or toppling,around an axis.However,the variable dips of the reversed strata from the foot to the top slope indicated different degrees of rotation.The strata are heavily weathered just below top slope,whereas less weathered in deeper portion on the excavated slope.Sandstones were blocky,and mud-stones were soft,loose,and sometimes disintegrated in heavily weathered portions.Consequently,rock mass condition of the toppled block is loose as a whole.The distribution of weathering is almost con-cordant to natural slope.As both normal and over-turned strata form the natural slope,this distribution of weathering has been produced after toppling of strata.Joint planes were well developed within the sand-stones.Especially prominent were three systems:bed-ding joint (N75j E/60j NW),cross-joint (N68j E/42j SE)and vertical joint (N83j W/83j NE)independ-ent of the directions of strike of the strata.The bedding joints were spaced 0.5to 2m apart and were open between 1and 2cm.The cross-joints were spaced 30to 40cm apart and were usually open (2to 5cm).The vertical joints were spaced 5to 10cm apart,and the 0.2-to 1.0-cm apertures were filled with clayey material.The cross-joints might have been formed by overburden pressure before tilting,whereas the vertical joints may be more recent.Sandstones of the toppled block comprised open and wide (4–5cm)fractures nearly parallel to the strike and perpendicular to the dip of the strata.These fractures (i.e.,flexural cracks)might have formed due to tensile stress during the process of toppling.On the top slope of the southern margin of the block,numer-ous tension cracks tens of meters long and trending E–W were recognized,including open cracks tens of centimeters wide and sometimes accompanied by low scarplets with the same strike.These may be related to the sliding movements of the block,and they indicate that the block may be still actively sliding,as dis-cussed later.4.Toppling rotation of the block and its sliding movementsFig.6shows stereographic projections of strikes and dips of normal and reversed strata measuredinFig.6.Stereographic projection of strikes and dips of normal and toppled strata:(a)plotted poles of strata on upper hemisphere,(b)a contoured diagram,and (c)average direction of toppling indicated by the arrow and rotation angle estimated based on the clustering of the poles.N.K.Tamrakar et al./Engineering Geology 64(2002)339–350345this area.They show two distinct clusters (Fig.6(b)).If toppling rotation was responsible for the reversed strata,these two clusters may correspond to the average directions of normal and reversed strata,respectively.Therefore,we can determine the rotation angle,axis of rotation and the direction of toppling based on these clusters.From these projections,the average direction of toppling was N12j W,and the rotation angle was from 74j to 118j (Fig.6(c)).Di-rection and plunge of the rotation axis were 256j to 260j (S76j W–S80j W)and 1j to 3j ,respectively.Boundaries between normal and reversed strata confirmed in many locations were used to restore the configuration of the bottom surface of thetoppledFig.7.Configuration and extent of toppled and slipped block.Broken lines show the bottom surface of the toppled block.Arrows indicate slip directions roughly.N.K.Tamrakar et al./Engineering Geology 64(2002)339–350346block,which was expressed by contour lines (Fig.7).On the whole,the surface dipped northwestward,but the dip was variable (Fig.7).The surface became gently dipping downslope and almost flat at 290m elevation east of the narrow ridge in the central portion.The surface further dipped downslope from the gentle portion approximately 17j and 42j ,towards N10j E and N60j W,respectively.From the overall configuration of the bottom sur-face,the toppled block moved in two directions after toppling.The first direction,N10j E,slightly differs from the toppling direction as a whole (N12j W).The latter direction indicates that the toppled block slid with a westward translation after toppling.Evidence,such as tension cracking and scarplets on the top slope as described above,is concordant with these move-ments and indicates that landsliding is still occurring.5.Mechanisms and processes forming a topple structureIn general,toppling phenomena are well known on steep slopes composed of phyllitic or jointed rocks (for example,Chigira,1992),which tend to creep due to gravity.However,the present example is a case of relatively younger alternating beds of sandstones and mudstones,which is a similar example reported by De Freitas and Watters (1973).Therefore,the occurrence of toppling may closely depend on extraordinary topographic and geologic conditions in the region,which is characterized by active tectonics and rapid stream dissection.A schematic diagram of a toppling rotation of alternating beds of sandstones and mudstones is shown in Fig.8.In general,easy shearing alongbedding planes is required for toppling to occur in columnar beds (Goodman,1989).Considering the high strength contrast of the order of 102between the sandstones and mudstones (Tamrakar,1999)and the high proportion of mudstones in this area,shearing may tend to occur along individual bedding planes between sandstone and mudstone,as shown in Fig.8.Then,the slabby to blocky sandstones may experience flexural slip along the bedding planes.Consequently,topographical and geological con-ditions that favored the toppling are as follows:(i)steeply dipping strata,(ii)a high proportion of mud-stone to sandstone in alternating sequence,(iii)a high contrast in strength between sandstone and mudstone,and (iv)rapid stream dissection at the foot of slopes.If the strata are vertical or steeply dipping to the mountain,they tend to be susceptible to toppling due to gravity.However,the bedrocks in this area dip mostly downslope.Therefore,it is not easy for the strata to topple under natural conditions,although toppling could occur on cataclinal underdip slope (Cruden,1989;Cruden and Hu,1994).Probably,the rapid tilting due to thrusting and seismic vibration also acted on the strata as the initial disturbance.Buckling with slipping along bedding planes is also expected at this stage.According to Goodman (1989),a buckling failure tends to occur in steeply dipping strata parallel to the dip slope and involving planar movement along the discontinuity.Because of the buckling and slippage along the bedding planes,block flexural toppling of columns of sandstone and mudstone might have occurred as a result of movement along cross-joints since such joints are well developed within the sand-stones.Also,movement along the flexural cracks,produced simultaneously with toppling,must have contributed to the overturning of the columns.During this stage,sliding and toppling may occur simulta-neously because the toppling steepens cross-joints (Goodman and Bray,1976).Repeated toe erosion and valley incision by the river as a result of uplift related to thrusting caused the sliding of the toppled block by removing the lateral support of the slope.The valley incision may have occurred after burial of the toppled block.Toppling and sliding may be contributed by (i)loading of colluvial debris in the cracks developed on the head of the slope and (ii)forming flexural cracks during toppling.A similar process has been reportedbyFig.8.A schematic diagram showing toppling of a portion of alternating strata of sandstone (stipples)and mudstone (dashes).Shearing between each stratum is required for toppling to occur.N.K.Tamrakar et al./Engineering Geology 64(2002)339–350347Cruden et al.(1993).Slip planes could have devel-oped parallel to the flexural cracks and some cross-joints causing sliding of the block.From our overall observation and mapping,we interpreted the landslide structure as a complex rock topple–rock slide (Var-nes,1978;Cruden and Varnes,1996).6.Processes from northward toppling to westward slidingIf forward toppling was the major phenomenon that produced the overturned structure,we must also evaluate the series of processes from the initial insta-bility to the northward toppling and westward sliding.Fig.9is a series of schematic diagrams that depict the changes from toppling to sliding described above.The Surai Khola River flows southward because of tec-tonic uplift and tilting (Fig.9(a)).As uplift proceeded,the river began dissecting the terrain.These events could have occurred after the early Pleistocene;thus,the toppling might be as young as late Pleistocene.Other clues to the date of the toppling include (a)the occurrence of a possibly Holocene terraces,corre-sponding to the terrace surfaces of the Surai Khola River,on the toppled block (Fig.4),(b)the presence of weathering structures concordant with the natural slope of the toppled mass (therefore weathering occurred after toppling),and (3)the occurrence of recent surficial landslide events (Figs.4and 5).The original slope of the topple–slide block has become two concave slopes,west-facing slope and north-facing ter events,including shallow landslides and weathering,and the undisturbed veg-etation cover indicate the older nature of the topple–slide event.Hence,it shows dormant-old to dormant-young (Keaton and DeGraff,1996)activity inferred to have occurred approximately in the late Pleisto-cene to the late Holocene.Sliding of the toppled block probably followed the toppling event and theformation of the terraces,and therefore,should be younger (late Holocene)than the formation of the surface.Based on these analyses,we reconstructed the toppling processes and the reactivated sliding as follows.The strata in the area had tilted gradually with time due to regional tectonic movements (Fig.9(a)).The initial toppling might have occurred at any stage (Fig.9(b)).Considering that the strata are not so rigid and have high contrast in strength between sandstone and mudstone layers,buckling of steeply dipping strata due to gravity might have occurred as initial processes leading to the deformation of the layers.The buckled layers then underwent flexural slip along joints.Flexural toppling of the strata (Fig.9(c))probably occurred in the late Pleistocene.Gully erosion must have swept away the mudstones between the steeply dipping strata and contributed to the separation of the layers downslope from those upslope (Fig.9(c)).Debris brought from upslope probably accumulated,thereby loading the head of the slope (Fig.9(c)),which probably induced further toppling.Weathering proceeded in the toppled block before burial of the portion in the Chor Khola valley by riverbed deposits,and the terrace formed on the toppled block (Fig.9(d)),probably in the Holocene.The Chor Khola River re-incised the valley afterward during rapid uplift.The toppled block slid due to (a)formation of a slip plane parallel to the cross-joints and flexural cracks,(b)toe erosion of the slope,which caused increased instability,and (c)loading of the head by colluvial debris,which increased surcharge.These events could have occurred in the late Holocene (Fig.9(e)).The slip plane may also have developed as a result of the toppling and opening up of the cross-joints and the flexural cracks.Hence,the block slid downslope as dissection of the valley proceeded.Numerous tension cracks must have occurred on the head of the slope (Fig.9(e)).Fig.9.Schematic diagrams showing changes from toppling to sliding.Refer to text for explanation.N.K.Tamrakar et al./Engineering Geology 64(2002)339–350348River deposits buried the toe of the slide(Fig.9(f)) and increased stability.A new slip plane developed on the crest of the toppled mass,along which debris slid to give rise to the debris flow deposits(Fig.5(b)). Such deposits have widely covered the topple–slide block on the northern slope.Even more recent debris flows have since occurred.Currently,the river is un-dercutting its banks,thus increasing instability.Wide-open tension cracks indicate activity of the unstable block.Therefore,this block tends to be unstable with the removal of stabilizing terrace sediments on the western slope.7.DiscussionThe structure described here is a complex rock topple–rock slide.It is the first structure of this kind to be described in Nepal,and may represent one type of landslide found in the Siwalik Hills.The above-men-tioned structure recognized along the highway provides clues that thorough geological and engineering geo-logical studies of the Siwalik Hills are required before choosing the route of a highway and other structure.The structure has experienced a long history from tilting and toppling to sliding.However,between top-pling and major sliding,river–valley dissection,deve-lopment of geomorphic surfaces,and re-incision of the valley may have occurred.This series of events is associated with the history of the geological processes in and around the structure.It is clear,on one hand, that the toppling was a local geological event occur-ring since the late Pleistocene,and that this event and other similar events may relate to active tectonic movements and also to seismic vibration as an initial trigger for toppling.The presence of the geomorphic surface(elevation290–315m;Fig.4)may indicate that major river incision,cessation and re-incision occurred successively,perhaps due to changes in the local base level.On the other hand,the existence of such a structure crossed by the highway is a threat to such infrastruc-ture.Sliding is still active,although recent river-terrace deposits have stabilized some parts of the toe.How-ever,the river has also been eroding the stabilizing deposits.Therefore,the structure will become unstable as toe erosion proceeds.We suggest mitigating possi-ble landsliding movements by constructing walls to protect the toe and by properly managing the river flow and drainage conditions in the moved mass.8.ConclusionsA complex rock topple–rock slide structure dis-tinguished on excavated slopes in the Siwalik Hills, midwestern Nepal,was investigated and analyzed to understand the formative processes,mechanisms and causes.We obtained the following results.(1)The slope where the rock topple–rock slide structure was distinguished is located in a tectonic zone in Nepal bounded by two major thrusts.The strata were alternating beds of sandstones and mud-stones of the late Miocene Siwalik Group,and they dipped steeply northward in the area of the structure.(2)The toppled structure was characterized by a distinct contrast in the dips of the strata:strata in the upper portion dipped southward,whereas those in the foot dipped northward.The former were overturned strata.Therefore,the structure was formed by the for-ward rotation around an axis,which is toppling.(3)The northward direction of the toppling and the angle and axis of rotation of the block were deter-mined from the data on the attitudes of the strata.(4)Major causes for the toppling were(i)steeply dipping strata,(ii)a high proportion of mudstone to sandstone,(iii)a high contrast in strength between sandstone and mudstone,and(iv)rapid river dissec-tion at the foot of the slope.(5)The shape of the bottom surface of the toppled block showed that the block slid westward because of river erosion after the toppling.Consequently,the struc-ture was regarded as a rock topple–rock slide structure.(6)Steeply dipping strata with alternating beds of mudstones and sandstones are widely distributed in this region;therefore,the existence in many places of similar structures was inferred.Similar investigations are required to evaluate unstable blocks before any construction projects.AcknowledgementsWe are thankful to Profs.T.Tokuoka,K.Takayasu, and S.Yamauchi of the Department of Geoscience, Shimane University,for their fruitful discussions andN.K.Tamrakar et al./Engineering Geology64(2002)339–350349。