Strongly enhanced mode selection in a thin dielectric-coated layered microcavity laser

Strongly enhanced mode selection in a thindielectric-coated layered microcavity laserHee-Jong Moon*and Dong-Yun KangDepartment of Optical Engineering,Sejong University,Seoul143-747,Korea*Corresponding author:hjmoon@sejong.ac.krReceived January5,2007;revised March29,2007;accepted April1,2007;posted April4,2007(Doc.ID78540);published May15,2007Strong mode selection through an enhanced interferential coupling effect was observed in a thin dielectric-coated layered cylindrical microcavity laser.The strong coupling effect was induced owing to an enhanced reflectivity of around50%at the dielectric-coated inner boundary of a fused silica capillaryfilled with a dye-doped liquid.At an optimized coating thickness of about0.4␮m,the lasing peaks appeared only at the wavelengths corresponding to the constructive interference condition,whereas those from a bare capillary were weakly modulated.©2007Optical Society of AmericaOCIS codes:140.3410,290.4210.Circular microcavities have attracted much interest because high-Q whispering gallery modes(WGMs) can exist.A layered circular microcavity introduces interesting complexities in the WGM structure due to an additional boundary.WGMs can be specified by angular mode number,radial mode order,and polar-izations(TM or TE).WGM-based microcavities are considered to be potential building blocks for active or passive optoelectronic devices such as low-threshold microlasers and channel-dropfilters for op-tical communication[1–4].Various studies on the layered microcavity have been conducted with spherical shells,ring microdisks,and capillary tubes [5,6].Among them,the interferential modulation ef-fect on the lasing spectrum is of practical importance because it can provide a scheme for mode selection in a broad-gain microcavity laser.Thefirst observation of the interference modulation from a layered micro-cavity laser was reported by Knight et al.[7],who used a thick fused-silica capillaryfilled with dye-doped liquid of high refractive index.Recently,a modified version of an efficient mode selection scheme was reported by Moon et al.[8],who used a similar configuration with a thin capillaryfilled with dye-doped liquid.Figure1(a)shows the previous schematic of our layered microcavity with inner radius a and outer ra-dius b[8].The refractive indices of the inner region and the outer layer are m1and m2,respectively(m1Ͻm2,m0=1.0in air).The outer boundary͑r=b͒andthe inner boundary͑r=a͒of the capillary will be de-noted S2and S1,respectively.Suppose the gain is uniformly imposed only in the inner region.If b/a is close to1,a high-order WGM can penetrate into the inner region so that its distribution extends over both regions and corresponding rays can obtain gain from the inner region.The incident angles on S1are de-noted␪2(or␪1).To form high-Q WGMs,the outer ray should be totally reflected at S2.The half-angle␺of the penetrating ray and the half-angle␾of the outer reflected ray are determined by m1,m2,a,b,and␪1 (or␪2)[8].If␺and␾are not equal,as shown in the upper part of Fig.1(a),the ray experiences reflectionloss upon each refraction,resulting in an accumu-lated loss sufficient to suppress laser oscillation.Un-der the condition␺=␾,a WGM is composed of two split rays,as shown in the lower part of Fig.1(a).Thus both rays interact with each other upon each reflection or refraction at S1,so an interferential cou-pling exists between them with no loss.Therefore the condition␺=␾should be satisfied for laser oscilla-tion.The difference in phase(or optical path length ⌬L=2bm2sin͑␾͒/sin͑␪2͒−2am1sin͑␺͒for␾=␺)be-tween the two rays per one reflection–refraction cycle should determine the period of constructive interfer-ence.The coupling strength strongly depends on the splitting ratio(or reflectivity)of the two split rays. Because the difference between m1and m2was small in the previous configuration,the reflectivity amounted to only about5%,resulting in insufficient mode selection in laser oscillation due to a poor cou-pling strength[8].To attain the strongest coupling strength,the split rays should have similar intensi-ties(or the reflectivity should be increased up to ϳ50%).We propose and demonstrate here a strongly en-hanced mode selection scheme in a layered microcav-ity laser in which a thin dielectric is coated in such away that the reflectivity at the inner boundary amounts toϳ50%.The selectivity of the lasingpeaksFig.1.(a)Schematic of mode selection in a layered micro-cavity.(b)Multiple reflection–refraction in a thinflat coating.1554OPTICS LETTERS/Vol.32,No.11/June1,20070146-9592/07/111554-3/$15.00©2007Optical Society of Americawas remarkably enhanced compared with the casewith no coating.The proposed new scheme is the same as Fig.1(a) except with a thin dielectric coating on S1.Thereflection–refraction from the coated circular bound-ary S1can be approximated as those from aflat coat-ing layer as shown in Fig.1(b).Consider the refrac-tive index of coating medium3to be m3and its thickness to be d.A plane wave(free-space wave-length of␭)with the incident angle␪1from medium1 will undergo multiple reflection–refraction on coating medium3.Now assume that the incident wave is spolarized(the electricfield is vertical to the plane of incidence).The total reflection coefficient r is ex-pressed by the Fresnel formula as[9]r=r13+r32e2j␤1+r13r32e2j␤,͑1͒where␤=2␲m3d cos␪3/␭=2␲dͱm32−m12sin2␪1/␭,r13 is expressed asr13=m1cos␪1−m3cos␪3 m1cos␪1+m3cos␪3=m1cos␪1−ͱm32−m12sin␪1m1cos␪1+ͱm32−m12sin␪1,͑2͒and r32has an expression similar to Eq.(2).The total reflectivity R of the thin coating can be expressed as [9]R=͉r͉2=r132+r322+2r13r32cos2␤1+r132r322+2r13r32cos2␤.͑3͒R is strongly dependent on d and␪1for the given re-fractive indices͑m1,m3,m2͒.Even with a very small coating thickness,R can amount toϳ50%at a cer-tain␪1if an appropriate set of refractive indices is given.Hence a strong interferential coupling effect can be expected when a thin dielectric coating on S1 is added.Our experimental setup to demonstrate the new scheme is similar to that of Ref.[8].We used a fused-silica capillary(2a=250␮m,2b=320␮m,m2Ӎ1.458) as a layered circular cavity.To fabricate a thin coat-ing on S1,we used a coating process similar to that of Ref.[10].The chosen coating material was Teflon AF 1600,whose refractive index was m3Ӎ1.31[11].The solution for AF1600was FC-77.By evaporating FC-77with forced airflow,we could obtain a thin AF 1600coating on the inner wall of the capillary.The coating thickness d can be adjusted by changing the AF1600concentration.d was measured from scan-ning electron micrograph(SEM)images of the cross section of the coated capillary.After coating we flowed an ethanol solution(refractive index m1 =1.361),doped by Rhodamine6G with a concentra-tion of0.5mM/l,into the capillary to form the gain region.A frequency-doubled Q-switched Nd:YAG pulse laser(pulse width of10ns,wavelength of532nm)was used as a pump source.The single-shot spectrum was measured with a photodiode array at-tached to a spectrometer.To determine the optimal coating thickness d,we have calculated the total reflectivity R with respect to ␪1.Figure2shows the␪1dependence of R at variousd calculated by using Eq.(3).The condition␺=␾in Fig.1(a)occurs at␪1=71.4°in our setup,which is in-dicated by the dotted vertical line in Fig.2.Hence,R along the dotted line is of practical importance to de-termine the optimal coating thickness.If d=0(no coating),R is as small as about5%,which is far from 50%.As d increases to0.3,0.4,0.5,and0.7␮m,R at ␪1=71.4°increases to about33%,42%,47%,and48%, respectively.In all cases,R is considerably enhanced compared with the case with no coating.From this simulation,strong mode selection is expected for d ജ0.4␮m because R exceeds40%.However,the large coating thickness is not preferable in our setup.Be-cause m1is larger than m3,WGMs originating by to-tal internal reflection at the m1–m3boundary(criti-cal angle of␪1Ӎ74.3°)and distributed only beneath S1exist if d is considerably larger than the wave-length of light.Because the tunneling probability de-creases as d increases,high-Q WGMs can exist if d is large.These WGMs may be dominantly lased instead of the preferred WGMs distributed over both regions as in Fig.1(a).Therefore the optimal d for strong mode selection must be small enough.In our setup, we concluded that a thickness of dϳ0.4␮m is opti-mal in practice because R=42%is sufficiently high for strong interferential coupling.Figure3shows a typical lasing spectrum from a bare capillary with no coating͑d=0͒.The lasing peaks were considerably modulated as a clustered pattern.Strong peaks appeared nearly periodically, but the interference period was not clearly defined. In our setup,the difference of optical path length⌬L deduced by the ray-optics model was calculated to be about59␮m;hence the interference period ⌬␭͑Ӎ␭2/⌬L͒was estimated to be about5.5nm for␭Ӎ570nm.In Fig.3,the wavelength distance between twolocally strong peaks was measured to beFig.2.(Color online)Calculated total reflectivity with re-spect to␪1at various d from a capillary coated with AF 1600.(2a=250␮m,2b=320␮m).The dotted vertical line indicates␪1=71.4°,at which␺=␾.June1,2007/Vol.32,No.11/OPTICS LETTERS1555ϳ5.3nm,which was approximately the same as the calculated ⌬␭.The measured polarization of the las-ing peaks was s -polarization.In this case,the reflec-tivity R was calculated to be as small as ϳ5%in Fig.2.Hence the modulation effect was not sufficient to suppress the side lasing peaks around constructively interferential peaks.Figure 4shows a typical lasing spectrum from a coated capillary with an optimal coating thickness ͑d =0.4␮m ͒.In Fig.4,a SEM image of a part of a capillary cross section is also shown.The coated cap-illary was carefully cut and metal coated on the cross section of the capillary for the SEM image.As shown,AF 1600was uniformly coated on the inner wall with a thickness of about 0.4␮m.As expected above,the lasing spectrum showed a strong mode selection.The lasing peaks appeared as a simplified pattern com-pared with that in Fig.3.That is,only strong peaks corresponding to the wavelengths for constructive in-terference were dominantly observed.Though a weak side peak also appeared just around the strong peak,the lasing spectrum showed a remarkably enhanced mode selection compared with the case with no coat-ing.⌬␭could be well determined to be about 5.3nm,which is the same as that in Fig.3.This means thatthe thin coating did not affect general features of the interferential coupling mechanism.The thin coating affects only the splitting ratio (or reflectivity)and the phase shift.Therefore the thin coating induces a strong coupling strength,though the wavelengths of constructive interference may change because of the phase shift during multiple reflection–refraction.Because the coating thicknesses or coating state varied slightly along the capillary axis,the observed lasing spectrum was slightly changed when the illu-mination position of the pump beam moved along the capillary.Nevertheless,the modulation pattern of lasing peaks was distinguishably simpler than the case with no coating.We have tested other capillaries having different coating thicknesses and found that the strongest mode selection occurred at around d ϳ0.4␮m as expected in Fig.2.When d ജ0.7␮m,las-ing by WGMs distributed only beneath S 1was also simultaneously observed as described above.At a cer-tain illumination position,even a nearly single con-structive peak (or single mode lasing)could be ob-served.Therefore we could conclude that our newly proposed scheme using a thin coating on S 1was an excellent method for strong mode selection of lasing peaks.This implies that an appropriate coating on the inner boundary of a layered microcavity acts as an efficient etalon for the circular microcavity laser.In conclusion,we have demonstrated remarkably strong mode selection in a layered cylindrical micro-cavity laser in which a thin dielectric was coated on the inner wall.This scheme might be expanded to semiconductor-based layered microdisk-type lasers for practical applications.This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD)(KRF-2005-041-C00198).References1.S.L.McCall,A.F.J.Levi,R.E.Slusher,S.J.Pearton,and R.A.Logan,Appl.Phys.Lett.60,289(1992).2.A.Polman,B.Min,J.Kalkman,T.J.Kippenberg,and K.J.Vahala,Appl.Phys.Lett.84,1037(2004).3.J.E.Heebner,N.N.Lepeshkin,A.Schweinsber,G.W.Wicks,R.W.Boyd,R.Grover,and P .-T.Ho,Opt.Lett.29,769(2004).4.Y.Huang,G.T.Paloczi,J.Scheuer,and A.Yariv,Opt.Express 11,2452(2003).5.M.V .Artemyev,U.Woggon,and R.Wannemacher,Appl.Phys.Lett.78,1032(2001).6.H.J.Moon,G.W.Park,S.B.Lee,K.An,and J.H.Lee,mun.235,401(2004).7.J.C.Knight,H.S.T.Driver,and G.N.Robertson,Opt.Lett.18,1296(1993).8.H.J.Moon and K.An,Appl.Phys.Lett.80,3250(2002).9.M.Born and E.Wolf,Principles of Optics (Cambrdge U.Press,1999),p.65.10.H.J.Moon,G.W.Park,S.B.Lee,K.An,and J.H.Lee,Appl.Phys.Lett.84,4547(2004).11.J.H.Lowry,J.S.Mendlowitz,and N.S.Subramanian,Opt.Eng.31,1982(1992).Fig.4.Typical lasing spectrum from a capillary coated by AF 1600with a thickness of 0.4␮m.Fig.3.Typical lasing spectrum from a bare capillary with no coating.1556OPTICS LETTERS /Vol.32,No.11/June 1,2007。