Influence of color centers on optical and lasing properties of the gadolinium gallium garnet single

Journal of Luminescence 82(1999)9}15

In #uence of color centers on optical and lasing properties

of the gadolinium gallium garnet single crystals

doped with Nd >ions

D.Sugak ,A.Matkovskii ,A.Durygin ,A.Suchocki *,I.Solskii ,

S.Ubizskii ,K.Kopczynski ,Z.Mierczyk ,P.Potera

Institute of Materials,SRC Carat,202Stryjska Str.,290031L v i v ,Ukraine State Uni v ersity L v i v ska Politechnika,12Bandera Str.,290646L v i v ,Ukraine

Institute of Physics,Polish Academy of Sciences,Al.Lotniko &w 32/46,02-668Warsaw,Poland

Institute of Optoelectronics,MUT,2Kaliski Str.,01-489Warsaw,Poland Institute of Physics,HPS,ul.Rejtana 16A,35-310Rzeszo &w,Poland Received 11November 1998;received in revised form 10March 1999;accepted 10March 1999

Abstract

The mechanisms of creation of the stable color centers generated in GGG-Nd crystals under the in #uence of UV light

and various kind of ionizing radiation (gamma-rays and electrons)are examined.The models of stable color centers in these crystals are proposed.In #uence of gamma-irradiation on the GGG-Nd crystal laser generation characteristics is studied.Absorption spectra and decay kinetics of the transient color centers,induced in GGG-Nd by pulsed UV light,are investigated. 1999Published by Elsevier Science B.V.All rights reserved.

PACS:42.70.Hj;42.55.Rz;78.55!m

Keywords:Defects in crystals;Garnet crystals;Optical absorption;Optical properties;Rare-earth ions

1.Introduction

The gadolinium gallium garnet (Gd Ga O )single crystals doped with Nd >ions (GGG-Nd)are of interest for fabrication of solid state laser active elements lasing in a 1.06}1.42 m spectral range [1}4].This material possesses several ad-vantages in comparison with Y Al O -Nd

*Corresponding author.Fax:#48-22-8430926.

E-mail address:andrzej.suchocki @uam.es (A.Suchocki)(YAG-Nd)single crystals,widely used for 1.06 m wavelength generation:the melting temperature is lower,the growth rate is faster,and it is possible to obtain crystals with large dimensions,high optical quality,and higher concentration of activator ions.Although the thermooptical and thermomechani-cal characteristics are not as favorable as those for YAG-Nd [5],the GGG crystals are very attractive for fabrication of solid state lasers working in the 1.3}1.42 m range [1}4].

It is well known [6]that the color centers stable at room temperature (SCC)and those that are

0022-2313/99/$-see front matter 1999Published by Elsevier Science B.V.All rights reserved.PII:S 0022-2313(99)00030-7

transient(TCC)with lifetimes shorter than1s at

room temperature(RT),are induced in laser crys-

tals under the in#uence of ionizing and ultraviolet

(UV)https://www.360docs.net/doc/4212619597.html,ually,the color centers are detri-

mental for laser generation e$ciency.For example,

in YAP:Nd crystals the SCC absorb pump light

radiation diminishing pumping e$ciency,and the

TCC re-absorb the laser radiation[7].The in#u-

ence of defects induced by ionizing radiation(IR),

on the GGG-Nd lasing characteristics have not

been thoroughly studied,in contrast to the other

oxide laser crystals as YAG[8],YAP[7]and

GSGG[9].

In this work the mechanisms of SCC creation

have been examined and models of SCC generated

in GGG-Nd crystals under the in#uence of UV

light and di!erent kinds of irradiation(gamma-rays

and electrons)are proposed.The in#uence of

gamma-irradiation on the GGG-Nd crystal gen-

eration characteristics is reported.The absorption

spectra and decay kinetics of the TCC generated in

GGG-Nd under the in#uence of pulsed UV light

are investigated.The results are compared with

data obtained for unirradiated GGG crystals.

2.Experiment

The crystals were grown in the[111]direction

by the Czochralski method from iridium crucibles

in98%Ar#2%O atmosphere.Samples for op-tical measurements were made in the form of

plane-parallel polished plates of0.5}2mm thick-

ness.The transmission and induced absorption(IA)

spectra were recorded with a SPECORD-M40

spectrophotometer(Carl Zeiss,Germany).The in-

duced absorption value was determined as

K"1

d ln

1

1

,(1)

where d is the sample thickness,and1 and1 are the crystal transmission before and after irradia-tion,respectively.

Two GGG-Nd rods(4mm in diameter and 48mm long)without antire#ection coatings at the end faces were used for lasing investigations.The rods were placed into the plane-parallel22cm laser cavity with output mirror transmission of36% for "1.06 m and9%for "1.32 m.The ellip-tical chamber covered with gold and single xenon discharge Xe lamp with diameter of4mm has been utilized for optical pumping.A pump pulse duration of120 s was used with energies vary-ing between7and60J.The GGG-Nd crystals demonstrated high sensitivity for UV radiation present in the Xe lamp spectrum.A special"lter cutting o!the UV radiation with wavelengths shorter than350nm(sodium glass)was placed in the resonator.The energy of laser pulses was mea-sured by a Gen-Tec radiometer with ED-200gauge head.

The samples were irradiated with:

E c.w.and pulsed radiation of Ar>-laser(351nm wavelength);

E gamma-radiation from a Co source with aver-age energy of1.25MeV up to2.6;10 Gy of absorbed doses;

E electron beam with energy about4MeV up to #uences( )of10 cm\ .

For studying the TCC,the samples were illumin-ated by Ar>-laser light( "351nm)pulses with duration of about2ms.Changes in sample absorp-tion were estimated from variations of the probe light intensity,measured in a set-up with a GDM 1000monochromator and a photomultiplier with an S20type cathode.The decay kinetics of the induced absorption were stored in an SR430multi-channel scaler.

3.Results and discussion

The induced absorption spectra of GGG-Nd crystals after irradiation with UV light,gamma-rays,and electrons are presented in Fig.1.A wide and relatively weak absorption band appears in the range of34000}11000cm\ .Bleaching of crystals near the fundamental absorption edge is observed if the crystals are irradiated with UV light or gamma radiation.The induced absorption spec-trum consists of at least two overlapping bands with maxima around29000and24000cm\ (Fig. 1,line1).The absorption value increases initially with the irradiation dose and it saturates for the gamma-quanta absorbed doses of10 }10 Gy.Its

10 D.Sugak et al./Journal of Luminescence82(1999)9}15

Fig.1.The induced absorption spectra of the GGG-Nd crystals after gamma-irradiation with dose10 Gy(1),and after electron irradiation with "10 cm\ (2).In the inset:dose dependencies of the absorption value in bands:24000cm\ for gamma-irradiated (a),and33500cm\ for electron irradiated(b)GGG-Nd crystals.

value does not exceed1cm\ (see Fig.1,inset a). The saturation of the induced absorption observed for higher doses of gamma-rays means that the mechanism of the CC formation under the in#u-ence of this type of irradiation is most probably associated with a process of recharging of the de-fects existing in the crystal whose number does not increase during irradiation.The induced absorp-tion spectra of irradiated GGG-Nd crystals are identical to those observed in pure GGG crystals, exposed to similar radiation[10].Thus,we expect that similar defects are associated with the ob-served induced absorption as in the pure crystals. Therefore this absorption is associated with crystal growth defects modi"ed under in#uence of radi-ation.Oxygen vacancies in di!erent charge states and`antisite a defects}(rare-earth ions replacing gallium ions in octahedral positions[9,11])could constitute these defects.The concentration of anti-site defects in rare earth gallium garnet crystals may be larger than10 cm\ [11].Earlier studies of the thermoelectric power,thermal conductivity and thermal destruction of induced absorption of irradiated nominally pure GGG crystals suggest that the29000}32000cm\ absorption band is associated with O\}centers,possibly localized near antisite defects[10].We associate the absorp-tion band in the24000}21000cm\ region with the F-type centers.F-center absorption in the 24000}20000cm\ region was observed in the other oxide crystals(LiNbO ,KNbO ,YAlO ) [12}14].

Irradiation of the GGG-Nd crystals with elec-trons at#uences below10 cm\ a!ects the GGG-Nd crystals in a very similar way as the UV and gamma radiation.Irradiation of the crystals with electron#uences higher than10 cm\ leads to important changes of the induced absorption spectrum.As the irradiation dose increases,a new band with maximum at33500cm\ appears in the induced absorption spectra close to the funda-mental absorption edge.This band replaces the bleaching observed previously under the in#uence of UV and gamma irradiation.The band intensity increases with#uence and at#uence "

10 cm\ it reaches a value of about15cm\ with

D.Sugak et al./Journal of Luminescence82(1999)9}1511

no tendency to saturate.The induced absorption spectrum of the GGG:Nd crystal after electron irradiation and its dose dependence are also shown in Fig.1(line2and inset b,respectively).The nature of observed changes in the induced absorption spectra in the GGG-Nd crystals exposed to irradia-tion with electrons with the large energy and high #uences is most probably associated with the cre-ation of new radiation defects,which are produced by the impact mechanism.

The total concentration of displacements in the sublattice of atoms of j-type can be expressed in the form

n B

H " )

G

n G) 2 G2 G d NG(E,1)d1) GH(1G)d1G ,(2)

where is the#uence of radiation particles;n G is the concentration of i-type atoms in the crystals;

d NG/d1is th

e di!erential cross section for transfer-ring the recoil energy1G to the primary knocked-out atom o

f i-type;1 is the maximum possible recoil energy;1B is a threshold energy for ion dis-placement and GH(1G)is the cascade function,i.e.the number of displaced atoms of j-type for a single knocked-out i-type atom with energy1G.The cas-cade function GH(1G)was calculated by Monte Carlo simulation usin

g assumptions of Kinchin}Pease model[15]and taking into account the energy loss of knocked-out atoms due to overcoming of the potential energy barrier(lattice recoil).The crystal structure was not taken into account.It means that the`partners selection a for collisions was com-pletely accidental and was determined only by the ratios of cross sections of di!erent collisions and the number of atoms of eac

h type.The following values of threshold energy for the GGG atoms were used in calculations:66eV for RE,56eV for Ga and40eV for O ions[16].

Computed dependencies of concentration of the displaced ion on one particle of primary radi-ation in GGG-Nd crystals vs.electron beam energy are presented in Fig.2.The oxygen ions are most prone to the displacement.The concentration of displaced oxygen ions at4MeV electron beam en-ergy for one impinging particle( "1cm\ )is equal to1.36cm\ .Concentrations of displaced Ga and RE ions are lower(0.71and0.53cm\ -

for Fig.2.Dependence of displaced ion concentrations in GGG-Nd

crystals on electron beam energy.

Ge and Re ions,respectively),though their total

concentration is close to the number of displaced

O ions.The total concentration of displaced ions at

#uence "10 cm\ adds up to2.6;10 cm\ .

Thus,it is possible that the radiation defects that

broaden the fundamental absorption edge and

are the source of the absorption band with a max-

imum at33500cm\ ,are related to both oxygen

ion vacancies(<-)and more complex defects,name-ly the displacement defects in anion and cation

sublattices,which are mutually charge compen-

sated.

The concentration of radiation defects under the

in#uence of high-energy electrons is too small for

detection at#uences of electrons below10 cm\ .

At#uences larger than3;10 cm\ the linear

growth of concentration of CC having absorption

at33500cm\ is observed.This corresponds to

radiation defect concentration larger than

&10 cm\ .The process of annihilation of new-born defects with those created earlier,which could have resulted in a deviation from a linear depend-ence of the defect concentration,does not occur for the range of electron#uences under investigation. Thus the dose dependence of the induced absorp-tion band at33500cm\ does not saturate in Fig.1(inset b).

An irradiation of the GGG-Nd laser rods

with gamma-quanta(dose2.6;10 Gy)leads to a

12 D.Sugak et al./Journal of Luminescence82(1999)9}15

signi "cant (about two times)decrease of the laser radiation energy output at both 1.06and 1.32 m wavelengths.However,in contrast to YAP-Nd crystals for which after irradiation with a dose of 10 Gy the generation completely ceases [7],for the GGG-Nd active elements the laser generation takes place also for doses higher than 2.6;10 Gy.Restoration of the initial characteristics of the GGG-Nd active elements,may be obtained by an-nealing the crystals in air at a temperature 473K for 1h.The energy characteristics of the GGG-Nd active elements before and after gamma-irradiation are shown in Fig.3.Observed degradation of

the

Fig.3.The energy characteristics of the GGG-Nd laser rods before (1)and after gamma-irradiation with a dose of 2.6;10 Gy (2)for generating wavelengths of 1.06(a)and 1.32 m (b).The intracavity sodium glass "lter was used;(3)the energy characteristics of the non-irradiated GGG-Nd laser rods without intracavity sodium glass "lter.

lasing properties of the active elements after gamma-irradiation is due to absorption of pump-ing illumination by SCC produced by the ir-radiation.Annealing of active elements destroys SCC and restores initial lasing properties of the crystals.

There is an additional reason that causes degra-dation of generation characteristics of laser rods.It is the creation of the TCC under the in #uence of radiation,including a short-wavelength part of pumping illumination as well.The dependencies of the GGG-Nd laser output energy on pumping lamp energy for pumping with the full spectrum of the pump lamp (without cutting o !"lter for the UV part of the spectrum)are also shown in Fig.3.The output laser energy increases signi "cantly if the laser rods are pumped by "ltered light (without the UV part of the Xe-lamp spectrum).A study of the TCC absorption reveals that their spectrum is a wide band with smeared maxima in the range of 22000}15000cm \ .Its long-wavelength wing ex-tends to the wavelength longer than 1000nm (see Fig.4).The decay kinetics of the TCC-induced absorption (Fig.5)are well approximated by the sum of two exponents with lifetimes equal to 2and 20ms.Two-exponential decay of the TCC suggests that two types of centers are created by the UV radiation.These centers are most probably associated with oxygen ion vacancies in di !

erent

Fig.4.The absorption spectra of TCC induced by the pulsed Ar >-laser radiation with "351nm.

D.Sugak et al./Journal of Luminescence 82(1999)9}1513

Fig.5.Decay kinetics of the TCC absorption in GGG-Nd crystal at room temperature,recorded at555nm(18000cm\ , approx.).

structural positions,as it is in the other oxide crys-tals(LiNbO ,YAlO [12,14]).

4.Conclusions

The absorption is induced in the GGG-Nd crys-tals upon irradiation with UV light,gamma-quanta and electron beams with#uences below101 cm\ . They are associated with the creation of stable CC due to the recharging process of growth defects. Both O\hole centers likely to be localized near defects of cation sublattice(the induced absorption band in the range29000}32000cm\ )and F-type centers(the induced absorption band in the range 24000}21000cm\ )make contributions to the induced absorption.The SCC reduces the per-formance of the laser active elements of the laser,but the radiation resistance of GGG-Nd is much better than this for YAP-Nd and YAG-Nd crystals and laser generation takes place even with gamma-quanta irradiation with doses above 10 Gy.

High-energy electrons with large doses in#uence the GGG-Nd crystals in di!erent ways.They create new defects due to impact interaction with the host ions.This results in broadening of the funda-mental absorption edge and a new absorption band with the peak at33500cm\ .The oxygen va-cancies and more complex displacement defects in anion and cation sublattices mutually charge com-pensating each other from,most probably,these defects.

A degradation of the laser generation perfor-mance of the GGG-Nd active elements under in#u-ence of pulsed UV illumination is determined to be due to TCC creation.The TCC absorption spec-trum is a relatively broad band with maximum near 18000cm\ .Most probably two di!erent defect centers with lifetimes of2and20ms at room tem-perature are associated with this absorption.The F>-type centers at oxygen ion vacancies in di!er-ent structural positions could explain the nature of this absorption.

Acknowledgements

This work was supported by the Ukrainian Min-istry of Science(Grant N2M/1857-97)and by the Polish Committee for Scienti"c Research(Grant 8T11B05213).

References

[1]M.Doroshenko,V.Osiko,V.Sigachev,M.Timoshechkin,

Kvantovaja Elektron.18(1991)298(in Russian).

[2]M.Doroshenko,V.Osiko,V.Sigachev,M.Timoshechkin,

Sov.J.Quantum Electron.21(1991)724.

[3]M.Doroshenko,V.Osiko,V.Sigachev,M.Timoshechkin,

Sov.J.Quantum Electron.23(1993)490.

[4]M.Doroshenko,V.Osiko,V.Sigachev,V.Strelov,

M.Timoshechkin,Proc.SPIE1839(1992)12.

[5]G.M.Zverev,Yu.D.Goliayev,Lazery na kristallakh

i ih primenenie,Radio i Sviaz,Moscow,1994(in

Russian).

[6]A.Matkovskii, D.Sugak,S.Ubizskii,O.Shpotiuk,

E.Chernyi,N.Vakiv,V.Mokritskii,In#uence of Ionizing

Radiation on Electronic Technique Materials,Svit,Lvov, 1994(in Russian).

[7]D.Sugak,A.Matkovskii,V.Grabovskii,V.Prokhorenko,

A.Suchocki,A.Durygin,I.Solskii,A.Shakhov,Acta

Physica Polonica A93(1998)643.

[8]T.S.Rose,M.S.Hopkins,R.A.Fields,IEEE J.Quant.

Electron.31(1995)1593.

[9]A.Divak,I.Kuratev,N.Kovyzhenko,Yu.Tsvetkov,

V.Yanchuk, B.Zaharkin,V.Nikolaev, A.Kalagin, V.Sorokin,Lazernaja Technika i Optoelektronika1-2 (1992)56(in Russian).

[10]A.Matkovskii, D.Sugak,S.Ubizskii,U.Ulmanis,

A.Shakhov,Physica Status Solidi A128(1991)21.

14 D.Sugak et al./Journal of Luminescence82(1999)9}15

[11]G.Kuzmicheva,S.Kozlikin,E.Zharikov,Zhurnal Neor-

ganicheskoj Khimii33(1988)2200(in Russian).

[12]E.Kotomin,R.Eglitis,A.Popov,J.Phys.:Condens.Mat-

ter9(1997)965.

[13]E.K.Hodgson,F.Agullo-Lopez,Solid State Commun.64

(1987)965.[14]A.Matkovskii, A.Durygin, A.Suchocki, D.Sugak,

G.Neuroth,F.Wallrafen,V.Grabovskii,I.Solskii,Op-

tical Materials,1998,to be published.

[15]G.W.Kinchin,R.S.Pease,Rep.Prog.Phys.18(1955)1.

[16]Ya.Dovgii,I.Kityk,A.Matkovskii,D.Sugak,S.Ubizskii,

Sov.Phys.Solid State34(1992)575.

D.Sugak et al./Journal of Luminescence82(1999)9}1515