Laser Damage Studies of Silver Gallium Sulfide Single Crystals
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ABSTRACT Although AgGaS 2 single crystals have desirable optical and nonlinear properties, the low laser damage threshold has limited their application for tunable infrared laser sources. Crystals have been studied by ESCA and Auger techniques and the surface of polished crystals has been found to be silver deficient, while the bulk composition is closer to stoichiometry. INTRODUCTION AgGaS 2 single crystals have exceptional fundamental optical properties with a nonlinear optical susceptibility d 36= 12-29pmi/V1 3 and a transmission range of 0.45 to 13ixm. No other nonlinear crystal that is transparent in this region can be phase-matched with a 1.061am pump. Such a combination of properties makes AgGaS 2 a very attractive candidate for a variety of nonlinear optical devices, such as an OPO operating from 2.5 to 11mm with a 1.06am as a pump. AgGaS 2 has a lower thermal conductivity (K %0.014-0.015 Wxcm'×xK" 4 1)4 than ZnGeP2 (K t, 0.35-0.36 Wxcm-'xK'l) . Low thermal conductivity results in more stringent conditions on the optical absorption coefficient, (x,of AgGaS 2; a value for a of 0.001 to 0.003 cm-1 is desirable in order to minimize thermal lensing at high average powers. AgGaS 2 crystals experience laser damage thresholds far below the values that are needed for useful OPO systems. Laser damage occurs at the surface of the crystal, rather than in the bulk. This property, which we do not believe to be intrinsic, limits the usefulness of AgGaS 2. Fundamentally, the surface laser damage should be linked to the chemical and phase composition of the crystal surface and to its defect structure. Mechanical damage, adsorption of common contaminants such as carbon, oxygen and sodium, deviation from stoichiometry, presence of scattering centers, or surface instability in the presence of the high electric fields of the pump laser could contribute to the observed surface laser damage. Due to its retrograde homogeneity region, all melt-grown AgGaS 2 single crystals appear "milky" due to dispersed second phase precipitates' 7 . An isothermal annealing in an excess of Ag2 S has been commonly used to reduce the precipitates ' 7. The annealed crystals are, however, not completely defect-free. Even the best samples exhibit a certain degree of residual optical absorption and scattering 5. The impressive potential for AgGaS2 nonlinear devices will not be realized until the surface laser damage threshold is substantially increased.
519 Mat. Res. Soc. Symp. Proc. Vol. 484 ©1998 Materials Research Society
EXPERIMENT Single Crystal Growth Synthesis The two-temperature vapor transport synthesis technique was used to synthesize pure AgGaS 2 for feedstock for crystal growth. The essence of this method is control of the speed of the synthesis reactions and composition of the condensed phase by the pressure of the volatile component. A sealed ampoule, loaded with Ag, Ga and S, is placed in a two-zone furnace with silver and gallium in the "hot" zone and sulfur in the "cold" zone. Sulfur vapor reacts with the molten Ag+Ga charge, while
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