Control of Melt Decomposition for the Growth of High Quality AgGaGeS₄ Single Crystals for Mid-IR Laser Applications

Abstract

AgGaGeS 4 (AGGS) is a promising nonlinear crystal for mid-IR laser applications which could fulfill the lack of material able to convert a 1.064 µm pump signal (Nd:YAG laser) into wavelengths higher than 4 µm up to 11 µm. The processing steps of this material are presented in this study. The key issue of AGGS crystal processing is the control of decomposition at high temperature due to the high volatility of GeS 2. This study present the solutions to obtain high quality single crystal. AGGS crystals with 28 mm diameter and 70 mm length were grown by the Bridgman-Stockbarger method. The crystals have good homogeneity and high transparency in the 0.5-11.5 µm spectral range making it suitable for optical experiments. The influence of GeS 2 volatility on melt stoichiometry during the AgGaGeS 4 processing is outlined and solutions to improve the crystals quality are presented.

Homogeneity characterization in AgGaGeS4, a single crystal for nonlinear mid-IR laser applications

“…In the present work, annealing was performed under static vacuum with addition of polycrystalline powder because of volatility of GeS2. Indeed, as previously shown [42,60], volatility of GeS2 can lead to stoichiometry deviation in the melt which increases the final optical absorption coefficient of the crystal. In consequence, annealing under vacuum would increase GeS2 depletion and deteriorate crystal quality.…”

“…After 500 h at 600 °C, crystal color changed from brown/orange to yellow as shown on the Figure 6. Measurements of absorption coefficient before and after annealing presented in [42] showed blue shift of the cut-off wavelength, from 0.49 µm to 0.45 µm, before and after annealing respectively which explains the color change.…”

“…As sulfur and derivatives are volatile components, the pressure increases inside the ampoule during heating which can lead to the ampoule explosion. Thus, the synthesis process was carried out using the two zones temperature method as described in [42]. First, temperature was raised in the lower zone up to 950°C were the compounds are melted and the chemical reaction takes place.…”

“…Indeed, AgGaGeS4 phase has a narrow stability whose range is comprised between 48-55 mol % GeS2 [41]. Moreover, it has been outlined that volatility of AgGaGeS4 derivatives compounds and particularly germanium sulfide (GeS2) induces stoichiometry deviations in the melt during the growth process which move crystal composition away from the stoichiometry and decreases its optical quality [42,43,44]. Consequently, this can drastically reduce the conversion efficiency of the sample [45,46,47].…”

Non-stoichiometry and point native defects in non-oxide non-linear optical large single crystals: advantages and problems

“…In other experiments, complex molecular composition of the overheated melt was also observed via temperature of its overcooling to 86 K. In this case, even with high-quality AgGaGeS 4 polycrystals, the crystals grown by the BS method showed good homogeneity but had a value of α ∼ 0.1 cm −1 in the 0.9–8 μm spectral range. 29 It is possible that from the inhomogeneous melt, crystals also grow as multiphase. The dissociation of the GeS 2 species in the melt was not surprising because dissociation above 1123 K is a truly intrinsic property of GeS 2 species.…”