Orthorhombic nonlinear crystals of AgxGaxGe1−xSe2 for the mid-infrared spectral range

We study the birefringence and nonlinearity of quaternary semiconductors of the type AgGaGenSe2(n+1), solid solutions in the system AgGaSe2-nGeSe2. The birefringence, e.g. na-nc at 1064.2 nm, increases from 0.114 for n = 2 (AgGaGe2Se6) to 0.149 for n = 5 (AgGaGe5Se12) which substantially exceeds the birefringence of the uniaxial AgGaSe2 (∼0.022), the parent compound in the limit n = 0. Sellmeier equations valid in the 0.6-11.5 μm range are constructed for the solid solutions with n = 2…5. All four quaternary compounds are optically negative biaxial crystals. The calculated second-harmonic generation (SHG) limit (minimum fundamental wavelength) is ≈1470 nm for AgGaGe2Se6 and ≈1240 nm for AgGaGe5Se12, for type-I interaction and propagation along the Y principal optical axis. These limits are much lower than the ≈3120 nm limit for type-I interaction in AgGaSe2. Thus, the AgGaGenSe2(n+1) orthorhombic crystals can be used for SHG down to their band-edge. The results for the nonlinear coefficients of AgGaGenSe2(n+1) (n = 3, 4 and 5), obtained from phase-matched SHG, indicate weak dependence on the composition. On the average, the larger nonlinear coefficient d31 is very close to d36 of AgGaSe2 (∼30 pm/V) while d32 is roughly two times smaller.