Linear, nonlinear optical properties and birefringence of AgGaX2 (X=S, Se, Te) compounds

We report calculations of the electronic, linear and nonlinear optical properties of AgGaX2 (X=S, Se, and Te) compounds using the full potential linear augmented plane wave (FP-LAPW) method. We present results for the band structure, density of states, and imaginary part of the frequency-dependent linear and nonlinear optical response. Our calculations show that the energy band gap of these compounds decreases when S is replaced by Se and Se by Te. This can be attributed to the fact that the conduction band minimum has strong cation-s states whereas the other states in the conduction band are more heavily mixed with other atomic orbitals such as anion-p states. In the conduction bands, shifting Ag-s states have small effect whereas shifting Ga-s states have a strong effect in opening the gap, while leaving the valence bands unchanged. Hence, the conduction bands shift towards Fermi energy (EF) when we move from S to Se to Te causing a reduction in the energy gap. We have calculated ε2(ω) and the birefringence of these compounds. We found that the birefringence is negative for AgGaS2 and AgGaSe2 whereas it is positive for AgGaTe2, in agreement with the experimental data. The intra- and inter-band contributions of the second harmonic generation increase when moving from S to Se to Te. The smaller energy band gap compounds have larger values of χ123(2)(0), in agreement with the experimental measurements and other theoretical calculations.