Linear, nonlinear optical properties and birefringence of AgGaX 2 (X=S, Se, Te) compounds

Abstract

We report calculations of the electronic, linear and nonlinear optical properties of AgGaX 2 (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 ( E F) when we move from S to Se to Te causing a reduction in the energy gap. We have calculated ε 2 ( ω ) and the birefringence of these compounds. We found that the birefringence is negative for AgGaS 2 and AgGaSe 2 whereas it is positive for AgGaTe 2, 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 χ 123 ( 2 ) ( 0 ) , in agreement with the experimental measurements and other theoretical calculations.