Influence of Replacing Si by Ge in the Chalcogenide Quaternary Sulfides Ag2In2Si(Ge)S6 on the Chemical Bonding, Linear and Nonlinear Optical Susceptibilities, and Hyperpolarizability

The linear and nonlinear optical properties of Ag2In2SiS6 and Ag2In2GeS6 are calculated so as to obtain further insight into the electronic properties. The influence of using different exchange correlation potentials and the effect of replacing Si by Ge on the geometry, chemical bonding, and on the optical properties are presented. There is notable increasing in the energy band gap when moving from LDA to GGA, EVGGA then to mBJ. The effect of replacing Si by Ge atom causes a geometric change, which leads to large changes in the linear as well as the nonlinear optical susceptibilities. For the linear optical properties, it causes to increase the amplitude of the left-hand hump of epsilon(average)(2)(omega) as well as a small shift of the main peak to lower energies. We have evaluated epsilon(average)(1)(0) and find that a smaller energy gap yields a larger epsilon(1)(0) value. From the calculated refractive indices we obtained the birefringence, which is important for second harmonic generation (SHG) and optical parametric oscillation (OPO) as it is defined by the phase-matching condition. The second-order nonlinear optical susceptibilities, namely, the SHG are investigated for chi((2))(111)(omega), chi((2))(122)(omega), and chi(323)1(a)). We find that 1423 (co) is the dominant component. The microscopic second order hyperpolarizability, film for the dominant component iiii(w) was obtained. We should emphasize that replacing Si by Ge enhances the linear and nonlinear optical susceptibilities so that Ag2In2GeS6 shows higher values of the linear and nonlinear optical susceptibilities and An in comparison to Ag2In2SiS6.