Calculation of second-order optical response in semiconductors

We present a first-principles calculation of two second-order optical response functions as well as the dielectric function for GaAs and GaP. Specifically, we evaluate the dielectric function epsilon(omega) and the second-harmonic generation response coefficient chi((2))(-2 omega;omega,omega) over a large frequency range. The electronic linear electro-optic susceptibility chi((2))(-omega;omega,0) is also evaluated below the band gap. These results are based on a series of self-consistent LDA calculations using the full-potential linearized augmented plane wave method. Self-energy corrections are included at the level of the ''scissors'' approximation, which corrects for the underestimation of the local density approximation band gap and produces a change in the velocity matrix elements. The analytic expressions for the second-order response functions are free of the unphysically divergent terms at zero frequency that have previously plagued such calculations. Results for chi((2))(-omega;omega,0) are in good agreement with experiment below the band gap and those for chi((2))(-2 omega;omega,omega) are compared with experimental data where available. We note that despite the equivalence of both of these second-order response functions at zero frequency, there seems to be some discrepancy between the experimental results for these functions in this regime.