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 elec tro-optic susceptibility chi((2))(-omega;omega,0) is also evaluated be low the band gap. These results are based on a series of self-consiste nt LDA calculations using the full-potential linearized augmented plan e wave method. Self-energy corrections are included at the level of th e ''scissors'' approximation, which corrects for the underestimation o f the local density approximation band gap and produces a change in th e velocity matrix elements. The analytic expressions for the second-or der response functions are free of the unphysically divergent terms at zero frequency that have previously plagued such calculations. Result s for chi((2))(-omega;omega,0) are in good agreement with experiment b elow the band gap and those for chi((2))(-2 omega;omega,omega) are com pared 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.