Ab initio study of the nonlinear optics of III-V semiconductors in the terahertz regime

We compute from first principles the infrared dispersion of the nonlinear susceptibility chi((2)) in zinc-blende semiconductors. At terahertz frequencies the nonlinear susceptibility depends not only on the purely electronic response chi((2))(infinity), but also on three other parameters C-1, C-2, and C-3 describing the contributions from ionic motion. They relate to the TO Raman polarizability, the second-order displacement-induced dielectric polarization, and the third-order lattice potential. Contrary to previous theory, we find that mechanical anharmonicity (C-3) dominates over electrical anharmonicity (C-2), which is consistent with recent experiments on GaAs. We predict that the sharp minimum in the intensity of second-harmonic generation recently observed for GaAs between omega(TO)/2 and omega(TO) does not occur for several other III-V compounds.