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 χ(2) in zinc-blende semiconductors. At terahertz frequencies the nonlinear susceptibility depends not only on the purely electronic response χ∞ (2), but also on three other parameters C1, C2, and C3 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 (C3) dominates over electrical anharmonicity (C2), 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 ωTO /2 and ωTO does not occur for several other III-V compounds.