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 ω_TO∕2 and ω_TO does not occur for several other III-V compounds.