Nonlinear optics of III-V semiconductors in the terahertz regime: an ab-initio study

We compute from first principles the infrared dispersion of the nonlinear susceptibility χ (2) in zincblende semiconductors. At terahertz frequencies the nonlinear susceptibility depends not only on the purely electronic response χ (2) ∞ , 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 ω TO / 2 and ω TO does not occur for several other III-V compounds.