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 $\chi^{(2)}$ in zincblende semiconductors. At terahertz frequencies the nonlinear susceptibility depends not only on the purely electronic response $\chi^{(2)}_{\infty}$, 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 $\omega_{\rm TO}/2$ and $\omega_{\rm TO}$ does not occur for several other III-V compounds.Comment: 9 pages, 3 figures; updated bibliograph

First-principles study of the electrooptic effect in ferroelectric oxides

We present a method to compute the electrooptic tensor from first principles, explicitly taking into account the electronic, ionic and piezoelectric contributions. It allows us to study the non-linear optic behavior of three ferroelectric ABO_3 compounds : LiNbO_3, BaTiO_3 and PbTiO_3. Our calculations reveal the dominant contribution of the soft mode to the electrooptic coefficients in LiNbO_3 and BaTiO_3 and identify the coupling between the electric field and the polar atomic displacements along the B-O chains as the origin of the large electrooptic response in these compounds.Comment: accepted for publication in Phys. Rev. Let

First-principles study of filled and unfilled antimony skutterudites

Using a first-principles approach based on density-functional theory, the electronic, dielectric and dynamical properties of the skutterudites CoSb3 and TlFeCo3Sb12 are studied. In particular, the electron localization tensor, static and dynamic effective charges, static and optical dielectric constants and phonon dispersion curves are computed. The Born effective charges are found to be significantly larger than the static charges of the ions. Moreover, the static dielectric constant of TlFeCo3Sb12 is found to be significantly larger than that of CoSb3. The analysis of the phonon dispersion curves reveals a low-energy mode due to coupled vibrations of Tl and Sb. This mode is at the origin of a well-defined peak in the phonon density of states of TlFeCo3Sb12 and its mode effective charge is related to the increase of the dielectric constant in TlFeCo3Sb12. Our results are compared to recent experiments performed on CoSb3 and TlFeCo3Sb12, and differences between the lattice dynamics of TlFeCo3Sb12 and other filled skutterudites are highlighted