Fanqi Gan

The linear and nonlinear optical responses in a large number of cubic insulators are studied by means of first-principles local-density calculations. Complete results on band structures, frequency-dependent dielectric functions, and frequency-dependent third-order nonlinear susceptibilities ${\mathrm{\ensuremath{\chi}}}^{(3)}$(\ensuremath{\omega}) (in the simplest form as the third-harmonic generations) are presented for 27 alkali halides, alkali-earth fluorides, oxides, and sulfides. They are LiF, LiCl, LiBr, LiI, NaF, NaCl, NaBr, NaI, KF, KCl, KBr, KI, RbF, RbCl, RbBr, RbI, ${\mathrm{CaF}}_{2}$, ${\mathrm{SrF}}_{2}$, ${\mathrm{CdF}}_{2}$, ${\mathrm{BaF}}_{2}$, MgO, CaO, SrO, BaO, MgS, CaS, and SrS. The results are compared with the existing experimental data and other calculations. The effectiveness of using a ``scissor operator'' to correct the gap underestimation in the local-density-approximation theory is assessed. It is shown that the full band-structure approach for the ${\mathrm{\ensuremath{\chi}}}^{(3)}$(0) calculation in these crystals gives results in very good agreement with experimental data, especially in the anisotropic coefficient of the nonvanishing tensor elements.

The electronic structure, the charge-density distribution, and the optical-absorption spectrum of a ${\mathrm{CaF}}_{2}$ crystal are studied by means of a first-principles local-density calculation. The calculated imaginary part of the dielectric function is in good agreement with experimental measurement up to 27.0 eV. We have also used a simplified self-interaction-correction (SIC) model to address the problem of band-gap underestimation in the local-density calculation. Although some marginal improvement in the optical result has been achieved, there is no strong evidence of unequivocal rectification other than an increment in the band gap. This study shows additional (non-SIC) correction to the conduction-band states of ${\mathrm{CaF}}_{2}$ may also be necessary.

The electronic structure and optical properties of crystalline C 60 and their pressure dependence have been studied by first-principles local density calculations. It is shown that fcc C 60 has a low dielectric constant and an optical spectrum rich in structures. The spectrum shows five disconnected absorption bands in the 1.4 to 7.0 eV region with sharp structures in each band that can be attributed to critical point transitions. This is a manifestation of the localized molecular structure coupled with long range crystalline order unique to the C 60 crystal. At a sufficient high pressure, the structures in the optical spectrum start to merge due to the merging of the bands. These results are in good agreement with some recent experimental measurements.