Large crystal local-field effects in second-harmonic generation of a Si/CaF ₂ interface: An ab initio study

In this work we present the ab initio study of crystal local-field effects in second-harmonic generation spectroscopy for an interface material such as Si/CaF ₂. Starting from an independent particle picture, we demonstrate the fundamental importance of the polarization effects at the interface discontinuity. The estimation of the magnitude of crystal local-field effects for second-order nonlinear response in Si/CaF ₂ interface was done by a comparative study with the absorption spectroscopy in the linear response. In both cases, we observe that the microscopic fluctuations due to the inhomogeneities of the system cause a decrease of the intensities of the spectra. However, for second-harmonic generation the decrease is selective and completely inhomogeneous while for absorption it is almost rigid. We also compare our theoretical study with experimental data showing unambiguously that only when crystal local fields are included, it is possible to correctly interpret experimental results.