Large crystal local-field effects in second-harmonic generation of a Si/CaF2 interface:An ab initio study

In this work we present the ab initio study of crystal local-field effects in second-harmonic generationspectroscopy for an interface material such as Si/CaF2. Starting from an independent particle picture, wedemonstrate the fundamental importance of the polarization effects at the interface discontinuity. The estimationof the magnitude of crystal local-field effects for second-order nonlinear response in Si/CaF2 interface was doneby a comparative study with the absorption spectroscopy in the linear response. In both cases, we observe that themicroscopic 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 absorptionit is almost rigid.We also compare our theoretical study with experimental data showing unambiguouslythat only when crystal local fields are included, it is possible to correctly interpret experimental results.