Spectral, tensor, and ab initio theoretical analysis of optical second harmonic generation from therutile TiO2(110) and (001) faces

Abstract

We give an overview of our recent experimental study on the optical second harmonic (SH) response of therutile TiO2(110) and (001) faces, and the analysis of these results by phenomenologicalelectromagnetic theory using nonlinear susceptibility tensors and by abinitio theory using the self-consistent full potential linearized augmentedplane-wave (FLAPW) method within the local-density approximation. Since bulk rutileTiO2 has a uniaxial crystal structure of symmetryD4h14, the nonlinear optical response of its surface and bulk showed remarkable anisotropy. TheTiO2(110) face exhibited stronger reflected SH response when the incident electricfield was directed parallel than perpendicular to the [001] axis, while theTiO2(001) face exhibited relatively isotropic SH response. The anisotropy of the SH intensity patternsdepended remarkably on the incident photon energy and the polarization combination. Byusing a phenomenological electromagnetic theory, we performed a simultaneous analysis ofthe SH intensity patterns from the (110) and (001) faces as a function of thesample rotation angle around its surface normal. As a result we could separatethe contributions from the surface second-order and bulk higher-order nonlinearsusceptibilities. We also found that the SH intensity spectra as a function of the SHphoton energy depended strongly on the sample rotation angle and the polarizationcombination of the fundamental and SH light. The onset of the SH resonance of theTiO2(110) face was located at eV when the induced nonlinear polarization was perpendicular to the surface. It waslocated at eV when the induced nonlinear polarization is parallel to the [001] direction in the surfaceplane. These onset energies were higher than the onset energy of the bulk linear absorptionat 3.0 eV. On the other hand, the onset energy of the SH resonance of the (001) face wasfound at eV. A discussion is given on the physical meaning of the observed SH intensity spectra.Furthermore, an ab initio calculation of the nonlinear optical response from theTiO2(110) surface using the FLAPW method was performed. The calculated results agreed very wellwith the experimental SH intensity patterns and spectra. We found both from thephenomenological and ab initio calculation that the main SH response from theTiO2(110) surface originated from the Ti–O–Ti–O– zigzag chains on theTiO2(110) surface.