Strain-induced second harmonic generation in ultrathin films

Abstract

The Nonlinear optical methods are widely used for the investigation of real solids, and are usually more powerful than linear optical methods. Among nonlinear optical phenomena second-harmonic generation (SHG) is very efficient for an investigation of thin films and interfaces. The SHG signal is very sensitive to a change of symmetry induced by various physical actions--for example, by strain. Special kinds of internal strain lead to a lowering of the symmetry of a crystal, i.e., to a missing of the inversion center. As a result, bulk dipole active optical SHG should be observed for transparent materials in the transmission geometry. From another side, surfaces and interfaces of centrosymmetric bulk materials are characterized by a lower point-symmetry group even in the absence of strain. In this case a more effective method to observe strain-induced changes of a surface is the measurement of optical SHG in the reflection geometry. In both cases, a method using three-photon phenomena is an effective tool for investigating strain-induced effects in solids and solid-state structures. For development of strain-induced SHG as an applicable method for investigation, both theoretical and experimental approach have been made in this thesis. Strain-induced three-photon effects such as SHG and hyper-Rayleigh light scattering, characterized by electromagnetic radiation at the double frequency of an incident light, are phenomenologically investigated by adopting a nonlinear photoelastic interaction. The relations between the strain and the nonlinear optical susceptibility for crystal surfaces with point symmetries of 4mm and 3m are described by a symmetry analysis of the nonlinear photoelastic tensor. We theoretically demonstrate a possibility of determining the strain components by measuring the rotational anisotropy of radiation at the second harmonic frequency. Hyper-Rayleigh light scattering by dislocation strain is also described using a nonlinear photoelastic tenso...