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 tensor. The angular dependencies of light scattered at the double frequency of an incident light for different scattering geometries are analyzed.
As application examples, SHG from a thin crystalline film on a substrate is theoretically investigated for both s and p polarized incident light. The contributions of lattice misfit strain as well as of misfit dislocation strain to the second-order nonlinear optical susceptibility are described using a nonlinear photoelastic tensor and can be separated by a polarization analysis of the scattered light at the second harmonic frequency. For the s-s and p-s scattering geometries, the nonlinear optical signal will be determined by dislocation strain only, whereas for the s-p and p-p geometries both lattice misfit strain and misfit dislocation strain will contribute. This argument is extended to a magnetic film on a nonmagnetic substrate. The contributions of lattice misfit strain as well as of misfit dislocation strain to second-order nonlinear optical susceptibility are described using a nonlinear photoelastic tensor, together with the contribution of magnetization described by a nonlinear magneto-optical tensor.
For completeness of this study, we have measured strain-induced SHG signals with the $Co_{0.25}Pd_{0.75}$ alloy film deposited on PZT substrate. With the help of PZT characteristic(piezoelectric effect), we can control the strain in the film substrate interface. We have obtained stable strain-induced SHG signals with sweeping the voltages across the PZT substrate. From the voltage hysteresis, it can be concluded that SHG can be a possible and unique tool for investigation of strain at interfaces in ultrathin films.
