Doctor of Philosophy (PhD)
The Department of Physics and Astronomy
Nonlinear optical studies have revealed that surfaces and interfaces possess unique optical properties due to the lack of inversion symmetry. These properties are significantly different from those of the bulk material. Second harmonic generation (SHG), a second-order nonlinear optical phenomenon, has emerged as a surface-specific technique to study these unique properties. The powerful selection rule allows generation of second harmonic light only in the bulk of noncentrosymmetric media and the regions with broken inversion symmetry, including surfaces and interfaces. The generated second harmonic provides significant information on the electronic and structural properties of the microscopic surface layer at the boundary and the noncentrosymmetric bulk. This thesis investigates the weak surface contribution to SHG, which is suppressed by the bulk contribution in noncentrosymmetric crystals such as GaAs and GaP. It suggests that amplifying the SHG signal originating from the surface can be achieved by irradiating the surface with an amplifier laser. In another study, the SHG technique is employed to probe the underlying structural symmetry at the interface between two perovskite oxides. The SHG measurements from SrTiO3 thin film deposited on NdGaO3 substrate demonstrated a mixed symmetry at the interface. Moreover, the spectrum of the heterostructure and the NdGaO3 substrate exhibited a two-photon excitation fluorescence emission, originating from luminescent Nd3+ ions in the substrate. This observation led to the investigation of the electronic transitions associated with fluorescence emissions. Second harmonic generation measurements from the surface of centrosymmetric crystals that are in contact with air molecules is far from ideal since the contamination on the surface can lead to inaccurate results. Inspired by Hsieh et al. experiments, a scattering-plane rotational SHG viii setup is proposed to be coupled to an ultra-high vacuum (UHV) chamber. This approach, which includes rotation of scattering plane rather than the sample allows nonlinear optical studies of materials under UHV conditions. Lastly, the ultrafast optical pump-probe reflectivity technique is employed to study the laser-induced damage threshold of gold thin films as a function of film thickness under systematic variation of the incident pump power. Together, these nonlinear experiments reveal new insights in the study of surfaces and interfaces.
Dehghani, Mojgan, "NONLINEAR OPTICAL STUDIES AT SURFACES AND INTERFACES" (2023). LSU Doctoral Dissertations. 6242.
Available for download on Tuesday, August 27, 2024