| Optical spectroscopy studies the interaction between light (photon) and matter. During such interaction, different processes such as reflection, transmission, scattering, absorption or fluorescence can occur. Among all the optical spectroscopic techniques, infrared (IR) and Raman spectroscopy are most commonly used. In an Infrared process, photons are absorbed. The required light source emits polychromatic Infrared light and when it passes through or being reflected by the sample the light is partially absorbed. The frequency dependent absorption allows one to study the electronic and vibrational structure of the sample. On the other hand, the Raman spectroscopy is second order in nature where the photon is scattered instead of being absorbed. A monochromatic light source is used instead of a continuous spectrum. Generally, the dominate effect in an optical process is absorption and transmission but a (small) portion of photons are scattered. A small fraction of photons change their energy/wavelength during the scattering. Depending on the scale of the change in energy, those inelastic scatterings can be categorized into Brillouin scattering and Raman scattering. Although sharing the same mechanism, different energy scale require completely different experimental setups for Brillouin scattering and Raman scattering.;In the study of infrared and Raman spectroscopy, group theory is a very helpful tool. The calculation of absolute intensity of an optical transition is rather difficult and sometimes infeasible, especially for crystal vibrations. Group theory is the mathematical language that describes the symmetry property of the physical system. Selection rules based on symmetry consideration had been predicted. Group theory, especially representation theory, is an important branch of condensed matter physics.;Both theoretical and experimental results of my PhD research are presented. The topics being covered are: infrared study of iron based superconductor BaFe1.85Co0.15As2; the study of Raman scattering with Laguerre-Gaussian (LG) beam. |
