Nonlinear Optics of Semiconductors Using a Tunable Free-Electron Laser

The short-pulse and ultrahigh intensity characteristics of the Vanderbilt free-electron laser (FEL) make it ideal for studying nonlinear optical properties of small band gap semiconductors. The FEL wavelength range also fortuitously corresponds to the band gaps of many technologically interesting semiconductors. For example, the FEL is a perfect light source for studying multiphoton processes, which can induce multiphoton transitions in measurable amounts. Multiphoton interaction techniques are important in the field of optical spectroscopy since they allow us to probe transition forbidden to single photon processes. This dissertation focuses on measurements using the FEL and other light sources to study Ge, InAs, and GaAs/AlAs asymmetric multiple quantum wells (MQWs). Several techniques were used, which include: photoluminescence, photoluminescence excitation spectroscopy, and Z-scan. The two-photon absorption in bulk Ge in the wavelength region near the two-photon absorption threshold was measured. In addition to its intensity, the FEL's tunability makes it possible optically pump a specific state. This technique also can be used for studying heterojunctions and MQWs. The interband and intersubband transitions of GaAs/AlAs asymmetric MQWs were measured. Another important area of materials research concerns chemical vapor deposited (CVD) diamond. Cathodoluminescence from annealed and unannealed CVD diamond films were measured using a low-energy electron beam. The measurements exhibit a pronounced difference between the luminescence spectra of annealed and unannealed CVD diamond. A significant temperature dependence in the luminescence has been observed.