| Nonlinear optical techniques have become very useful diagnostic methods for surface and interface properties of materials and for developing new materials. However, it is still very difficulty to interpret nonlinear optical data. Here, I continued to develop the simplified bond hyperpolarizability model (SBHM). I extended the theory to describe the bulk-forbidden second harmonic generation (SHG), bulk third harmonic generation (THG), and fourth harmonic generation (FHG). I clarified the relative importance of bulk-forbidden contribution and surface dipole contribution to SHG. I showed the transverse-bond-direction motion was necessary to correctly describe THG data. And I presented a way to evaluate the surface roughness effects on FHG.; In addition, from the principle of SBHM, I developed a simple model to interpret the origin of terahertz radiation from semiconductor surfaces trigged by ultrashort laser pulse. I unified the mechanisms of terahertz generation---difference frequency mixing and current surge mechanism at the field level.; Finally, I showed spectroscopic SHG could give us more information than single frequency azimuthal dependent data. |
