Nonlinear optical studies of ultrafast carrier dynamics in silicon and a molecular thin film on a silver surface

Transient grating (TG) scattering and second harmonic generation (SHG) were used to study dynamical and structural properties of semiconductors and molecular thin films on metal surfaces. In the first part of this dissertation, the dynamics of photoexcited carriers generated near a silicon surface were studied on femtosecond timescales using transient grating scattering in a reflection configuration. Experiments were conducted to study both carrier concentration and orientation gratings. In the concentration grating study, the lifetime for hot carrier relaxation for electrons excited 1.4 ± 0.6 eV above the conduction bandedge and their corresponding holes was determined to be 240 fs at carrier densities below 1020 cm–3 . At carrier densities higher than 5 × 1020 cm –3, the relaxation lifetime increased sharply providing the first direct evidence for charge screening of carrier-phonon scattering in silicon. In the orientation grating study, a transient grating signal was observed whose origin is believed to be from anisotropic state-filling. The time profile of the transient grating signal mimics the observed time profile for the input pulses measured from correlation experiments. This indicates that the momentum reorientation time in silicon is much faster than the input pulsewidth of 100 fs. TG studies were also conducted to measure the ambipolar diffusivity and carrier recombination rate at very high carrier densities (> 1020 cm–3). In the second part of this dissertation, second harmonic generation was used to study thin pyridine films on a silver surface (Ag(111)). For films grown at 100 K, an irreversible phase transition from a disordered solid to an ordered solid was observed when the films were annealed above 125 K. An analytical model of the second harmonic generation from the annealed pyridine films is presented. It was determined that the annealed films were composed of randomly oriented crystallites with an average length distribution greater than 700 nm.