Femtosecond laser-gas-solid interactions


C. Wu. 2000. “Femtosecond laser-gas-solid interactions”. Thesis Type: phd.


This dissertation discusses two sets of experiments. The first set of experiments investigates the interaction of femtosecond laser pulses with silicon in the environment of a halogen-containing gas. We find that upon irradiation, sharp spikes are formed on the surface. Spike formation strongly depends on the laser and gas conditions. The spikes are crystalline and contain a high density of both structural and chemical defects. The spikes are very strong light absorbers and exhibit absorption exceeding A > 0.9 for ultraviolet (0.25 m) to near-infrared (2.5 m) wavelengths. Spiked avalanche photodiodes show a more than threefold increase in quantum efficiency upon illumination with 1.06 m and 1.31 m radiation. Unlike ordinary silicon, microstructured silicon luminesces strongly in the visible. The photoluminescence intensity and peak wavelength depend on the laser conditions used to produce the luminescent surfaces. In the second set of experiments, we study the oxidation of CO from CO/O2/Pt(111) using femtosecond laser pulses. We observe a nonlinear dependence of both the CO oxidation and O2-desorption yields on the laser fluence. The yields depend strongly on the laser wavelength. Nonthermal electrons must play an important role in the excitation mechanism leading to desorption of O2 and production of CO2. Oxidation of CO from CO/O2/Pt(111) induced with femtosecond laser pulses most likely proceeds via the formation of an intermediate CO3* complex.