R. W. Schoenlein

Femtosecond laser-tissue interactions: Retinal injury studies

We report the first study of laser-tissue interaction in the femtosecond time regime. Retinal damage thresholds and mechanisms produced by exposure to high-intensity femtosecond laser pulses were investigated in chinchilla grey rabbits. Exposures were performed using single laser pulses of 80 fs duration at 625 nm. ED 50 injury thresholds of 0.75 and 4.5 μJ were measured using fluorescein angiographic and ophthalmoscopic visibility criteria evaluating 204 laser exposures. Ultrastructural studies including light and electron microscopy were performed on selected lesions. Results suggest that the primary energy deposition in the retina occurs in melanin, However, in contrast to laser injuries produced by longer pulses, exposures of more than 100 × threshold in the 50-100 \mu J range did not produce significantly more severe lesions or hemorrhage. This suggests the presence of a nonlinear damage limiting mechanics in tissue exposed to femtosecond laser pulses.

Femtosecond hot‐carrier energy relaxation in GaAs

Excited carrier dynamics in GaAs and Al0.2Ga0.8As are investigated using femtosecond pump and continuum probe techniques. We observe absorption spectral hole burning arising from excited carriers generated by transitions from the split‐off band as well as the heavy‐ and light‐hole bands. Transient absorption saturation measurements indicate that the initial nonthermal carrier distribution thermalizes on a time scale of several tens of femtoseconds.

Amplification of femtosecond pulses in Ti:Al 2 O 3 using an injection-seeded laser

A 440-fsec, 0.1-pJ pulse from a dye laser is injected into a high-repetition-rate Ti:Al(2)O(3) laser pumped by a copper-vapor laser to study the amplification and pulse broadening of femtosecond pulses in Ti:Al(2)O(3). Gains of 2 x 10(7) are achieved with output pulse durations of 1.1 psec. After recompression with a grating pair to compensate dispersion broadening, pulses as short as 275 fsec are obtained.

Femtosecond hot carrier energy redistribution in GaAs and AlGaAs

Excited carrier dynamics in GaAs and Al.2Ga.8As are investigated using femtosecond pump and continuum probe techniques. Absorption saturation measurements provide evidence for transient spectral hole burning due to split-off as well as heavy and light hole valence to conduction band transitions. The initial nonthermal carrier distribution thermalizes and assumes a broad energy distribution on a femtosecond time scale.

Femtosecond studies of intervalley scattering in GaAs and AlxGa1-xAs

Abstract Results are presented from ensemble Monte Carlo simulations of the relaxation of photoexcited electrons and holes. The results are compared directly with three types of femtosecond optical experiments: transient absorption saturation, pump and continuum probe, and tunable pump-probe experiments. For these experiments we find that intervalley scattering has a dominant effect for the first several hundred femtoseconds, and that electron-electron scattering is only important at later times.

Femtosecond Photoemission Studies of Image Potential and Electron Dynamics in Metals

The combination of femtosecond laser generation techniques with photoemission electron spectroscopy can permit the measurement of surface state and electron dynamics on a femtosecond time scale. We describe the application of these techniques to perform the first time resolved measurements of image potential state dynamics in metals. Image potential states occur when an electron outside a metal surface is bound in a Coulombic potential produced by its image charge in the metal. These states form a Rydberg-like energy series and resemble a two dimensional electron gas. The lifetimes of the n = 1 and n = 2 states in Ag(100) are 15–35 fs and ~ 200 fs, respectively. In Ag(111) the lifetimes of the n = 1 and n = 2 states are less than ~ 20 fs. Experimental results are in qualitative agreement with theoretical models of image potential electron dynamics.

Femtosecond Studies Of Excited Carrier Energy Relaxation And Intervalley Scattering In GaAs and AlGaAs

We report the investigation of excited carrier scattering, energy relaxation, and intervalley scattering in GaAs and AlGaAs. Pump and continuum probe absorption saturation measurements provide evidence for femtosecond transient nonthermal carrier distributions and permit a measurement of carrier cooling processes. Measurements performed using a tunable femotsecond laser allow an investigation of intervalley scattering.