Picosecond-laser-excited photoluminescence spectra in GaSe at room temperature have been measured and analyzed.Both spontaneous and stimulated emissions were attributed to the excitonexciton scattering process in GaSe through a careful band-shape analysis. The exciton-photon coupling coefficient~a/@=3. 3)&10 ' was obtained from the spontaneous emission spectra fit. The stimulated emission is well explained by the optical gain mechanism arising from the reverse absorption process of the exciton-exciton scattering.

No abstract

A frequency sweeping (chirping) mechanism governed by the lasing gas pressure and composition has been observed in a long-pulse TE CO2 laser for the first time. The mechanism was detected in heterodyned data obtained from photomixing the pulsed laser output with a cw local oscillator. A theory has been developed which links this chirp-governing mechanism to the vibration-translation (V-T) transfer rate from the lower laser level to the ground state. This new theory extends the existing theories on chirp in pulsed CO2 lasers into the long-pulse regime.

Dye-laser efficiency measurements were performed on four Coumarin dyes: C-485, C-498, C-500, and C-503, all operating near 500 nm. They were pumped with a 16-ns pulse-length KrF laser. The most efficient of the Coumarin dyes tested was C-498. The intrinsic efficiency of this dye was found to increase with pump rate. At a pump rate of 2 MW/cm(2), an intrinsic efficiency of > 20% was measured. Longer KrF laser pulses (> 1.2 µs) were also used to pump C-498 and resulted in comparable laser efficiencies at similar pump rates. The temporal relationship between the input KrF laser pump pulse and the output dye-laser pulse was studied and was found to differ significantly for long and short pulses. Significant improvement in long-pulse laser performance was observed when the laser-cavity configuration incorporated a skip mode.

The generation of intense picosecond laser- driven shock waves in the layered GaSe semiconductor and simultaneous probing of the sample using another picosecond laser to study the shock- induced phenomena are discussed. In unshock condition, at low probe intensity, spontaneous emission is observed while at high probe intensity stimulated emission is observed. Under laser- induced shock pressure, 20-nm spectral shift of spontaneous emission and sixfold decrease in the intensity of stimulated emission have been detected. By careful band-shape analysis, both spontaneous and stimulated emission is attributed to the exciton-exciton scattering process. The red shift of the spontaneous emission is caused by the shrinkage of GaSe band gap under shock pressure. The quenching of the stimulated emission is explained on the basis of stimulated absorption by the electron-hole recombination process.