Numerical investigation of recombination gain in the Li III transition to ground state.

We present a numerical investigation of the parameters characterizing the 2-->1 transition recombination gain in Li III ions ( 13.5 nm ). The numerical model includes the initial optical field ionization of the plasma by an intense 100 fs laser pulse, taking into account above threshold ionization heating, particle collisions, and spatial effects. Gain is then calculated during the process of recombination as the plasma expands and cools. We show that by taking into account the non-Maxwellian nature of the electron distribution function in the plasma and its spatial distribution, high gain in the 2-->1 transition of Li III is feasible under certain initial conditions, even though initial estimates based on the energy absorption during the ionization predict very low gain. We characterize the behavior of the gain under different pumping parameters and initial plasma conditions.

Reaching the nonlinear regime of Raman amplification of ultrashort laser pulses.

The intensity of a subpicosecond laser pulse was amplified by a factor of up to 1000 using the Raman backscatter interaction in a 2 mm long gas jet plasma. The process of Raman amplification reached the nonlinear regime, with the intensity of the amplified pulse exceeding that of the pump pulse by more than an order of magnitude. Features unique to the nonlinear regime such as gain saturation, bandwidth broadening, and pulse shortening were observed. Simulation and theory are in qualitative agreement with the measurements.