Kinetic model of stimulated emission created by resonance pumping of aluminum laser-induced plasma

摘要

Stimulated emission observed experimentally in an aluminum laser induced plasma is modeled via a kinetic approach. The simulated emission at several cascade transitions is created by a pump laser guided through the plasma at several microseconds after its creation and tuned in resonance with the strong 3s~23p-3s~24s transition at 266 nm. A two-dimensional space-time collisional radiative plasma model explains the creation of the population inversion and lasing at wavelengths of 2100 nm and 396.1 nm. The population inversion for lasing at 2100 nm is created by depopulation of the ground 3s~23p state and population of the 3s~25s state via the absorption of the resonant radiation at 266 nm. The population inversion for lasing at 396.1 nm occurs during the laser pulse via the decay of the population of the pumped 3s~25s state to the excited 3s~24s state via cascade transitions driven optically and by collisions. In particular, efficient are the mixing transitions between neighboring states separated by small gaps on the order of kT at plasma temperatures of 5000-10 000 K. The model predicts that the population inversion and corresponding gain may reach high values even at very moderate pump energy of several μJ per pulse. The efficiency of lasing at 2100 nm and 396.1 nm is estimated to be ~3% and 0.05%, correspondingly with respect to the pump laser intensity. The gain for lasing at 396.1 nm can reach as high as ~40cm~(-1). The polarization effect that the pump radiation at 266 nm imposes on the stimulated emission at 396.1 nm is discussed. The calculated results are favorably compared to experimental data.