Laser-induced plasma formation in water at nanosecond to femtosecond time scales: Calculation of thresholds, absorption coefficients, and energy density

The generation of plasmas in water by high-power laser pulses was investigated for pulse durations between 100 ns and 100 fs on the basis of a rate equation for the free electron density. The rate equation was numerically solved to calculate the evolution of the electron density during the laser pulse and to determine the absorption coefficient and energy density of the plasma. For nanosecond laser pulses, the generation of free electrons in distilled water is initiated by multiphoton ionization but then dominated by cascade ionization. For shorter laser pulses, multiphoton ionization gains ever more importance, and collision and recombination losses during breakdown diminish. The corresponding changes in the evolution of the free carrier density explain the reduction of the energy threshold for breakdown and of the plasma energy density observed with decreasing laser pulse duration. By solving the rate equation, we could also explain the complex pulse duration dependence of plasma transmission found in previous experiments. Good quantitative agreement was found between calculated and measured values for the breakdown threshold, plasma absorption coefficient, and plasma energy density.