Energy reservoir of femtosecond filament and plasma channel formation in focused beams

Propagation of high-power femtosecond laser pulse in transparent dielectric medium leads to beam filamentation [1]. Filamentation of collimated beams is widely studied by different scientific groups [1], unlike the propagation of tightly focused laser pulses. Experiment [2] showed that very tight beam focusing leads to dense plasma region generation in geometrical focal area. Also tight focusing [3] can result in plasma channels passing through system focal plane. Hypothesis of energy reservoir, sustaining extended filament existence in case of collimated beam, was proved in experiment [4]: placing of a small diaphragm, passing only high energy beam core and absorbing peripheral parts of the pulse, led to filament termination. Energy reservoir role in filamentation of focused beam has not been studied before, so we experimentally and numerically studied the reservoir influence on plasma channel formation in case of tight beam focusing.