Filamentation of focused femtosecond laser pulse and plasma channel formation in the vicinity of geometric focus

Filamentation of focused femtosecond laser pulse in the vicinity of the geometric focus is studied both theoretically and experimentally. Keywords—femtosecond pulse; geometric focus, plasma channel; filamentation; nonlinear optics High power femtosecond laser pulse propagates through the transparent medium with pulse energy localizati on accompanied with thin plasma channels (1). The phenomenon of filamentation occurs owing to the dynamic balanc e between two major physical effects: Kerr self-focusing and plasma defocusing of the beam. The paper (2, 3) demonstrat ed that main part energy contained in background reservoir during the propagation of femtosecond laser pulses in air. The filamentation process was terminated immediately when filament passed through a pinhole (3). It was shown in (4) that for collimated and tightly focused laser beam the f ilament parameters (radius of the filament, laser pulse int ensity, electron density in plasma channel) are strongly di fferent. But there are no data about background reservoir behavi or in the vicinity of the geometric focus with propagation of tightly focused laser beam. In this paper the some features of self- focusing and plasma channel formation with femtosecond laser pulse filamentation in focused laser beam at the area closed to the geometric focus are studied and discu ssed both experimentally and theoretically. Femtosecond laser pulse (740 nm, 100 fs) focused by concave dielectric mirrors (for the experiment desc ribed below the focal length was 50 cm). In the vicinity of the geometric focus a pinhole (diameter was about 300 μm) was placed. Linear plasma density was measured by hemisphere electrodes. When plasma channel formed by laser pulse was appeared between the electrodes the recharge curren t of the capacitor was measured by oscilloscope. The current was proportional to linear plasma density in the plasma channel. The laser pulse fluence distribution was visualized along the pulse propagation in filamentation area by a CCD-ca mera also.