Hydrodynamic phenomena in optical discharges in liquids under self-focusing of periodic-pulse laser radiation

Abstract

Quasi-stationary flows under the effect of focused periodic pulse femtosecond laser radiation were generated and observed in liquid solvents: water, heavy water, alcohols, ketones, chloromethanes. The mechanism inducing directional flows appears to be directional collapse of the gas bubbles produced by multiphoton dissociation in a focused laser beam. Laser pulses of 450 fs length, up to 220 μJ pulse energy at repetition rates up to 10 kHz have induced stationary flows of liquid originated from the laser beam waist directed along or transversely to the beam axis. The streams along the beam axis were observed under low pulse power (10-20 μJ), provided precise lens adjustment. Lens displacement transversely to the beam axis led to splitting beam waist in two astigmatic foci. Both foci generate the streams along the beam axis. Counter directed streams have collided in the gap between foci, forming the flow spreading transversely to the laser beam. The increase of the pulse energy was followed by formation of the filament of self-focusing. Repeating cycles of focusing and defocusing along the filament produced several beam energy dissipation zones, each one generating separate streams along the beam axis. Colliding of the counter directed streams gave rise to complex flow pattern transversely and upward with respect to the beam axis.