Laser-Induced Streams in Liquids Depending on Focusing Conditions of Pulse-Periodic Femtosecond Laser Radiation

Abstract

The paper is devoted to generation and observing quasi-stationary flows under the effect of the focused pulse-periodic femtosecond laser radiation on liquid solvents: water, heavy water, al-cohols, ketones, chlormetanes. The mechanism inducing directional flows appears to be directional collapse of the gas bubbles produced by multiphoton dissociation in a focused laser beam. Formation of the flows had being observed with schlieren technique and or framing small gas bubbles illuminated by visible laser beam converted to the line. Absorption coeffi-cients of focused and unfocused femtosecond laser radiation in the fluids were also measured. In experiments with laser pulses of 450 fs at 1.023 nm wavelength with pulse energy from 10 to 220 uJ and repetition rate 1.43 ÷ 10 kHz stationary flows of fluids were originated from the laser beam waist directed along or transversely (in most cases) to the laser beam. The streams along the laser axis in both directions were observed under low pulse power (10 ÷ 20 uJ) pro-vided precise lens adjustment without astigmatism or self-focusing. The tests show that single beam waist of 10 ÷ 20 um in diameter and 100 ÷ 200 um long generates two narrow jets along the beam axis in both directions from the waist. Lens displacement transversely to the beam leads to splitting beam waist in two astigmatic foci. Both foci generate axial flows directed toward each other that collide in the gap between foci, forming synthetic transverse flow. The increase of the pulse energy and focal point intensity were followed by nonlinear self-focusing forming filament like beam waist up to 14 mm long. Repeating cycles of self-focusing and defocusing produce several energy deposition zones along the filament beam waist, each one generating separate streams along the beam axis. Colliding of the contra-directed streams gave rise to complex flow pattern along and transversely to the beam axis.