Microscale nanosecond laser-induced optical breakdown in water

Abstract

Microscale optical breakdown induced in bulk pure water by high-power nanosecond KrF laser pulses was studied using optical transmission and contact broadband photoacoustic techniques. The breakdown has been identified as a sharp transmission drop coinciding with the appearance of unipolar compressive acoustic pulses, both indicating a thresholdlike rise of local intrinsic absorption in the micrometer-scale laser focal volume. The acoustic pulses, which are much broader than the exciting laser pulse and show a strongly reduced far-field diffraction effect, result from breakdown-induced millimeter-sized steam bubbles. The acoustic pulse amplitudes exhibit a sub-linear ( proportional, variantI(3/4)) pressure dependence on the laser intensity I characteristic of subcritical electron-ion plasma and demonstrating the avalanche enhancement of two-photon ionization above the breakdown threshold until the appearance of the critical plasma. In the critical plasma regime, where the transmission and the acoustic signals slowly vary as a function of laser intensity, the main acoustic pulse is preceded by nanosecond and sub- micros prepulses, where the first one represents a GPa-level plasma-driven shock wave and the second one adjacent to the main pulse appears due to weak submillimeter-long heating of water surrounding the hot plasma by its bremsstrahlung radiation, indicating significant dissociation of water molecules in the plasma.

Pulsed laser ablation in liquid for micro-/nanostructure generation

“…I p depends on the laser wavelength. For example, I p of water is ∼10 13 W/cm 2 for a focused 1064 nm laser [63]; while for a focused 248 nm laser, I p of water is just ∼2 × 10 9 W/cm 2 [64]. The expansion of plasma associated with the breakdown pushes the surrounding liquid, resulting in a cavitation bubble.…”

“…Several routes may generate atomic or molecular hydrogen and oxygen: the evolution of free radicals produced by photodissociation [64] or collapsing of bubbles [71], the original soluble gas in the liquid [72], and the reaction byproducts of ablated materials with the liquid. When considering gas-filled bubbles in a unsaturated liquid, the collapse is determined by mass diffusion of the gas into the liquid, and the time t cs for complete solution is [72] …”

Laser Control of the Shock Wave Shape by Tightly Focused Femtosecond Laser Radiation

“…For example, in crystals focused femtosecond laser radiation generates phonons and shock waves and create microvoids, in liquids it generates cavitation bubble and shock waves. Shock waves define impact of laser irradiation in a far-field, because plasma is localized in a few microns [3][4][5][6][7][8]. These post effects have important practical usage.…”

Underwater laser plasma with reactive fuels

“…The bubble dynamics that stem from a laser focused in water [1][2][3] and at a metal target surface 4 underwater are critical to the understanding of energy release in each scenario. Spectroscopic measurements have also been conducted 5 in some cases to measure the products from such an event.…”