Femtosecond laser-induced optical breakdown and cavitation dynamics in water imaged with an x-ray free-electron laser

We investigate the ultrafast dynamics of plasma formation by optical breakdown, filamentation, and cavitation in water, using high spatiotemporal resolution offered by x-ray free-electron laser (XFEL) radiation. A femtosecond infrared laser pulse is focused in a water-filled cuvette and probed by a single femtosecond x-ray pulse, with a time delay covering nearly four orders of magnitude. By exploiting the quantitative contrast values obtained by phase retrieval, we can follow the transition from plasma to gas in terms of a continuous decrease of mass density in the cavity. At the same time, we image the emission of a cylindrical shock wave for the scenario of a single elongated breakdown filament with a high degree of symmetry. Contrarily, the regime of multiple breakdown spots deviates from cylindrical symmetry and the idealized picture expected for a Gaussian beam. Here different scenarios of cavitation and (collective) expansion dynamics as well as bubble fusion are observed. Specifically, we quantify the decrease of the expansion velocity with the number of auxiliary cavitation events due to a redistribution of the deposited laser energy. We also report events with (multi)filamentation reflecting instabilities in the initial distribution of the laser intensity upon formation of the plasma. Filaments with submicron diameter and few-micrometer spacing are observed, as well as the phenomena of filament emergence, splitting, and termination. The different regimes of heterogeneous optical breakdown and cavitation can be distinguished depending on the laser pulse energy. Altogether, the experiments demonstrate the potential of single-pulse XFEL imaging for the investigation of optical breakdown and ultrafast hydrodynamics. The future application of the imaging approach to soft matter environments, tissue, glasses, and opaque materials seems straightforward.