Influence of inertial confinement on laser-induced bubble generation and shock wave emission

Abstract

Laser-induced breakdown with ultrashort laser pulses is isochoric and inertially confined. It is characterized by a sequence of nonlinear energy deposition and hydrodynamics events such as shock wave emission and cavitation bubble formation. With nanosecond pulses, inertial confinement is lost especially during micro- and nanobubble generation and energy deposition and hydrodynamic events occur concurrently. The onset of bubble expansion during the laser pulse reduces peak pressure, bubble wall velocity, conversion into mechanical energy, and prevents shock wave formation. Here we present an extension of the Gilmore model of bubble dynamics in a compressible liquid that enables to describe the interplay between particle velocity during acoustic transient emission and bubble wall acceleration in the inertial fluid at any degree of confinement. Energy deposition during a finite laser pulse duration is encoded in the time evolution of the bubble's equilibrium radius such that no explicit description of phase transitions is required. The model is used to simulate bubble generation, acoustic transient emission and energy partitioning as a function of laser pulse duration and bubble size at fixed plasma energy density and ambient pressure. It turns out that bubble formation with femtosecond laser pulses is more disruptive than with nanosecond pulses. This applies mainly for micro- and nano-cavitation but to a lesser degree also for millimeter-sized bubbles. We discuss implications for process control in microsurgery and microfluidic manipulation with free-focused laser pulses and via nanoparticle-mediated energy deposition.