Laser-Induced Shocks in Solid Helium

We investigate laser-induced cavitation and phase-transition dynamics in solid helium, using ultra-high-speed video imaging at frame rates up to 7 million frames per second. Experiments are conducted at temperatures between 1.2 K and 2 K and pressures ranging from the melting point (~25 atm) to 39 atm, spanning both HCP and BCC crystal structures of solid helium. A pulsed 532 nm, 6 ns Nd:YAG laser is focused into the solid using a parabolic mirror inside an optical-access cryostat, which includes five windows for laser entry, backlighting, and imaging. The focused laser pulse generates a high-pressure plasma that rapidly vaporizes helium, forming a spherical void and inducing local melting around the bubble. A thin liquid layer develops, which eventually resolidifies. The initial shockwave propagates only a short distance before dissipating, while the melting front expands at speeds exceeding 100 m/s before freezing back over several seconds.



To interpret the pressure dynamics, we model the event as a spherically symmetric high-pressure source in a compressible medium and derive a closed-form solution to the spherical wave equation. This reveals an outward-propagating pressure front followed by a rarefaction zone, where the pressure drops below ambient—creating favorable conditions for phase transitions, such as solid-to-vapor conversion. The solution exhibits N-wave structure, ensures mass conservation, and estimates the pressure field surrounding the event. The theoretical predictions are consistent with observed dynamics, including shock dissipation, void growth, and interface re-solidification, providing new insight into ultrafast phase transitions and interfacial behavior in quantum solids.