Ultra-high-speed imaging of laser-induced interfacial dynamics in superfluid and solid helium*

Superfluid helium (He-II) becomes a solid at pressures of about 25 atmospheres. The interface between solid and the superfluid exhibits intriguing behavior, with wavy time-evolving oscillations on its surface (K. Keshishev, JETP Lett, 30. 1). It can thereby act similar to a free surface (S. Balibar, RevModPhys. 77. 1. 327). Herein, we use ultra-high-speed imaging at frame rates up to 7 million frames per second to experimentally investigate laser-induced interfacial dynamics at temperatures between 1.2 K and 2 K and at pressures starting from the melting pressure of about 25 atm and up to 39 atm, which spans both HCP and BCC structure of the solid.  The system (N. Speirs, PhysRevFluids. 5. 044001), has five optical windows which we use to pass the pulsed laser-beam through and capture the dynamics using ultra-high speed cameras, watching through a different set of windows. We use a 532 nm, pulsed Nd:YAG laser with a duration of ~ 6 ns and variable pulse energy up to 120 mJ to disturb the interface, focused by a parabolic mirror within the cell. We characterize melting and resolidification as well as track interface motions and shockwaves inside the different phases.