Brillouin Spectroscopy of Metastable Superfluid Helium-4

We report measurements of the Brillouin frequencies of acoustically driven metastable states of superfluid ${}^4\mathrm{He}$ at $T\sim 1\mathrm{K}$. This is done using a stimulated Brillouin gain spectrometer operating on a confined spatiotemporal domain ($\sim 40\mu \mathrm{m}$, 200 ns). Our work questions previous experimental results regarding the metastable states of liquid ${}^4\mathrm{He}$ and suggests that thermal effects or vortices can play a significant role in the cavitation process. Thanks to our time-resolved measurements, we also find that it is possible to probe metastable superfluid ${}^4\mathrm{He}$ beyond the commonly defined cavitation threshold.

Figure 1

Left: scheme of the experiment. A piezoelectric transducer focuses 1.14 MHz acoustic waves into bulk liquid helium ($T\sim 1\mathrm{K}$) producing metastable states during the negative swings of the wave. A SB gain spectrometer consisting of two crossed laser beams, a pulsed pump and a frequency tuneable continuous wave probe, is used to measure time-resolved Brillouin frequencies of the liquid at acoustic focus. $\theta$ is the crossing angle between the lasers. Right: timeline of the experiment. Orange: 10 Hz logical clock signal delivered by the SB pulsed pump laser. Blue: driving piezo transducer voltage (amplitude $V$, frequency $f_{\mathrm{ac}}$). Gray: Brillouin frequency at acoustic focus. Red: Pump SB laser intensity $I_1$.

Figure 2

Time evolution of the Brillouin frequencies ($f_B$) at acoustic focus. Orange circles: driving voltage $V=150\mathrm{mV}$. Red circles: $V=540\mathrm{mV}$. The point linking lines are guides to the eye. Horizontal dashed line: static value of the Brillouin frequency $f_B^0=317.8(3)\mathrm{MHz}$. (A), (B), (C), (D) mark the different local negative swings of the Brillouin frequency. Inset: Brillouin gain spectrum obtained for $V=540\mathrm{mV}$ at $t=28.7\mu \mathrm{s}$; full circles, data; solid line, Gaussian fit [12].

Figure 3

Local time minima of the Brillouin frequencies $f_{B_{\min }}$ as a function of the AFG driving voltage $V$. Blue, yellow, red, gray symbols: minimum Brillouin frequency of swing (A), (B), (C), (D), respectively (see Fig. 2). The different symbols (crosses, squares, diamonds, circles, triangles) correspond to different runs. Vertical dashed lines: see text. Inset: bubbles creation probability $\mathrm{\Sigma }$ as a function of $V$ defining $V_c$: $\mathrm{\Sigma }(V_c)=1/2$.