Brillouin Spectroscopy of Metastable Superfluid Helium-4

Abstract

We report measurements of the Brillouin frequencies of acoustically driven metastable states of superfluid 4 He at T ∼ 1 K. This is done using a stimulated Brillouin gain spectrometer operating on a confined spatio-temporal domain (∼ 40 µm, 200 ns). Our work questions previous experimental results regarding the metastable states of liquid 4 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 He beyond the commonly defined cavitation threshold.

Measurement of the Equation of State of Superfluid Helium-4 at Negative Pressure

“…We found that available theoretical 0 K EOS of liquid 4 He at negative pressure correctly reproduce experimental data for the pressure range 0 > P > −1 bar. This set up will be used to measure the EOS of deep metastable states of liquid 4 He at ∼ 1 K in the close vicinity of the destabilization (cavitation) threshold where deviations between the actual EOS and the theoretical one are suspected [19].…”

“…To minimize the interaction volume, they are crossed at an angle θ ∼ 90 • and then superimposed on the acoustic focus, see Fig. 2a) [19]. Brillouin frequencies are measured on a space scale of (2w) 3 = 40 3 µm 3 .…”

“…The measurement of the Brillouin frequencies in the metastable states of superfluid 4 He is though more challenging. We have newly succeeded in doing so by synchronizing the Brillouin frequency measurement with the acoustic wave excitation [19].…”