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We report stimulated Brillouin gain spectra of bulk superfluid helium-4. Stimulated Brillouin gain spectroscopy is a powerful optical method to probe the acoustic properties of liquid helium in a narrow spatio-temporal domain with moderate optical power and fast acquisition rates. We determine the velocity of first sound of superfluid helium-4 from the stimulated Brillouin gain spectra between 0.90 and 2.17 K and compare it to well-known available data, which validate our experimental method.

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 spatiotemporal 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.

Using a stimulated Brillouin gain spectroscopy technique, we have measured the equation of state of bulk liquid helium-4 at $\ensuremath{\sim}1$ K between 0 and 10 bars. Our results are in very good agreement with previous measurements and with theoretical equations of state. However, our specific method allows one to determine it on space/timescales as narrow as 35 $\ensuremath{\mu}\mathrm{m}$/190 ns. This technique is of particular interest for solving a remaining debate about the value of the homogeneous cavitation pressure of liquid $^{4}\mathrm{He}$ at $T\ensuremath{\sim}1$ K.

Experiments and theories describing the metastable phases of liquid and solid $$^4$$ He are presented and discussed. For the case of metastable liquid $$^4$$ He with respect to its gaseous phase, it is shown that different measurements of its destabilization threshold (cavitation threshold) and their comparisons to available theories reveal that the nucleation mechanism is not totally understood. Then experiments measuring typical lifetime of cavitation bubbles in He I and He II are shortly considered showing the important role of heat transport mechanism during their collapse. Finally for liquid $$^4$$ He, its metastability with respect to its solid phase and the possibility of the liquid destabilization due to the vanishing of the roton gap is presented. The last part of the review is devoted to metastable solid $$^4$$ He with respect to its liquid phase. The first experimental production of such a state is described and its destabilization limit possibly invoking the creation and proliferation of crystalline defects is discussed.