RESUMO
We report measurements of the Brillouin frequencies of acoustically driven metastable states of superfluid ^{4}He at Tâ¼1 K. This is done using a stimulated Brillouin gain spectrometer operating on a confined spatiotemporal domain (â¼40 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.
RESUMO
Light carries momentum which induces on atoms a recoil for each photon absorbed. In vacuum, for a monochromatic beam of frequency ν, the global momentum per photon is bounded by general principles and is smaller than hν/c leading to a limit on the recoil. However, locally this limit can be broken. In this Letter, we give a general formula to calculate the recoil in vacuum. We show that in a laser beam with a distorted optical field, there are regions where the recoil can be higher than this limit. Using atoms placed in those regions we are able to measure directly the extra recoil.
