Optical Measurement of the Non-linear Focusing of Sound in Liquid Helium 4

We have measured the amplitude of 1MHz acoustic waves focused in liquid helium 4. Our resolution is 10 nanoseconds in time and 15 micrometers in space. The waves are focused onto a flat glass plate. We measure the reflection of light at the glass/helium interface, which depends on the refractive index of the liquid, consequently on its density which we could measure with an accuracy of ±10 -4 g/cm 3 . At large amplitude, strong non-linear effects are observed, in good agreement with a numerical calculation.