Acoustic microscopy and nonlinear effects in pressurized superfluid helium

The operation of an acoustic microscope having a resolution of 15 nm has been demonstrated. It uses as a coupling medium superfluid ⁴ He colder than 0.9 K and pressurized to greater than 20 bar. The microscope is now being used to image objects that show little or no contrast on a scanning electron microscope. In addition, it is being used to study the properties of sound propagation in the coupling fluid. At low acoustic intensities, the coupling fluid has very low acoustic attention at the microscope´s operating frequency (15.3 GHz), but near the focal point the acoustic intensity can be high enough that the helium behaves with extreme nonlinearity. In fact, this medium is capable of entering new regimes of nonlinear interaction. Plots of the received signal versus input power display a nearly complete pump depletion at certain input power levels and a reconversion to the pump frequency at higher power levels