Acoustic microscopy at 15.3 GHz in pressurized superfluid helium

An acoustic microscope operating at 15.3 GHz and using superfluid 4He as the coupling medium has been developed. The helium is colder than 0.9 K to achieve low acoustic attenuation, and is pressurized to greater than 20 bar to eliminate three-phonon processes which saturate the received signal as the input power is increased. A resolution of 150 Å has been achieved.