Advances in superfluid helium acoustic microscopy

An acoustic microscope utilizing low-temperature (0.1 K) superfluid helium as the coupling medium has been operated at 8 GHz. At this frequency the sound wavelength in helium is 300 Å. Pulse compression techniques have been used to improve the signal-to-noise ratio of the system. A theoretical and experimental analysis of acoustic transducers operating at 8 GHz is presented. The transducer performance has been optimized for operation in liquid helium with the pulse compression system. The images formed with the microscope demonstrate a lateral resolution of 200 Å with a signal-to-noise ratio of 15 dB. A comparison of acoustic and scanning electron microscope or transmission electron microscope images is given.