Acoustic Microscopy at Temperatures less than 0.2°K

Abstract

The resolving power of the acoustic microscope is set primarily by the wavelength of the transmitted sound in the coupling fluid. The shortest wavelength that can be used is, in turn, determined by the acoustic loss in the fluid, which typically increases rapidly with frequency. To our knowledge, the shortest wavelength yet used for imaging in water is approximately 4000 A at 3.8 GHz.1 Since the attenuation at this frequency is extreme, approximately 14,000 dB/cm, the total path length through the fluid must be very short, typically less than 40 µm . To achieve shorter wavelengths and higher resolution, one can use a coupling fluid with lower acoustic attenuation and/or lower velocity than water. Among the fluids that potentially offer improved resolution performance are the cryogenic liquids2 and high pressure inert gases.3