Acoustic microscope using pressurized superfluid 4He

Abstract

An acoustic microscope has been developed which makes use of pressurized superfluid helium as an acoustic coupling medium. Acoustic micrographs were taken using pure superfluid 4He under saturated vapor pressure at 0.1 K and under 25 atm at 0.1 and 1.9 K, for the frequency of 380 MHz. By pressurizing liquid 4He, the acoustic attenuation decreases, the efficiency of the antireflection coating increases, and the power level of saturation caused by nonlinearity of the superfluid 4He is raised. A good signal-to-noise ratio of imaging is thus obtained. The entire imaging instrument set is within a closed cell. The focusing mechanism is composed of two voice coils of superconducting wire. Using a servocontrol mechanism with a sensor and drive coil, Z-direction stability was better than 0.05 μm. The manipulation sensitivity is 0.5 μm/mV, and the whole stroke is about 2 mm.