Since the aberrations of the lens are negligible, the resolution of the acoustic microscope is essentially limited by diffraction. If the lens has a high numerical aperture the resolution is comparable with the acoustic wavelength. The smallest wavelength which can be used is determined by attenuation in the coupling fluid, which generally varies as the square of the frequency. Of all the candidate fluids, one of the best is warm water, which has low attenuation and offers good coupling to the elastic properties of most samples. Superfluid helium offers lower attenuation, and a grating of period 200 nm has been imaged at 15.3 GHz, but for most practical purposes water is used.
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