Cryogenic Acoustic Microscopy.

This dissertation describes the design and applications of a scanning acoustic microscope which uses cryogenic liquids as acoustic coupling media. Cryogenic liquids are particularly well suited for use in acoustic microscopy because their low velocity of sound and low attenuation allow propagation of short wavelength sound waves. The liquids that were used include liquid nitrogen at 77 K, liquid argon at 85 K and superfluid helium at 1.95 K. The cryogenic acoustic microscope currently offers the best resolution in acoustic imaging. The achieved resolution is as good as or better than that of high quality optical instruments equipped with oil-immersion objective lenses. The mechanical and electronic design of the microscope is discussed. Particular attention is given to the design of a two-dimensional mechanical scanning device capable of 1000 (ANGSTROM) scan accuracy at cryogenic temperatures. For the microscope to operate in superfluid helium with satisfactory signal-to-noise ratio, acoustic impedance matching layers were developed to couple sound efficiently from the high impedance lens material (sapphire) into the low impedance liquid. Images of test objects taken at 840 MHz in helium are presented. The corresponding wavelength in the liquid is 0.27 (mu)m. A grating with spatial period of 0.25 (mu)m is easily resolved. Liquid nitrogen and liquid argon were successfully used as acoustic coupling media at frequencies as high as 2.8 GHz. The corresponding acoustic wavelength is 0.30 (mu)m. Objects studied with the microscope include resolution test objects, field effect transistors, human metaphase chromosomes and chick heart fibroblasts. The imaging properties of high intensity focused acoustic beams are investigated. A nonlinear effect which improves resolution beyond the linear diffraction limit was discovered. Using this effect in liquid nitrogen, a 0.21 (mu)m period grating was successfully resolved with 0.43 (mu)m radiation. The theory of propagation of finite amplitude focused acoustic waves in a nonlinear medium is discussed. In particular it is shown that after passage through the focus, a portion of the power in the second and higher harmonics is transferred back into the fundamental beam.