Generation and detection of high-order harmonics in liquids using a scanning acoustic microscope

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Abstract
The nonlinear generation of harmonics by high‐power focused acoustic beams in liquids has been investigated. A transmission mode acoustic microscope using focal planedetection has been used for high‐harmonic signal sensitivity. Detection of up to the 14th harmonic in ethanol has been achieved at an emission frequency of 15 MHz. Measurements show that signal in all harmonics tends to saturate at a very high emitted power. At lower power, harmonics varied as P n 1, where P 1 is the emitted power and n the harmonic number. Resolution improvement of the acoustic microscope by receiving at harmonic signals has also been demonstrated. The resolution has been shown to increase as (n)1/2 for reception at the nth harmonic, in good agreement with focused Gaussian beam theory. A nonlinear resolution improvement by a factor of 3 has been obtained.

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