Resolution beyond the diffraction limit in the acoustic microscope: A nonlinear effect

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Abstract
By operating the reflection mode scanning acoustic microscope at nonlinear power levels, resolution beyond the linear diffraction limit can be achieved. To demonstrate this effect, imaging experiments were performed in liquid nitrogen, liquid argon, and water at frequencies between 2000 and 2800 MHz. The spatial frequency response of the nonlinear imaging system was found to have a cutoff frequency which is at least 1.4 times higher than the cutoff frequency of the linear system. It is proposed that the resolution improvement is the result of interaction between the transmitted fundamental beam and harmonics. The harmonics are generated in the converging portion of the beam and have focal spots which are significantly smaller than the focal spot of the fundamental beam. The harmonic power reflected by the object influences the detected fundamental power due to nonlinear (parametric) interaction and results in the improved imaging resolution. A simple physical model and a gaussian beam analysis are presented to elucidate energy exchange between the focused fundamental beam and the higher harmonics.

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