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Acoustical Microscopy can be performed using laser ultrasonics by feeding both the generation and detection optical beams through the microscope objective. The figure below shows a microelectronic circuit where a pulsed or chopped laser beam (red) thermoelastically heats a local region (focused to around 10 micron diameter). A second laser beam (green) detects the local surface ultrasonic motion by interferometric detection (either with passive or active interferometers). Surface waves (Rayleigh) and bulk waves (longitudinal) can be observed traveling through the microelectronic circuit allowing for various nondestructive evaluation measurements of film thickness, substrate bonding, substrate flaws, etc.

Photo of the acoustical microscope

Sensitivities of 6x10-4 nm @ 880 MHz have been demonstrated with over 4 orders of magnitude dynamic range using 532 nm optical wavelength. The calibration curve shown was obtained using a piezoelectric generator and the optical detection using lock-in phase sensitive methods. The source laser can be focussed to about 1-2 microns diameter providing for generation and detection at frequencies of 1 GHz. At GHz frequencies, the acoustical wavelengths are on the order of a few microns. This microscope capability is being developed for investigation of ultrasonic wave propagation in material microstructures at the individual grain level.

Reference:

Ken L. Telschow, Vance A. Deason, David L. Cottle and John D. Larson III, “UHF Acoustic Microscopic Imaging of Resonator Motion,” The IEEE 2000 Ultrasonics Symposium in Puerto Rico, October 22-25, 2000 — 66kB PDF