Photoacoustics on a microscopic scale

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Abstract
We have modified a transmission acoustic microscope by replacing the input lens with an optical counterpart. The input to this system comes from a mode‐locked and Q‐switched Nd : YAG laser at 1.06 μm. Acoustic signals arising from thermoelastic coupling are generated in metallic films at frequencies determined by the modulation envelope of the optical beam. The output acoustic lens and transducer responsive to sound waves at 840 MHz detect the acoustic energy which comes from the heated volume (∼2‐μm diameter) near the focal region. The sample is mechanically scanned through the focal point in a raster pattern to record the photoacoustic images. We suggest this as a method for collecting new information on microscopic structures.