Subsurface structures of solids by scanning photoacoustic microscopy

Definitive experimental evidence is given for the nondestructive detection of subsurface structures of solids by the technique of scanning photoacoustic microscopy (SPAM). In the case of the ceramic materials studied, the thickness of the surface layer probed is dictated by the thermal diffusion length δth which scales as f−1/2. For a chopping frequency f∼100 Hz, δth is on the order of 100 μm. This demonstrates the advantages of SPAM over conventional microscopy in investigating structures beneath the surface.