Nondestructive Evaluation and Materials Characterization Using Photothermal-Optical-Beam-Deflection Imaging

Photothermal-optical-beam-deflection (PTOBD) imaging involves use of a focused, modulated laser beam to locally heat a sample and a second laser beam to probe the resulting changes in sample temperature. For opaque samples, the photothermal heating occurs essentially at the surface and temperature changes in the bulk occur via thermal diffusion to a depth below the surface of the order of a thermal-diffusion length, δ = (2K/PCω)½, where K is the thermal conductivity, p is the density, C is the specific heat, and ω is the modulation frequency. Since the limiting factor in definition/resolution is not the thermal diffusion length [1,2], features of the order of the heating-laser-beam diameter or smaller can be investigated, even at relatively low modulation frequencies. Lateral spatial resolution for subsurface features much closer to the surface than a diffusion length is independent of the diffusion length and is determined principally by the diameter of the heating beam, although the diameter of the probe beam can affect the resolution [1–4]. Generally, the limiting factors on spatial resolution in PTOBD imaging are the heating-beam diameter, the shape and depth of the subsurface feature being imaged, and the modulation frequency (to a lesser extent) [2,4]. This paper presents images of a number of important classes of materials with resolution less than 10 urn. Some contrast issues and underlying materials issues are discussed qualitatively