Scanned Photothermal Imaging of Subsurface Structure
Photothermal imaging using laser beam deflection (PTLBD imaging) has recently shown considerable potential for nondestructive detection of subsurface structure in solids.1 In this technique localized heating of a sample by a modulated laser beam produces a periodic temperature gradient in the fluid near the sample surface. The thermally induced gradient in the index of refraction of the fluid causes a periodic deflection of a laserprobe beam passing through the heated surface region. As the sample is translated relative to the beam intersection point, the magnitude and phase of the deflection signal give information about local variations in sample optical and thermal properties. Preliminary indication of the detection of subsurface structure with this technique has been given.2,3 The PTLBD technique has proved successful in studying surface structure, 2-4 and has also been used in spectroscopic studies.5 The potential advantages of PTLBD imaging over related photothermal approaches have already been identifieid. When compared to photoacoustic cell detection, this technique has much greater potential sensitivity,2 is not restricted to small sample size, and allows both excitation and detection to be spatially localized.7
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