ABSTRACT
I. INTRODUCTION Over the past decade, photoacoustic and photothermal phenomena have generated a class of scanning probe techniques that have been used in an extremely wide variety of applications [1-5]. If a beam of intensity-modulated optical radiation is focused onto the surface of a solid sample, nonradiative decay processes following light absorption will generate in the sample a spatially damped oscillating temperature field known as a thermal wave [5]. Because the damping distance of thermal waves is very short range in most materials and because this distance is readily controlled by varying the modulation frequency of the optical beam, subsurface or buried features in a thin sample may be visualized through their interaction with thermal waves. Photoacoustic and photothermal techniques that use thermal waves to probe materials have thus been employed in a wide range of problems involving the recovery of depth-resolved information from thin samples [6-8]. The scale of depth profiling available from photothermal methods ranges from a few hundred nanometers to a few millimeters [6], depending on the thermal properties of the sample. This depth range is not accessible to other spectroscopic techniques on a nondestructive basis. Applications have been reported in biology and biophysics, nondestructive evaluation, semiconductor and thin film imaging, polymer analysis, and materials science and include the imaging of ion implantations in semiconductors [10], the inspection of welds and metallurgical bonds [11], the evaluation and detection of subsurface defects in paints [12,13] and coatings [14-17], and the evaluation of thermal anisotropies in polymer films and injection-molded plastics [18]. A
number of major reviews and monographs detailing developments in photothermal technology have been reported over the last 5 years, and new applications continue to be reported [l-4,6,7].
