Photothermal measurements near a 90° edge. I. Mirage deflection by a free edge

Abstract

The presence of a 90° edge (corner) modifies substantially the photothermal signal generated by a focused and modulated laser beam incident on an opaque sample. A 3-D model has been developed in order to calculate the ac thermal fields in the sample and adjacent gas, as well as the mirage deviations in the gas. This model incorporates the effects introduced by the finite size of the Gaussian pump beam, and the height and size of the Gaussian probe beam. The experimental measurements of the mirage normal (φn) and transverse (φt) deflections confirm the theoretical predictions. The spatial symmetric forms, of both functions, relative to the exciting point, are broken by the thermal wave scattering produced by the edge. In particular, the inner (close to the edge) parts of φn and φt show an enhancement in their magnitude and phase. For imaging inspection (scanning the sample with probe and pump beams centered), there is an elevation in the magnitude of φn but a drop in its phase at distances smaller than the sample’s thermal diffusion length. However, the most prominent change takes place in φt (both in magnitude and phase), which gives the signature of the presence of the edge.