Detecting laser induced phase change at the surface of solids via latent heat of melting with a photothermal deflection technique

The detection of laser‐induced melt at the surface of a solid in real time is demonstrated using a photothermal deflection (PTD) technique. Experimental results for indium and tin show that a local maximum and minimum pair can occur in the temporal profile of the PTD signal when melt occurs. A local minimum does not occur without phase change. Analytical work is presented which explicitly shows the effect of the latent heat of melting,thermal properties, and probe‐beam size and offset on the shape of a PTD signal. Results are presented which demonstrate that the observed change in shape will not occur with planar heating, with or without phase transition. However, results derived for point‐source heating show that it is possible for the maximum/minimum pair to occur when melting with a focused laser beam. The ratio of the sensible heat to the latent heat, and the ratio of the thermal diffusivities of the target and deflecting medium are the key factors which govern the deflection response. Computations for a general class of materials are given to show when the effect of latent heat can be observed at a surface. Results are also given which consider the probe‐beam size and offset on the observations.