Photothermal Lens and Photothermal Mirror Techniques: Effects and Applications for Material Characterization

Abstract

The mode-mismatched dual-beam photothermal lens (PTL) and photothermal mirror (PTM) have been widely applied in the characterization of solid, liquid, and gas samples due to their remote, sensitive, and non-destructive characteristics. The PTL and PTM techniques detect a broad range of phenomena arising from the interaction of tightly focused laser beams and matter at different time scales. A pulsed or continuous laser beam may induce a thermal perturbation and momentum transfer from the light to the sample in these techniques. The effect is probed by monitoring the probe beam phase shift caused by the surface expansion of the heated area, the photoelastic effects, and the spatial distribution of the refractive index within the sample and in the fluid surrounding it. The transient signal is monitored at the far-field detector by analyzing the wavefront distortion of the probe beam. Thermal, optical, and mechanical properties can be quantitatively determined for solids, liquids, and gas. The applications involve material characterization of optical glasses, polymers, metals, alloys, semiconductors, fuels, and dyes. Here we show applications of PTL and PTM methods under continuous or pulsed Gaussian laser excitations and the advances in their theoretical description over the past few years. The advances comprise investigating the effects of radiation forces in liquids [1,2], generation and detection of pressure transients in liquids [3,4] and thermoelastic waves in metals [5], analytical description of sample–fluid heat coupling effect in photothermal techniques [6,7], and recent PTL results using mid-infrared (mid-IR) excitation for characterization of liquid samples.