Photothermal measurement of bulk and surface absorption of transparent infrared optical elements

Abstract

Photothermal probe beam deflection spectroscopy is used to separately measure bulk and surface absorption of transparent IR optical elements. Different approaches of this technique are considered. One approach consists in variation of the laser beam heat modulation frequency in the condition that the heating beam component is transverse to the path of the probe beam. Another approach consists in varying the heat and probe cross section measuring point position within the tested optical sample. It is theoretically and experimentally shown that in the second case, the spatial resolution and measurement locality are determined by the size of laser beam spots and the thermal diffusion length in a sample. The relative contribution of surface and bulk absorption to summary absorption can be derived from the ratio of the probe beam deflection signal, measured both near the surface and within the sample volume far from the surface, to thermal wavelength. Absolute calibration of absorption measurements can be carried out using laser calorimetry. A comparison between deflection and calorimetry data on volume absorption in a number of ZnSe laser optical elements is made and shows a good correlation.