Heat coupling effect on photothermal detection with a moving Gaussian excitation beam.

Abstract

A strong photothermal response is beneficial to the measurement of optical and thermal properties of optical materials using the laser-induced thermal mirror method. A highly sensitive asymmetrical thermal mirror method was recently proposed by employing a moving Gaussian excitation beam [Appl. Phys. Lett.114, 131902 (2019)APPLAB0003-695110.1063/1.5080163]. However, the heat transfer across the interface between the thermodynamic system and the surroundings is ignored, which will lead to an error in the absolute measurement of the material properties. To address the problem, we present a theoretical and experimental study of heat transfer within the heated sample and out to the air coupling fluid in the photothermal detection with a Gaussian excitation beam moving at a constant velocity. We analyze the dynamic temperature fields inside the sample and in the surrounding air, and the phase shifts induced by the thermoelastic displacement of the sample (thermal mirror) and the refractive index gradient of air (thermal lens), as well as the diffracted intensity profiles of the probe beam in the detection plane. The experiments are implemented under normal pressure and vacuum, respectively, for a fused silica glass-air heat coupling system to verify the theoretical model. The experimental results show that the thermal lens, due to the heat coupling effect, introduces a signal deviation approximately 4.2% of the total photothermal signal, which is close to the theoretical result of 5%.