Frequency-resolved photothermal lens: An alternative approach for thermal diffusivity measurements in weak absorbing thin samples

Abstract

In this work, a pump-probe frequency-resolved photothermal lens method, for measuring thermal diffusivity, has been presented. The method is based on measurement of induced thermal lens signal amplitude as a function of time taken from different modulation frequencies of the pump beam. The performance of the technique was verified by determination of thermal diffusivity of four standard weak absorbing reference thin samples such as: ethanol, water, soda-lime-glass and polymethylmethacrylate and comparing the results to thermal diffusivities reported by other conventional photothermal methods.Good agreement among the obtained results and reported literature values confirmed that the method is a reliable and accurate technique applicable for determination of thermal diffusivity of weak absorbing thin materials. Furthermore, the use of lock-in capabilities enables to achieve amplified noise-free signal using few mW of excitation power leading to favourable precision of determination (2%). Moreover, in measurements of samples with very low thermo-optical enhancement factor, the method offers higher sensitivity than conventional photothermal technique. Additionally, lowest power threshold for measuring thermal parameters is crucial for thin films as well as for photodegradable samples, making it an important advantage of the method.