EXPRESS: High-Accuracy Thermal Diffusivity Measurements by Steady-State and Time Resolved Thermal Lensing: Methodology and Application to Nanoparticle Dispersions

Thermal lens spectrometry offers an advantage over other optical spectroscopy methods due to its ability to record multiple signals and investigate the optical and thermal properties of samples. Highly accurate characterization of samples is only possible for highly diluted and low-absorbance (A < 0.001) homogeneous solutions. Accurate photothermal measurements for multicomponent homogeneous and heterogeneous systems are complicated by multifactorial metrological control. The dependence of the characteristic thermal lens development time on the irradiation time is highly sensitive to side effects, such as thermal (thermophoresis) and optical (dynamic changes in light absorption). This paper examines the behavior of the characteristic time of thermal field development for homogeneous dye solutions (Sudan 1 in ethanol and ferroin in water), nanoparticle dispersions (silicon oxide and graphene, mixed iron oxide, and polystyrene), and photosensitive systems (boron subporphyrazine and substituted lutetium phthalocyanine) depending on the nature of the sample and the most significant instrumental factors. It is shown that the choice of thermal lens pumping and dissipation times and the achievement of a steady state for time-resolved measurements play a key role in the accuracy of thermal-lens thermal diffusivity measurements. The potential for using the characteristic time function to verify the accuracy of photothermal measurements of various types of samples is demonstrated. It is established that the first 5-10 ms of the characteristic time function development occur due to the thermal properties of the solvent. The change in the characteristic development time of the thermal lens of the first 5-10 ms is the result of the influence of high optical absorption or non-optimal conditions of the photothermal experiment, leading to a systematic error of more than 5%. These methodological advances are particularly valuable for accurate characterization of nanoparticle thermal transport and are essential for developing advanced nanofluidic and photothermal systems where the accuracy of thermal properties directly impacts material performance.