Thermal-lens spectrometry is a sensitive technique for determination of physicochemical properties and thermophysical parameters of various materials including heterogeneous systems and nanoparticles. In this paper, we consider the issues of the correctness (trueness) of measurements of the characteristic time of the thermal-lens effect and, thus, of the thermal diffusivity determined by dual-beam mode-mismatching thermal lensing. As sources of systematic errors, major factors—radiation sources, sample-cell and detector parameters, and general measurement parameters—are considered using several configurations of the thermal-lens setups, and their contributions are quantified or estimated. Furthermore, with aqueous ferroin and Sudan I in ethanol as inert colorants, the effects of the intermolecular distance of the absorbing substance on the correctness of finding the thermophysical parameters are considered. The recommendations for checking the operation of the thermal-lens setup to ensure the maximum accuracy are given. The results obtained help reducing the impact of each investigated factor on the value of systematic error and correctly measure the thermophysical parameters using thermal-lens spectrometry.

The main issues and areas of application of photothermal and optoacoustic spectroscopy are reviewed. Progress in innovative techniques in the most actively developing areas is presented, including microspectroscopy, multispectral techniques, the measurements of single particles and objects with a resolution better than the diffraction limit (nanoscopy) by both optical and probe-based methods. Possible applications of photothermal and optoacoustic spectroscopy for determining the properties of materials, studying photochemistry and fluorescence, chemical reactions, and analytical and applied chemistry, and solving biomedical problems is discussed. Some prospects for the development of these methods are presented.

The growing interest in heat-conducting nanofluids requires highly sensitive methods for analyzing the thermal properties. Thermal lens spectrometry (TLS), despite its advantages over classical methods, does not have a general approach for measuring and interpreting results for dispersed systems. In this paper, for nanofluids of silicon oxide in water in a wide range of concentrations and sizes, the selection of measurement parameters for transient and steady-state thermal lensing is justified, and the interpretation of the results of thermal diffusivity measurements is substantiated. The features of the measurements of thermal diffusivity by TLS under stationary states for dispersed systems are considered. Using this approach, it is possible to detect and distinguish thermal effects with high accuracy. For dispersions of silicon oxide, with increasing concentrations, the thermal diffusivity passes through a minimum threshold. Silicon oxide dispersions can be used both as coolants or as heat-removing liquids by selecting the particle size and concentration.

A simultaneous steady-state and transient photothermal-lens modality was used for both the thermal and optical parameters of aqueous dispersed systems (carbon and silica nanoparticles, metal iodides, surfactants, heme proteins, albumin, and their complexes). Heat-transfer parameters (thermal diffusivity and thermal effusivity), the temperature gradient of the refractive index, light absorption, and concentration parameters were assessed. To simultaneously measure thermal and optical parameters, the time scale of thermal lensing (characteristic time, tc) should correspond to an excitation beam size of 60–300 µm, and the relative time intervals 0.5÷5tc and (5÷20)tc should be selected for transient and steady-state measurements, respectively. Dual-beam thermal-lens spectrometers in a mode-mismatched optical schematic at various excitation wavelengths were built. The spectrometers implement back-synchronized detection, providing different measurement conditions for the heating and cooling parts of the thermal-lens cycle. By varying the measurement parameters depending on the dispersed system, the conditions providing the suitable precision (replicability, repeatability, and reproducibility) of thermal-lens measurements were found; setups with a broad excitation beam (waist size, 150 and 300 μm) provide longer times to attain a thermal equilibrium and, thus, the better precision of measurements of thermal diffusivity.

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