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The model of a photoacoustic gas-microphone signal in the time domain recorded in a transmission configuration using a minimum volume cell is derived. This model takes into account the inertial thermal relaxations of both the sample and the gas filling the cell by means of the generalized hyperbolic theory of heat conduction. With the introduction of electro-thermal analogy for the thin sample and short microphone length cavity, characteristic quantities are defined, which can be used in solving the inverse problem in time-domain photoacoustic in both cases, when thermal relaxations are neglected as well as when they are considered. The derived model that includes thermal relaxation explains the experimentally observed occurrence of overshoots and undershoots as well as an oscillatory approach to the steady values of the recorded signal in high-resolution time-domain photoacoustic measurements of thin semiconductor membranes, which will be presented in detail in Paper II of the paper. [ABSTRACT FROM AUTHOR]

In order to make the photothermal effect detectable from a sample with low optical absorption, an optically opaque layer must be applied on its surface. A model of optically induced temperature variations for such a structure is derived, with a very thin coating applied on the non-illuminated (back) side of the sample and neglecting heat flux dissipation in the coating. Validation of the model is performed by comparing the obtained results with the results for the corresponding two-layer structure model. The influence of optical absorption on surface temperature variations of both optically transparent and semi-transparent film, whether the film is a good or poor thermal conductor, is analyzed. It is shown that the effects of optical absorption in the coated structure can be observed through variations in the measured temperature, despite the low absorbance of the sample. [ABSTRACT FROM AUTHOR]

In this paper, a self-consistent inverse procedure is developed for the estimation of linear thermal expansion coefficient and thermal diffusivity of solids from transmission photoacoustic measurement. This procedure consists of two single-parameter fitting processes applied alternately: phase data are fitted by shifting thermal diffusion coefficient, while amplitude data are fitted by shifting thermal expansion coefficient. Each fitting process uses the resulting parameter of the other, previously finished one, thus converging to the best solution-pair achievable. In numerical experiments, the convergence proves to be very fast. The achieved parameter estimation error is as low as 1%, and it can be lowered further more by increasing the fitting resolution in parameter space. The application of the procedure on experimentally obtained aluminum photoacoustic response (measurements) on three thickness levels returns the estimates of its thermal diffusion and thermal expansion coefficients within expected literature boundaries. [ABSTRACT FROM AUTHOR]

In this paper, a theoretical model of temperature variations is derived for a two-layer optically absorbing structure, including thermal memory effects. It is considered that the two-layer structure is surrounded by gas and illuminated on the front side by a harmonically modulated laser beam. This model is based on the hyperbolic theory of heat conduction and Beer–Lambert's law of absorption neglecting multiple optical reflections in each layer. The derived model represents the generalization of the current models in two aspects. First, the influence of thermal memory properties of both layers is accounted for, and second, both layers are regarded as volume absorbers of the incident light. Based on the derived model, the expressions for surface temperature variations are given and discussed for the special type of two-layer structures, irradiated on the coated side, which is a prominent configuration in photoacoustic and photothermal experiments. It is shown that there exists a frequency range in which the influence of the thermal properties of the coating cannot be neglected, especially at the high-frequency range in which thermal memory of coating becomes significant. It is an important result in terms of understanding experimentally measured photothermal and photoacoustic response and, consequently, accurate characterizations of various high optically reflected and/or optically transparent samples by using these experimental techniques. [ABSTRACT FROM AUTHOR]

This paper introduces the possibility of the determination of optical absorption and reflexivity coefficient of silicon samples using neural networks and reverse-back procedure based on the photoacoustics response in the frequency domain. Differences between neural network predictions and parameters obtained with standard photoacoustic signal correction procedures are used to adjust our experimental set-up due to the instability of the optical excitation source and the state (contamination) of the illuminated surface. It has been shown that the changes of the optical absorption values correspond to the light source wavelength fluctuations, while changes in the reflexivity coefficient, obtained in this way, correspond to the small effect of the ultrathin layer formation of SiO2 due to the natural process of surface oxidation. [ABSTRACT FROM AUTHOR]

In this article, a method for determination of photoacoustic detector transfer function as an accurate representation of microphone frequency response is presented. The method is based on supervised machine learning techniques, classification and regression, performed by two artificial neural networks. The transfer function is obtained by determining the microphone type and characteristic parameters closely related to its filtering properties. This knowledge is crucial within the signal correction procedure. The method is carefully designed in order to maintain requirements of photoacoustic experiment accuracy, reliability and real-time performance. The networks training is performed using large base of theoretical signals simulating frequency response of three types of commercial electret microphones frequently used in photoacoustic measurements extended with possible flat response of the so-called ideal microphone. The method test is performed with simulated and experimental signals assuming the usage of open-cell photoacoustic set-up. Experimental testing leads to the microphone transfer function determination used to correct the experimental signals, targeting the "true" undistorted photoacoustic response which can be further used in material characterization process. [ABSTRACT FROM AUTHOR]