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This article deals with the photopyroelectric (PPE) technique, a useful tool for optical spectroscopy and for measurement of thermal properties of condensed mater samples, in particular in the liquid state. It has been demonstrated here that, for constant laser power, the pyroelectric (PE) sensor response does not depend on the spatial distribution of the laser beam intensity. As a consequence of this, it follows that in the case of measurements in voltage mode, the signal amplitude is inversely proportional to the active area of the sensor. Then, if the active area is diminished, leaving the total area of the sensor constant, it is possible to increase the pyroelectric signal. On the basis of this result, a method is proposed to enhance the PPE signal measured in voltage mode by means of the optimization in the configuration of the sensor metal electrodes. It was experimentally demonstrated that using this improved technique, it is possible to increase up to ten times the amplitude of the signal without electrical noise rise. [ABSTRACT FROM AUTHOR]

Photothermal beam deflection is a well-established technique for measuring thermal diffusivity. In this technique, a pump laser beam generates temperature variations on the surface of the sample to be studied. These variations transfer heat to the surrounding medium, which may be air or any other fluid. The medium in turn experiences a change in the refractive index, which will be proportional to the temperature field on the sample surface when the distance to this surface is small. A probe laser beam will suffer a deflection due to the refractive index periodical changes, which is usually monitored by means of a quadrant photodetector or a similar device aided by lock-in amplification. A linear relationship that arises in this technique is that given by the phase lag of the thermal wave as a function of the distance to a punctual heat source when unidimensional heat diffusion can be guaranteed. This relationship is useful in the calculation of the sample’s thermal diffusivity, which can be obtained straightforwardly by the so-called slope method, if the pump beam modulation frequency is well-known. The measurement procedure requires the experimenter to displace the probe beam at a given distance from the heat source, measure the phase lag at that offset, and repeat this for as many points as desired. This process can be quite lengthy in dependence of the number points. In this paper, we propose a detection scheme, which overcomes this limitation and simplifies the experimental setup using a digital camera that substitutes all detection hardware utilizing motion detection techniques and software digital signal lock-in post-processing. In this work, the method is demonstrated using thin metallic filaments as samples. [ABSTRACT FROM AUTHOR]

This paper introduces a novel application of laser spot step heating thermography, namely the visualization of a convective phenomenon. A fluid confined in a measurement cell is in intimate contact with the inner face of a thin vertical metallic sheet, on whose external surface a laser beam is tightly focused generating heat that diffuses to the closest part of the fluid layer adjacent to the sample decreasing its density locally. This induces a density gradient that generates internal motion of the fluid affecting thermograms recorded on the external side of the sheet, which are visualized after a given time has elapsed since the laser is switched on. The results of these measurements agree with those obtained using the finite-element analysis simulation software COMSOL Multiphysics®. A simple model for free convection explains why the effect is only observed for some fluids. [Display omitted] • Laser spot step heating thermography is used to visualize convective flow. • Fluid is in contact with a metallic sheet with a superficial punctual heat source. • The induced motion of the fluid affects thermograms recorded on the sheet's surface. • Thermograms are recorded after a time has elapsed since the laser is switched on. • Asymmetries in vertical temperature profiles show the effects of convection. [ABSTRACT FROM AUTHOR]

In this work, a low detection limit for hexavalent chromium in water of parts per trillions (21 ng/L) was achieved using a micro-spatial thermal lens spectroscopy setup with coaxial counter-propagating pump and probe laser beams and an integrated passive optical Fabry-Perot resonator, aided with a well-established diphenyl carbazide colorimetric method. Cr-VI concentrations in the range of μg/L, i.e. well-below the toxicity thresholds in humans and animals (26 and 190 mg/L respectively) and below those delimited by international regulations for drink water (~0.05–0.5 mg/L), have been obtained by measurements in bottled and tap water samples. The developed thermal lens microscope is also capable to detect Cr-VI directly in potassium dichromate solutions using pump beam wavelengths within the very low optical absorption region in the visible part of the spectrum, i.e., without the use of any colorimetric method. [ABSTRACT FROM AUTHOR]