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Here, we present a novel application of the shadowgraph technique for obtaining the thermal diffusivity of an opaque solid sample, inspired by the orthogonal skimming photothermal beam deflection technique. This new variant utilizes the shadow projected by the sample when put against a collimated light source. The sample is then heated periodically by another light beam, giving rise to thermal waves, which propagate across it and through its surroundings. Changes in the refractive index of the surrounding media due to the heating distort the shadow. This phenomenon is recorded and lock-in amplified in order to determine the sample's thermal diffusivity. [ABSTRACT FROM AUTHOR]

• Facile doping method of VO 2 films with TiO 2 nanoparticles. • Ultraviolet–visible spectral operating range of VO 2 sensitive to TiO 2 doping. • Gaussian and Lorentzian oscillators adequately describe the experimental data. • Doping level changes size and morphology of agglomerates in thin films. A simple chemical method for the tunability of the optical properties of TiO 2 nanoparticles-doped VO 2 thin films is reported. Although deposited thin films revealed granular morphology, an increase in dopant concentration showed increased homogeneity (smaller crystallite size). The insulator-to-metal transition, monitored by UV–vis-NIR reflectivity, is adjusted through a simple model based on a sum of Lorentzians and Gaussians to follow the changes in the optical spectra, which in turn allows us to extract the corresponding refractive index and extinction coefficient. In addition, the reflection spectrum in low frequency range (< 700 nm), reveals new variations in the doped samples triggered by the changes in temperature, which allows an extended optical operating range, for instance, in smart windows applications. [ABSTRACT FROM AUTHOR]

We report on the use of an optical fiber sensor for the determination of concentration of contaminants in solutions below the micromolar level based on the photothermal lens effect. A pump laser beam has been added to a conventional configuration widely used as refractometer, so that the device is improved to detect tiny changes of the refractive index of a sample related to the amount of heat generated following optical absorption. In this way, concentrations as low as 0.1 μM of methylene blue in distilled water were measured. A simple experiment is also performed to demonstrate the photothermal effect. [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]

In this paper, we report about the use of an optical fiber sensor for measurements of the refractive index ( η ) of liquids as a function of wave-length ( λ ). The method uses only one optical fiber. Different laser diodes are used to carry out measurements at several wavelengths. The principle of measurement is based on the detection of the intensity of reflected light. Periodical intensity modulated light of a given λ is transmitted using an optical liquid fiber through the sample contained in a quartz glass cuvette, being reflected by a mirror located at the rear side of the cuvette back to the same fiber, through which it arrives to a photodetector after passing through a beam splitter that blocks the original beam. The signal amplitude is measured using a Lock-in amplifier synchronized at the modulation frequency as a function of the distance between the fiber tip and the cuvette for different samples of well-known η . A linear calibration curve is obtained by plotting the refractive index versus the amplitude maximum value, which can be used to measure the value of η for an unknown sample, as demonstrated by measurement of the refractive index of sodium chlorine and glucose in water solutions (in the range between 0 and 25 weight%) for different wave-lengths. Good agreement has been obtained with values reported and/or measured with an Abbe refractometer values. A dispersion relation, η ( λ ), is obtained for these samples. The described refractometer has a sensitivity of 0.56 V/RIU, its linearity is of more than 99% in the investigated range between 1 and 1.5 RIU. The limit of detection was estimated as 1.4×10 −3 RIU (RIU = Refractive Index Unit) at the used wave-lengths. [ABSTRACT FROM AUTHOR]