P. R. B. Pedreira

Time-resolved thermal mirror for nanoscale surface displacement detection in low absorbing solids

A time-resolved thermal mirror method for measurements of absolute thermo-optical-mechanical properties of low absorbing solids is presented. The thermoelastic equation for the surface displacement and an analytical expression for the probe beam intensity at the detector plane were derived. Experimental proofs were performed in an optical glass and the fitted parameters are in good agreement with previous literature data for thermal, optical, and mechanical properties, suggesting that the method is a useful tool for the characterization of a wide range of transparent materials.

Time-resolved thermal mirror method: A theoretical study

A general and complete theoretical model of the time-resolved thermal mirror method for the measurement of thermo-optical-mechanical properties of solid materials is developed. The laser-induced temperature profile in a sample and its thermoelastic surface displacement are derived. The center intensity of a probe beam at the detector plane is calculated using the Fresnel diffraction theory. Additionally, simplified models for high and low optical absorption samples are presented, and the suitability of the simplified models is also analyzed. The influence of experimental parameters on the sensitivity of the thermal mirror method is discussed for the optimization of the experimental apparatus. The presented model and the experimental technique can be used to quantitatively determine the physical properties of transparent and opaque solids.

Nanoscale surface displacement detection in high absorbing solids by time-resolved thermal mirror

Nanoscale surface displacement is used to determine thermo-optical–mechanical properties of high absorbing solids by means of the time-resolved thermal mirror method. The thermoelastic equation for the surface displacement and an expression for the probe beam intensity at the detector plane were derived. Experiments were performed in a high absorbing TiO2-doped low silica calcium aluminosilicate glass, and obtained the valuable values of the fluorescence quantum efficiency and thermal properties. The results indicate that this method is reliable for the characterization of semitransparent, high absorbing, and opaque materials.

Arrhenius behavior of hydrocarbon fuel photochemical reaction rates by thermal lens spectroscopy

The temperature dependence of thermo-optical and photochemical reaction properties of hydrocarbon fuels is investigated using thermal lens spectroscopy. We consider the time dependence of the absorption coefficient due to the photoinduced chemical reaction (PCR) and species diffusion to evaluate nonequilibrium characteristics of the samples. The measured temperature dependences of the reaction rates are found to follow the Arrhenius correlation. Experimental results for thermophysical properties of the samples and an analysis of the connection between PCR properties and the chemistry of the samples are also presented.

Real-time quantitative investigation of photochemical reaction using thermal lens measurements: Theory and experiment

In this work the time-resolved mode-mismatched thermal lens method is applied to investigate Cr(VI) species in water. An abnormal behavior of the thermal lens transient induced by a photochemical reaction was observed during optical excitation. With the purpose of better understanding this phenomenon, the existing theoretical model of thermal lens effect was generalized in order to take the time dependence of the absorbance of the sample into account due to the changes in concentration resulting from photochemical reaction and diffusion of absorbing species. Consequently, the photochemical reaction rate can be quantitatively evaluated by this technique with the generalized model. The adopted procedure demonstrates the usefulness of the time-resolved thermal lens method for the study of photochemical reactions under the presence of absorbing species diffusion with the advantage of monitoring the processes in a quantitative way and with a temporal resolution of a few milliseconds.

Thermal-lens study of photochemical reaction kinetics.

We consider the time dependence of the absorption coefficient due to the photoinduced chemical reaction (PCR) and species diffusion to calculate the temperature rise in the thermal-lens (TL) effect. The TL signal at the detector plane is also calculated. This theoretical approach removes the restriction that the PCR time constant is much greater than the characteristic TL time constant, which was assumed in a previously published model. Hydrocarbon fuel and aqueous Cr(VI) samples are investigated, and quantitative experimental results for the thermal, optical, and PCR properties are obtained. While similar results were obtained for the Cr(VI) solution using the previous and present models, the relative difference between the PCR time constants extracted from the same experimental data for a hydrocarbon fuel sample is found to be more than 220%. This demonstrates the significant difference of the two models.

Top-hat cw-laser-induced time-resolved mode-mismatched thermal lens spectroscopy for quantitative analysis of low-absorption materials

Thermal lens spectroscopy is a highly sensitive and versatile photothermal technique for material analysis, providing optical and thermal properties. To use less expensive multimode non-Gaussian lasers for quantitative analysis of low-absorption materials, this Letter presents a theoretical model for time-resolved mode-mismatched thermal lens spectroscopy induced by a cw laser with a top-hat profile. The temperature profile in a sample was calculated, and the intensity of the probe beam center at the detector plane was also derived using the Fresnel diffraction theory. Experimental validation was performed with glass samples, and the results were found well consistent with literature values of the thermo-optical properties of the samples.

Temperature dependence of the thermo-optical properties of water determined by thermal lens spectrometry

The high transparency of water makes conventional spectroscopic measurements very difficult to be performed. In this work thermal lens spectrometry is applied to determine the temperature dependence of the thermo-optical properties of distilled and de-ionized water. The experiments were performed in the temperature range between 22 and 70 °C using the mode mismatched thermal lens configuration. The results showed three anomalous regions for the thermo-optical coefficients around 39, 42, and 59 °C. These observations may contribute to a better understanding of the physical and chemical properties of water.

Time resolved thermal lens in edible oils

In this work time resolved thermal lens spectrometry is applied to investigate the optical properties of the following edible oils: soya, sunflower, canola, and corn oils. The experiments were performed at room temperature using the mode mismatched thermal lens configuration. The results showed that when the time resolved procedure is adopted the technique can be applied to investigate the photosensitivity of edible oils. Soya oil presented a stronger photochemical reaction as compared to the other investigated samples. This observation may be relevant for future studies evaluating edible oils storage conditions and also may contribute to a better understanding of the physical and chemical properties of this important foodstuff.

Time-resolved thermal mirror technique with top-hat cw laser excitation

A theoretical model was developed for time-resolved thermal mirror spectroscopy under top-hat cw laser excitation that induced a nanoscale surface displacement of a low absorption sample. An additional phase shift to the electrical field of a TEM(00) probe beam reflected from the surface displacement was derived, and Fresnel diffraction theory was used to calculate the propagation of the probe beam. With the theory, optical and thermal properties of three glasses were measured, and found to be consistent with literature values. With a top-hat excitation, an experimental apparatus was developed for either a single thermal mirror or a single thermal lens measurement. Furthermore, the apparatus was used for concurrent measurements of thermal mirror and thermal lens. More physical properties could be measured using the concurrent measurements.