J. Shen

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.

Soret effect and photochemical reaction in liquids with laser-induced local heating

We report a theoretical model and experimental results for laser-induced local heating in liquids, and propose a method to detect and quantify the contributions of photochemical and Soret effects in several different situations. The time-dependent thermal and mass diffusion equations in the presence and absence of laser excitation are solved. The two effects can produce similar transients for the laser-on refractive index gradient, but very different laser-off behavior. The Soret effect, also called thermal diffusion, and photochemical reaction contributions in photochemically reacting aqueous Cr(VI)-diphenylcarbazide, Eosin Y, and Eosin Y-doped micellar solutions, are decoupled in this work. The extensive use of lasers in various optical techniques suggests that the results may have significance extending from physical-chemical to biological applications.

A composite photothermal technique for the measurement of thermal properties of solids

In this work, a composite photothermal technique combining open photoacoustic cell and photothermal deflection methods for thermal characterization of opaque solids was developed. An excitation laser was employed to concurrently generate both photoacoustic and mirage effects. Thermal diffusivity and thermal effusivity of carbon-based samples were measured, and the values of thermal conductivities and specific heat were then deduced. The experimental results were found to be in good agreement with the literature values. The photothermal technique developed in this work permits a convenient and precise measurement of thermal properties of solids.

Thermophysical property measurements using time-resolved photothermal deflection spectrometry with step optical excitation

This letter reports the theoretical and experimental results of thermophysical property measurements using our recently developed time-resolved photothermal deflection spectrometry (PDS) with step optical excitation. One-dimensional heat conduction boundary conditions of the third kind were derived, and a theoretical model with the boundary conditions was proposed. Both thermal diffusivity and effusivity of two well-known samples were precisely measured simultaneously by fitting experimental data to the theory. Thermal conductivity and unit volume specific heat then can be deduced. Moreover, time-resolved PDS with step optical excitation has the advantages of simpler experimental apparatus and less time consuming measurement, compared with the traditional periodically modulated PDS.

Effective normalization method for sample-position-dependence effect in photoacoustic spectrometry.

Sample position dependence effect in photoacoustic (PA) spectrometry has been reported by several scientists. This effect must be taken into account in a PA application that requires a quantitative theoretical treatment. In this work, we experimentally investigated PA signal magnitude varying with sample-to-window distance in an MTEC Model 300 Photoacoustic Detector, which has a fixed empty (gas) volume in addition to the sample-to-window-distance-dependent gas volume. An operative method was introduced to obtain the coefficient, which considered the sample-to-window distance and the additional gas volume. With this coefficient, the one-dimensional PA model, developed by Aamodt, Murphy, and Parker, can be employed to quantitatively process PA experimental data, no matter what the sample-to-window distance is. Quantitative measurements of thermal effusivities of two samples were performed to prove this effective normalization method.

Thermal mirror and thermal lens techniques for semitransparent material characterization

The mode-mismatched thermal lens technique(TL) has been used to study many semitransparent materials. Its theoretical development considers weakly absorbing materials, which introduce restrictions on the samples optical thickness. However, the same equipment required by TL can be used to perform the thermal mirror (TM) experiment, which is useful to characterize materials with any optical absorption coefficient. In this work, we investigate a simple correction to be used in the TL model, making it possible to apply TL to a wide range of materials. Using TL and TM, we have determined the temperature coefficient of the optical path length (ds/dT) of a glass.

Material characterization with top-hat cw laser induced photothermal techniques: a short review

In this work, we present a short review of the recent development of the theoretical models for top-hat cw laser induced spectroscopies of thermal lens and thermal mirror. With the same probe and top-hat excitation lasers, an apparatus is set up to concurrently measure both thermal lens and thermal mirror effects of transparent samples. With the theoretical models and the experimental apparatus, not only optical and thermal properties are measured, but also the fluorescence quantum coefficient and the temperature coefficient of the optical path length of a fluorescent sample are simultaneously determined with no need of any reference sample. Mechanical properties also could be measured. Opaque samples are also studied using top-hat cw laser thermal mirror and top-hat photothermal deflection techniques to determine thermal properties (e.g., thermal conductivity and unit volume specific heat). This work shows that the combined top-hat cw laser photothermal techniques are useful for nondestructive evaluation of both transparent and opaque samples with a less expensive non-TEM00 Gaussian laser.