V. S. Zanuto

A Theoretical and Experimental Study of Time-Resolved Thermal Mirror with Non-Absorbing Heat-Coupling Fluids

A theoretical and experimental study taking sample-fluid heat coupling into account in time-resolved photothermal mirror experiments is presented. Thermoelastic equations were solved to obtain a semi-analytical solution to the phase shift induced by the sample and the surrounding fluid. The solution was used to model the thermal mirror effects and found to be in excellent agreement with the finite element method analysis and experiment. Heat transferred to the air-coupling fluid did not introduce important effects in the phase shift when compared with the solution obtained, without considering heat flux. However, when using water as the fluid, heat coupling led to a significant effect in fluid phase shift. Experimental results using stainless steel in air and water were used to demonstrate the potentiality of the thermal mirror technique to determine the thermal properties of both the sample and the fluid.

Quantitative assessment of radiation force effect at the dielectric air-liquid interface.

We induce nanometer-scale surface deformation by exploiting momentum conservation of the interaction between laser light and dielectric liquids. The effect of radiation force at the air-liquid interface is quantitatively assessed for fluids with different density, viscosity and surface tension. The imparted pressure on the liquids by continuous or pulsed laser light excitation is fully described by the Helmholtz electromagnetic force density.

Pulsed photothermal mirror technique: characterization of opaque materials

The time-resolved thermal mirror technique is developed under pulsed laser excitation for quantitative measurement of thermal and mechanical properties of opaque materials. Heat diffusion and thermoelastic equations are solved analytically for pulsed excitation assuming surface absorption and an instantaneous pulse. Analytical results for the temperature change and surface displacement in the sample are compared to all-numerical solutions using finite element method analysis accounting for the laser pulse width and sample geometry. Experiments are performed that validate the theoretical model and regression fitting is performed to obtain the thermal diffusivity and the linear thermal expansion coefficient of the samples. The values obtained for these properties are in agreement with literature data. The technique is shown to be useful for quantitative determinations of the physics properties of metals with high thermal diffusivity.

Pulsed-laser time-resolved thermal mirror technique in low-absorbance homogeneous linear elastic materials.

A theoretical model for a time-resolved photothermal mirror technique using pulsed-laser excitation was developed for low absorption samples. Analytical solutions to the temperature and thermoelastic deformation equations are found for three characteristic pulse profiles and are compared to finite element analysis methods results for finite samples. An analytical expression for the intensity of the center of a continuous probe laser at the detector plane is derived using the Fresnel diffraction theory, which allows modeling of experimental results. Experiments are performed in optical glasses, and the models are fitted to the data. The parameters of the fit are in good agreement with previous literature data for absorption, thermal diffusion, and thermal expansion of the materials tested. The combined modeling and experimental techniques are shown to be useful for quantitative determination of the physical properties of low absorption homogeneous linear elastic material samples.

Generation and detection of thermoelastic waves in metals by a photothermal mirror method

We investigate the thermoelastic waves launched by a localized heat deposition. Pulsed laser excitation is used to generate mechanical perturbations in metals that are detected using the photothermal mirror method. This method detects the wavefront distortion of the probe beam reflected from the perturbed sample surface. Nanometer scale expansion of the material is induced just under the irradiated surface releasing transient thermoelastic waves of much smaller amplitudes on the surface. Numerical predictions and the experimental results are in a good agreement and represent both the thermal diffusion of the large amplitude, long-lasting outward bulge, and the released elastic waves.

Enhanced and tunable white light emission from Ag nanoclusters and Eu3+-co-doped CaBAl glasses

Noble metal embedded glasses have been studied as promising candidates for a variety of technological applications, mainly due to their ability to enhance rare earth luminescence properties. In this work, Ag:Eu-co-doped calcium boroaluminate glasses were prepared and submitted to further heat treatment to form different Ag species. The optical and luminescence properties were investigated in terms of heat treatment times. Absorption spectra showed a successful Eu and Ag ion incorporation in the host, as well as Ag nanoparticle precipitation induced by heat treatment. Upon UV-light excitation, the co-doped glasses exhibited an intense wide emission band centered at about 500 nm, attributed to molecule-like silver species, which combined with the Eu3+ characteristic emission reaches a white light resultant emission. A new excitation band for Eu3+ at 335 nm and a silver luminescence lifetime decrease suggest an energy transfer process from molecule-like Ag to Eu3+ as being responsible for the enhanced PL properties in these glasses. An appropriate combination of a violet LED with the sample emission provides a route to achieve the ideal white light CIE color parameters. The relevant quality color results qualify these glasses as phosphors with high potential for white light emitting devices.

Accessing thermo-mechanical properties of semiconductors using a pump-probe surface displacement method

Description of the physical mechanism leading to laser induced thermal and electronic effects in semiconductors is crucial in both basic research and technological applications. In this paper, we present a thermal mirror technique to study the thermo-mechanical properties of semiconductors. A detailed theoretical investigation is presented, and the dominant effects are described in terms of the physical properties of the material. The effect of heat coupling between the sample and the surrounding fluid was taken into account and considerations on the time and spatial approximations to the photogenerated carriers profile were used to simplify the theoretical model. These approximations were then compared to numerical models and the results hold for high recombination rate semiconductors. Experiments were performed to validate the theoretical model, and the thermal diffusivity and photogenerated heat in the sample were determined. The values obtained for these properties were found to be in good agreement w...

Analysis of the Thermo-Reflectivity Coefficient Influence Using Photothermal Pump–Probe Techniques

Recent improvements in the modeling of photo-induced thermo–optical–mechanical effects have broadened the application of photothermal techniques to a large class of solids and fluids. During laser excitation, changes in optical reflectivity due to temperature variation may affect the photothermal signal. In this study, the influence of the reflectivity change due to heating is analyzed for two pump–probe photothermal techniques, thermal lens and thermal mirror. A linear equation for the temperature dependence of the reflectivity is derived, and the solution is tested using optical properties of semi-transparent and opaque materials. For semi-transparent materials, the influence of the reflectivity change in photothermal signals is less than 0.01%, while for opaque materials it is lower than 3%.

Application of Photoreactive Barium Titanate (BaTiO3) Beam Fanning to the Photothermal Mirror Technique: An Experimental Analysis

An adaptive spatial filter is used as an optical novelty filter to detect photothermal mirror (PM) signals in high absorbing materials using continuous wave laser excitation. The optical novelty filter uses an optical beam-fanning limiter based on single domain barium titanate (BaTiO3), cut and poled 45° relative to the c-axis. The optical novelty filter approach relaxes the requirement for high sample surface smoothness because the effect aperture adapts to the surface, reducing the stationary background from the optical signal and provides a means of developing the photothermal mirror signal. Time-dependent probe laser phase shifts due to photothermal surface deformation pass through the optical novelty filter and are detected as an intensity increase over the stationary or “mundane” signal. Experimental studies are performed using four well-characterized metals using both the conventional photothermal mirror and optical novelty filter apparatuses in order to understand the complicated signal behavior. Signal behavior is analyzed in different excitation intervals using pseudo-chopped sample excitation with different duty cycles. Optical novelty filter signals show fast response for changes in the spatial beam profile followed by long relaxation time. Reasons for the optical novelty filter response are described.