Photothermal Reflectance Microscopy Research Articles

Real-time imaging of surface chemical reactions by electrochemical photothermal reflectance microscopy.

Traditional electrochemical measurements based on either current or potential responses only present the average contribution of an entire electrode's surface. Here, we present an electrochemical photothermal reflectance microscope (EPRM) in which a potential-dependent nonlinear photothermal signal is exploited to map an electrochemical process with sub-micron spatial resolution. By using EPRM, we are able to monitor the photothermal signal of a Pt electrode during the electrochemical reaction at an imaging speed of 0.3 s per frame. The potential-dependent photothermal signal, which is sensitive to the free electron density, clearly revealed the evolution of surface species on the Pt surface. Our results agreed well with the reported spectroelectrochemical techniques under similar conditions but with a much faster imaging speed. We further mapped the potential oscillation during the oxidation of formic acid on the Pt surface. The photothermal images from the Pt electrode well matched the potential change. This technique opens new prospects for real-time imaging of surface chemical reaction to reveal the heterogeneity of electrochemical reactivity, which enables broad applications to the study of catalysis, energy storage, and light harvest systems.

Fast mapping of absorbing defects in optical materials by full-field photothermal reflectance microscopy

We report a technique for rapidly mapping absorbing defects in optical materials, which act as laser-induced damage precursors, based on full-field photothermal reflectance microscopy. An intensity-modulated pump beam heats absorbing defects in the optical sample, creating localized, modulated refractive-index variations around the defects. A probe beam then illuminates the defect sites, and the measured amplitude of the reflectance variation is used to map the distribution of defects in the medium. Measurements show that this method offers a faster defect mapping speed of about 0.03 mm(2) per minute and a detectivity of a few tens of nanometers comparable to that of conventional scanning photothermal deflection microscopy.

Theoretical calculation of the dispersion relation for polymeric thin films: Determination of the thermal diffusivity using photothermal microscopy

Photothermal reflectance microscopy can be used to characterize thin films grown on substrates in two different ways: (a) scanning modulation frequency with both the pump and probe beams coincident and (b) varying the pump–probe beam distance for several modulation frequencies. In the second case, the phase lag behavior, as a function of the beam separation, gives the effective wave number of the thermal wave along the surface. A plot of the wave number versus frequency provides a dispersion relation. Such a relation can be obtained by modeling the thermal problem of the system, and from the experimental data analysis one can determine the thermal parameters of the film and substrate. In the case of films with high thermal diffusivities, it is possible to find an analytical expression for the dispersion relation [A. A. Maznev et al., J. Appl. Phys. 78, 5266 (1995)]. However, in the case of films with low thermal diffusivities numerical calculation is needed. In this article we present the results of such a calculation for polymer films which have low diffusivities compared to the substrates (glass and silicon). The films were assumed to be opaque to both the pump and probe beams, i.e., both absorption and reflection occur at the film surface. It was observed that at high modulation frequencies the dispersion relation splits into two branches, one characteristic of the film, which is valid for small distances between the pump and probe, and the other characteristic of the substrate, valid for large distances.

Contrast and Sensitivity Enhancement in Photothermal Reflectance Microscopy through the Use of Specific Probing Wavelengths: Applications to Microelectronics

Contrast and Sensitivity Enhancement in Photothermal Reflectance Microscopy through the Use of Specific Probing Wavelengths: Applications to Microelectronics

Heat source distribution, vertical structure, and coating influences on the temperature of operating 0.98 μm laser diodes: Photothermal reflectance measurements

In the present work single-quantum-well laser diodes operating at 0.98 μm are investigated by photothermal reflectance microscopy. Temperature maps were obtained for the output facet of all devices studied. Furthermore, the temperature distribution was determined along the cavity (on the ridge) of lasers soldered with the junction side up. Near the facets, the measured temperature was found to be about seven times the bulk’s temperature, indicating the presence of an important surface heat source. The signal phase distribution of the laser facet shows the important role of the vertical structure on the heat confinement. Comparison between experiments and calculations shows that the confinement layers (GaAlAs and GaInP) thermal parameters are the principal responsible for the heat propagation in these structures near the active region. The same calculations show the role of the coating (Al2O3) in the heat propagation, and give a quantitative ratio between surface and bulk heat sources. Measurements made on the facet and on the ridge as a function of injection current were found to present a quite similar behavior, leading to the conclusion that thermal effects are strongly dominant in these measurements, masking any carrier or electroreflectance effects. Finally, measurements made under different light output power conditions and under the same injection current conditions showed that the surface heat source is caused by laser light absorption at the facets.

Analysis of local heat transfer properties of tape-cast AlN ceramics using photothermal reflectance microscopy

Photothermal reflectance microscopy was applied to the analysis of local thermal diffusivity on tape-cast AlN ceramics. The materials were obtained from three different commercial powders, two sintering temperatures (1750 and 1800 °C), and 3 wt % Y2O3 sintering aid. Owing to the high spatial resolution of the technique (∼30 μm in the present case), measurements in different positions on the sample surface were carried out. In this way a study of the homogeneity of thermal properties was performed.

Photothermal microscopy: Thermal contrast at grain interface in sintered metallic materials

A theoretical and experimental study of the influence of planar thermal barriers on photothermal reflectance microscopy signals is presented. An analytical solution is developed for the problem of vertical barriers in a semi-infinite solid and the signal contrast obtained when scanning through the barrier is discussed as a function of the thermal resistance, the thermal diffusion length, and the pump and probe beam dimensions. The shape and the width of the signal perturbation introduced by the barrier is also analyzed. For the case of slanted barriers results of finite-element calculations are presented, and the main feature of the signal when going from vertical to slanted barriers is thus shown. Finally, the theoretical predictions are compared with measurements made on Fe sintered samples. Scanning through grain interfaces revealed different signal shapes and contrast. Good agreement between theory and experiment was found when the optical contrast at the interface is negligible. Examples are shown where the thermal barrier model is no longer valid and an extended model seems to be necessary.

Photothermal reflectance microscopy : signal contrast in the case of thick and thin grain interfaces in solids

In this work a thick interface model is proposed in order to describe the photothermal signal perturbation introduced by grain interfaces. The validity of the model is discussed based upon photothermal reflectance microscopy results obtained on Fe sintered samples. A comparison with the thermal resistance model, which is valid only in the case of thin interfaces, is also presented.

Temperature measurements of telecommunication lasers on a micrometre scale

Temperature maps of laser diode facets obtained by photothermal reflectance microscopy are presented. Biased double heterojunction InGaAsP lasers were investigated. The temperature images reveal the influence of laser mode on the spatial distribution of the heat dissipation in the device. Moreover, a study of the modulated reflectance as a function of the injection current is achieved.