D. Fournier

Variable‐temperature photoacoustic effect: Application to phase transition

A low‐temperature‐adapted photoacoustic cell using an electret microphone and a MOSFET preamplifier is described. This cell can operate between 5 and 300 K in a gas‐flow cryostat. By way of illustration, it is used here to perform a new kind of photoacoustic measurement, i.e., to study the specific‐heat anomaly of various insulators in the neighborhood of magnetic phase transitions.

Thermal measurements on diamond and related materials

Abstract The outstanding thermal conductivity of CVD diamond gives rise to many potential industrial applications, necessitating the availability of reliable, standardized, thermal measurement methods. Although, in general, these measurements are in agreement for traditional materials, for CVD diamond different measurement methods can yield significantly different values for comparable or even identical samples. The aim of this work is to compare the methods and estimate to what extent these variations may be caused by the inhomogeneity and anisotropy of the samples themselves. Using a six-layer model to represent a typical CVD diamond layer, the potential signals for the principal methods are calculated and the results are compared.

Optical and thermal characterization of coatings

Abstract The mechanical, thermal and optical properties of thin coatings have been considerably improved these last years. For instance optical coatings for laser mirrors with sub ppm losses are now available. Moreover thermal diffusivity varying over more than 4 orders of magnitude can be found when going from polymers coatings to diamond ones. Reliable sensitive techniques are necessary to characterize them. First, we present two optical techniques: ellipsometry and polarization microscopy which lead to determination of optical (refractive index) and geometrical (thickness, homogeneity) parameters. Second, we describe a few optically based photothermal techniques in which heating and detection are contactless and non-destructive: photothermal microscopy, mirage detection, time resolved ellipsometry, etc. These techniques allow local (down to micron scale) to global determination of the optical properties (absorption with a few ppb sensitivity) as well as thermal properties (diffusivity, effusivity) and reveal inhomogeneities, adhesion as well as anisotropies.

Contrast enhancement in crack detection by stimulated differential infrared radiometry: modelisation and experiments

Abstract Usually sample surface structures generate large flunctuations of the photothermal response due to local variations of optical, thermal and geometrical properties which mask the crack signal when using single detection ir radiometry. Demonstrated experimentally and theoretically is the ability of a differential stimulated ir detection scheme to obtain a better contrast in crack detection on real samples. A finite element simulation confirms the magnitude and shape of the signals for vertical and slanted cracks.

Photoacoustic spectroscopic studies of methylene blue and malachite green adsorbed on silica

Abstract Spectra and calibration curves of methylene blue and malachite green adsorbed on silica show that photoacoustic spectroscopy can be used for the determination of these compounds. The problem of the existence of an equilibrium between monomer and dimer for methylene blue is considered.

Photothermal approach to local heating imaging: application to laser degradation

We use various probes to measure local temperature induced by internal or external heating of active or passive devices: Mirage detection can reveal heating of few ppb of the input power, whereas photothermal microscopy provides sub-micron spatial resolution. Temperature distribution is measured through periodic deflection or reflectivity mapping at frequencies high enough to confine heating near the source. Scanning InGaAsP/InP lasers facets, shows the weak influence of nonradiative recombination, in agreement with the high output power of these lasers before degradation. On strained-layer InGaAs quantum well lasers we obtained a drastic temperature rise, that we explain through simple model based on line heating for the laser cavity and point heating located at the facet. On a damaged laser the result, demonstrate clearly the existence of heating zones far from the facet along the laser cavity.

Automatic Measurements of Thermal Resistances by the Mirage Effect

An industrial mirage effect bench monitored by a microcomputer is used to measure automatically thermal resistances between metallic samples.