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International audienceThe first purpose of this paper is to underline a relevant colorimetric co-ordinate characterizing the colour of ochres within their extremely wide range, from pale yellow to dark red. The second purpose is to link together quantitatively the variations of this colorimetric co-ordinate and the various chemical compositions of the samples, mainly hematite, goethite and white pigments. A group of 30 modern ochres and a group of 20 ancient ochres have been investigated. All these natural pigments have been commercialized. Diffuse reflectance spectrometry allows to calculate the colorimetric co-ordinates in the CIE-L*a*b* space and the position of the absorption band of each sample. Physico-chemical analysis has been obtained by quantitative X-ray diffraction, scanning and transmitting electronic microscopy and particle-size analysis by laser diffraction. The positive a* co-ordinate (redness) has been underlined, for the first time, to be the only relevant colorimetric parameter to characterize the colour of the ochres. Its variations are quantitatively connected to the shift of the absorption band due to the charge transfer between the ligand (OH− or O2−) and the Fe3+ ion contained in goethite and/or hematite. For ochres containing both hematite and goethite, the a* co-ordinate linearly increases with the relative amount of hematite while the absorption band progressively shifts towards the high wavelengths. Such a linear shift of the absorption band has never been underlined before. For ochres containing only one iron oxide, a* linearly decreases with the amount of white pigments, whatever the nature of the white charges. Moreover, this study gives the opportunity to show that only the nature, the amount and the size distribution of the white charges allow to discriminate the ochres according to their geographic origin

We present a new micromechanical resonator designed for cavity optomechanics. We have used a micropillar geometry to obtain a high-frequency mechanical resonance with a low effective mass and a very high quality factor. We have coated a 60-$\mu$m diameter low-loss dielectric mirror on top of the pillar and are planning to use this micromirror as part of a high-finesse Fabry-Perot cavity, to laser cool the resonator down to its quantum ground state and to monitor its quantum position fluctuations by quantum-limited optical interferometry

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International audienceWe present a free-space optomechanical system operating in the 1-K range. The device is made ofa high mechanical quality factor micropillar with a high-reflectivity optical coating atop, combinedwith an ultra-small radius-of-curvature coupling mirror to form a high-finesse Fabry-Perot cavityembedded in a dilution refrigerator. The cavity environment as well as the cryostat have beendesigned to ensure low vibrations and to preserve micron-level alignment from room temperatur

Quantum mechanics has so far not been tested for mechanical objects at the scale of the Planck mass c/G 22 µg. We present an experiment where a 1 mm quartz micropillar resonating at 3.6 MHz with an effective mass of 30 µg is cooled to 500 mK with a dilution refrigerator, and further optomechanically sideband-cooled to an effective temperature of 3 mK, corresponding to a mode thermal occupancy of 20 phonons. This nearly 1000-fold increase in the mass of an optomechanical system with respect to previous experiments near the quantum ground state opens new perspectives in the exploration of the quantum/classical border.

Although the Vibrating Beam Accelerometer (VBA) shows excellent performances (resolution<1μg), its bias drift over temperature and temperature gradient is a main performance limitation. Numerical compensation can be performed with a temperature sensor in the accelerometer package, but this approach shows two drawbacks for fast measurements: first, there is a time delay between the temperature of the vibrating beam and the temperature of the package, and second the noise level is very high. In order to overcome these limitations, a new structure comprising a second resonator in torsional mode placed in the middle of the vibrating beam was designed. It shows good sensitivity and linearity over temperature and senses the instantaneous temperature of the vibrating beam with low noise, allowing a better compensation of thermal drifts. A new accelerometer with resonator-based thermal compensation was designed, realized and tested. Experimental results confirm the excellence of the torsional mode resonator used as a temperature sensor.I.

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