Catégorie : fr

Basile Pottier and Ludovic Bellon, Appl. Phys. Lett. 110, 094105 (2017)

doi: 10.1063/1.4977790

When measuring quadratic values representative of random fluctuations, such as the thermal noise of Atomic Force Microscopy (AFM) cantilevers, the background measurement noise cannot be averaged to zero. We present a signal processing method that allows to get rid of this limitation using the ubiquitous optical beam deflection sensor of standard AFMs. We demonstrate a two orders of magnitude enhancement of the signal to noise ratio in our experiment, allowing the calibration of stiff cantilevers or easy identification of higher order modes from thermal noise measurements.

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Felipe Aguilar Sandoval, Manuel Sepúlveda, Ludovic Bellon, and Francisco Melo, Sensors 2015, 27905-27916 (2015)

doi: 10.3390/s151127905

An interferometric method is implemented in order to accurately assess the thermal fluctuations of a micro-cantilever sensor in liquid environments. The power spectrum density (PSD) of thermal fluctuations together with Sader’s model of the cantilever allow for the indirect measurement of the liquid viscosity with good accuracy. The good quality of the deflection signal and the characteristic low noise of the instrument allow for the detection and corrections of drawbacks due to both the cantilever shape irregularities and the uncertainties on the position of the laser spot at the fluctuating end of the cantilever. Variation of viscosity below 0.03 mPa·s was detected with the alternative to achieve measurements with a volume as low as 50 μL.

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Felipe Aguilar Sandoval, Mickael Geitner, Éric Bertin, and Ludovic Bellon, Journal of Applied Physics 117, 234503 (2015)

doi: 10.1063/1.4922785

In optical detection setups to measure the deflection of micro-cantilevers, part of the sensing light is absorbed, heating the mechanical probe. We present experimental evidences of a frequency shift of the resonant modes of a cantilever when the light power of the optical measurement set-up is increased. This frequency shift is a signature of the temperature rise and presents a dependence on the mode number. An analytical model is derived to take into account the temperature profile along the cantilever; it shows that the frequency shifts are given by an average of the profile weighted by the local curvature for each resonant mode. We apply this framework to measurements in vacuum and demonstrate that huge temperatures can be reached with moderate light intensities: a 1000 °C with little more than 10 mW. We finally present some insight into the physical phenomena when the cantilever is in air instead of vacuum.

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Tianjun Li, Anthony Ayari and Ludovic Bellon,  Journal of Applied Physics 117, 164309 (2015)

doi:10.1063/1.4919355

The physics of adhesion of one-dimensional nano structures such as nanotubes, nano wires, and biopolymers on different substrates is of great interest for the study of biological adhesion and the development of nano electronics and nano mechanics. In this paper, we present force spectroscopy experiments of individual single wall carbon nanotube loops using a home-made interferometric atomic force microscope. Characteristic force plateaus during the peeling process allow the quantitative measurement of the adhesion energy per unit length on various substrates: graphite, mica, platinum, gold, and silicon. Moreover, using a time-frequency analysis of the deflection of the cantilever, we estimate the dynamic stiffness of the contact, providing more information on the nanotube configurations and its intrinsic mechanical properties.

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Roberto Pérez-Aparicio, Caroline Crauste-Thibierge, Marius Tanase, Pascal Metz, Ludovic Bellon, Antoine Naert, and Sergio Ciliberto,  Rev. Sci. Instrum. 86, 044702 (2015)

doi: 10.1063/1.4916260

We present an innovative technique which allows the simultaneous measurement of the dielectric constant of a material at many frequencies, spanning a four orders of magnitude range chosen between 10-2 Hz and 104 Hz. The sensitivity and accuracy are comparable to those obtained using standard single frequency techniques. The technique is based on three new and simple features: a) the precise real time correction of the amplification of a current amplifier; b) the specific shape of the excitation signal and its frequency spectrum; and c) the precise synchronization between the generation of the excitation signal and the acquisition of the dielectric response signal. This technique is useful in the case of relatively fast dynamical measurements when the knowledge of the time evolution of the dielectric constant is needed.

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M. Adier, F. Aguilar et al.,  General Relativity and Gravitation 46, 1749 (2014)

doi: 10.1007/s10714-014-1749-4

The Amaldi 10 Parallel Session C3 on Advanced Gravitational Wave detectors gave an overview of the status and several specific challenges and solutions relevant to the instruments planned for a mid-decade start of observation. Invited overview talks for the Virgo, LIGO, and KAGRA instruments were complemented by more detailed discussions in presentations and posters of some instrument features and designs. This article is a collection of the abstracts of this session, including the presentation by my collaborator Massimo Granata :

Results from Raman spectroscopy and direct thermal noise measurements on tantala and silica coatings (Presenter: M. Granata)

In order to isolate the mechanisms behind thermal-noise fluctuations in optical coatings, the Laboratoire des Matériaux Avancés has started a collaboration with the Institut Lumière Matière to investigate the Raman spectra of ion-beam sputtered tantala samples with different annealing history. Work is presently ongoing to understand the observed behaviours, in order to correlate the mechanical loss to the evolutions of the Raman spectra. The same analysis will be shortly carried out on fused silica coatings too.

A novel technique of direct thermal noise measurements, developed by the École Normale Supérieure de Lyon, is presented. Measured samples are tipless cantilevers with ion-beam sputtered coatings of tantala or fused silica, annealed before and after the coating. In all cases, the power spectral density of the measured noise is inversely proportional to the frequency, as predicted by the structural noise model with constant loss.

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C. Devailly, J. Laurent, A. Steinberger, L. Bellon, S. Ciliberto,  EPL 106, 54005 (2014)

doi: 10.1209/0295-5075/106/54005

We analyze the advantages and drawbacks of a method which measures the viscosity of liquids at microscales, using a thin glass fiber fixed on the tip of a cantilever of an ultra-low-noise Atomic Force Microscope (AFM). When the fiber is dipped into a liquid, the dissipation of the cantilever-fiber system, which is linked to the liquid viscosity, can be computed from the power spectral density of the thermal fluctuations of the cantilever deflection. The high sensitivity of the AFM allows us to show the existence and to develop a model of the coupling between the dynamics of the fiber and that of the cantilever. This model, which accurately fits the experimental data, gives also more insights into the dynamics of coupled microdevices in a viscous environment.

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Tianjun Li, Felipe A. Aguilar Sandoval, Mickael Geitner, Gianpietro Cagnoli, Vincent Dolique, Jérôme Degallaix, Raffaele Flaminio, Danièle Forest, Massimo Granata, Christophe Michel, Nazario Morgado, Laurent Pinard, and Ludovic Bellon,  Phys. Rev. D 89, 092004 (2014)

doi: 10.1103/PhysRevD.89.092004

In recent years, an increasing number of devices and experiments are shown to be limited by mechanical thermal noise. In particular, subhertz laser frequency stabilization and gravitational wave detectors that are able to measure fluctuations of 10^-18 m/rtHz or less are being limited by thermal noise in the dielectric coatings deposited on mirrors. In this paper, we present a new measurement of thermal noise in low absorption dielectric coatings deposited on microcantilevers, and we compare it with the results obtained from the mechanical loss measurements. The coating thermal noise is measured on the widest range of frequencies with the highest signal-to-noise ratio ever achieved. In addition, we present a novel technique to deduce the coating mechanical losses from the measurement of the mechanical quality factor which does not rely on the knowledge of the coating and substrate Young’s moduli. The dielectric coatings are deposited by ion beam sputtering. The results presented here give a frequency-independent loss angle of (4.7±0.2)x10^-4 with a Young’s modulus of 118 GPa for annealed tantala from 10 Hz to 20 kHz. For as- deposited silica, a weak frequency dependence (~ f^-0.025) is observed in this frequency range, with a Young’s modulus of 70 GPa and an internal damping of (6.0±0.3)x10^-4 at 16 kHz, but this value decreases by one order of magnitude after annealing, and the frequency dependence disappears.

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