Renaud Côte

Enhancing the dynamic range of targeted energy transfer in acoustics using several nonlinear membrane absorbers

Abstract In order to enhance the robustness and the energy range of efficiency of targeted energy transfer (TET) phenomena in acoustics, we discuss in this paper about the use of multiple nonlinear membrane absorbers (called nonlinear energy sinks or NES) placed in parallel. We show this way, mainly thanks to an experimental set-up with two membranes, that the different absorbers have additional effects that extend the efficiency and the possibilities of observation of TET. More precisely, we present the different behavior of the system under sinusoidal forcing and free oscillations, characterizing the phenomena for all input energies. The frequency responses are also presented, showing successive clipping of the original resonance peak of the system, and strongly modulated regimes (SMR). A model is finally used to generalize these results to more than two NES and to simulate the case of several very similar membranes in parallel which shows how to extend the existence zone of TET.

Wideband fluorescence-based thermometry by neural network recognition: Photothermal application with 10 ns time resolution

Neural network recognition of features of the fluorescence spectrum of a thermosensitive probe is exploited in order to achieve fluorescence-based thermometry with an accuracy of 200u2009mK with 100u2009MHz bandwidth, and with high robustness against fluctuations of the probe laser intensity used. The concept is implemented on a rhodamine B dyed mixture of copper chloride and glycerol, and the temperature dependent fluorescence is investigated in the temperature range between 234u2009K and 311u2009K. The spatial dependence of the calibrated amplitude and phase of photothermally induced temperature oscillations along the axis of the excitation laser are determined at different modulation frequencies. The spatial and frequency dependence of the extracted temperature signals is well fitted by a 1D multi-layer thermal diffusion model. In a time domain implementation of the approach, the gradual temperature rise due to the accumulation of the DC component of the heat flux supplied by repetitive laser pulses as well the immedi...

Experimental Study of an Acoustic System Coupled to a Nonlinear Membrane Under Multi-Frequency Forcing

In this paper, an experimental study of the coupling of a 2 degree-of-freedom acoustic medium to a nonlinear visco-elastic membrane under multi-frequency forcing is presented. When the excitation is sinusoidal, we observe classical results: non-linear effects tend to damp pressure in the linear system. When the excitation is quasi-periodic with two frequencies, various aspects of quasi-periodic regimes are observed. Some regimes exhibit only the two excitation frequencies in their spectrum, other display a more complicated, but mostly discrete, spectrum. These stable quasi-periodic regimes can disappear leaving to Strongly Modulated Quasi-Periodic Responses (SMQPR). Various structures have been extracted from SMQPRs using signal processing tools (RMS analysis, Fourier analysis, histogram analysis). They are compared with theory and the NES efficiency is assessed. The system behaves in general as expected: the periodic, quasi-periodic or SMQPR regimes correspond to theoretical findings.Copyright

Film thickness determination by laser ultrasonics

The thickness of films deposited on substrates is crucial for their thermal, electrical, optical behaviour. These properties are essential in thin film applications, especially in the field of microelectronics. In this study, we are interested in thickness determination of silver and gold films deposited by evaporation on a silicon substrate by using an ultrasonic non‐destructive technique. In particular, the well‐known laser ultrasonic technique is used to generate and detect the surface acoustic waves. Results obtained by two complementary methods allowing a non‐contact measurement in a large bandwidth (from 5 MHz to 200 MHz) are presented, and the dispersion of the Rayleigh wave propagation velocity is analyzed to determine the film thickness.

Stroboscopic interferometric full‐field imaging of laser‐induced surface acoustic waves

Surface acoustic waves (SAW) have the interesting property that they specifically interact with surface and sub‐surface regions, rendering them suitable for non‐contact investigation of sub‐surface properties and heterogeneities. Transmission, reflection and diffraction effects of SAW propagation are analysed in order to reveal information on the region over which they propagate. Laser excitation allows to efficiently excite SAW with short wavelengths, enhancing both the lateral and the depth resolution. Typically information on the waves is collected by scanning a part of the surface of interest with a fast laser probe, using the surface displacement or slope as a real time witness of the wave field. Here we present results obtained by a full field imaging technique [1], in which the sample is repetitively excited by a pulsed pump laser, and the wave displacement field is stroboscopically gathered by illuminating the sample surface with an electronically delayed pulsed probe laser, whose displacement ind...