Sébastien Euphrasie

Ultra-wide acoustic band gaps in pillar-based phononic crystal strips

An original approach for designing a one dimensional phononic crystal strip with an ultra-wide band gap is presented. The strip consists of periodic pillars erected on a tailored beam, enabling the generation of a band gap that is due to both Bragg scattering and local resonances. The optimized combination of both effects results in the lowering and the widening of the main band gap, ultimately leading to a gap-to-midgap ratio of 138%. The design method used to improve the band gap width is based on the flattening of phononic bands and relies on the study of the modal energy distribution within the unit cell. The computed transmission through a finite number of periods corroborates the dispersion diagram. The strong attenuation, in excess of 150 dB for only five periods, highlights the interest of such ultra-wide band gap phononic crystal strips.

DESIGN OF METALLIC MESH ABSORBERS FOR HIGH BANDWIDTH ELECTROMAGNETIC WAVES

In this paper, models of metallic absorbers for electromag- netic waves in the infrared to microwave frequency range are reported and discussed. The Hadleys formalism (1D model) of transmission, re∞ection and absorption for semi-inflnite layers, which allows to de- sign all conflgurations of unstructured absorber fllms and dielectrics is generalized. To make the micro-fabrication of the metallic absorbers easier (that means to have layers thick enough), the metallic layers need to be structured (grid for example). We developed a model that allows us to consider the structure of metal as a homogeneous layer, where the difiraction is negligible. This new layer can be used with the previous model. When difiraction efiects must be taken into account, we modifled an electrical model made by Ulrich. We further developed it for the conflguration of a dielectric before the metallic grid. The re- sults showed the importance to take into account all the dimensions of the grid, the dielectric layer parameters and the wavelength to design the best absorber.

Evidence of a broadband gap in a phononic crystal strip

ABSTRACT We experimentally demonstrate a very large ultrasonic band gap in a one‐dimensional phononic crystal. The structure consists of periodic tungsten pillars fixed to a tailored silicon strip with a layer of epoxy. Combining local resonances and Bragg scattering, the gap ranges from 450 kHz to 1250 kHz, which corresponds to a gap‐to‐midgap ratio of 94%, and the attenuation exceeds 35 dB with only three periods. Numerical calculations with the Finite Element Method are performed to support the analysis and provide a better understanding of the behavior of the structure. In particular, the role of the thin layer of epoxy is studied and is shown to have a strong influence on the dispersion. This phononic structure with a very large band gap can be considered as a new tool to design acoustic devices with high performances.

Electro-optic sensor for specific absorption rate measurements

We propose a sensor system to measure the specific absorption rate at radio frequencies, using a matrix of electro-optic (EO) sensors. The aim is to replace the conventional metallic antenna scanned by a robot inside an open phantom with a matrix of small probes positioned inside a closed phantom. Results are presented for a dual sensor, showing the feasibility of the proposed approach. The two EO probes interrogated by the same laser are shown to detect electric fields smaller than 1 V/m. Measurements are conducted both with the probes in air and inside the phantom liquid.

Thermal detectivity enhancement of visible and near infrared thermography by using super-resolution algorithm: Possibility to generalize the method to other domains

This paper reports on a method which allows a decrease in the minimal detectable temperature in visible and near infrared thermography. This original method permits an increase in the thermal sensitivity without loss of good spatial resolution. It is based on a binning operation and a super-resolution algorithm. The radiometric model and super-resolution method are presented. Measurements on two different samples show the enhancement of the thermal sensitivity and the capability of the method. Finally, the authors propose different ways in which the method can be applied.

Evidence of a large elastic band gap in a one-dimensional phononic crystal

The aim of this work is to study the mechanisms of enlargement of band gaps in phononic strips and to present the characterization of such a structure. Indeed, to our knowledge, the gap-to-midgap ratio of the PC strips used for applications in literature does not exceed 45%. Yet, large band gaps generally imply better performances for phononic devices and a stronger robustness to fabrication tolerances. Using three different structures based on tungsten pillars fixed on a tailored silicon strip, we show that large band gaps can be obtained by coupling Bragg scattering and local resonances. Optical measurements made on a structure of few millimeters show good agreement with numerical calculations. The three periods phononic strip exhibits a main band gap with a gap-to-midgap ratio close to 100% and a maximum attenuation of -40 dB.

Femtosecond heterodyne pump probe platform

We present a femtosecond heterodyne pump probe platform with electronically synchronized Ytterbium laser. The main goal of this platform is to provide thermal characterization at short space and time scales. Picosecond acoustic phenomena can also be observed and used to extract information such as acoustic wave velocities. Thermal conductivities and thermal interface resistances can be extracted for the different layers in the studied sample by fitting thermal models with experimental data. To characterize laterally structured samples, solid immersion lenses have been fabricated and used to increase the spatial resolution.

Piezoelectric and optical setup to measure an electrical field: application to the longitudinal near-field generated by a tapered coax.

We propose a new setup to measure an electrical field in one direction. This setup is made of a piezoelectric sintered lead zinconate titanate film and an optical interferometric probe. We used this setup to investigate how the shape of the extremity of a coaxial cable influences the longitudinal electrical near-field generated by it. For this application, we designed our setup to have a spatial resolution of 100 microm in the direction of the electrical field. Simulations and experiments are presented.