Jean-Michel Geffrin

Continuing with the Fresnel database: experimental setup and improvements in 3D scattering measurements

In this paper, the experimental setup and the improvements required to obtain further measurements for the third opus of the Fresnel Database are presented. The most original feature of those new datasets is the fact that they were obtained with three-dimensional targets instead of the two-dimensional ones used in the two previous opuses. The measurements were performed all around the targets under test to collect enough information about the objects to be able to perform inversion on their scattered fields. As the targets were small in comparison with the wavelength, the challenge here was to extract these small scattered fields from the measurements, and a specific post-processing procedure had to be designed to compensate for the drift errors. The five targets selected for the database are presented, including the Myster target, a hitherto undivulged target that is presented in this paper for the first time, i.e., at the same time as the submissions of all the other contributors to this special section. Some scattered field comparisons are also presented.

Controllable emission of a dipolar source coupled with a magneto-dielectric resonant subwavelength scatterer

We demonstrate experimentally and theoretically that a local excitation of a single scatterer of relative dielectric permittivity ε = 6 permits to excite broad dipolar and quadrupolar electric and magnetic resonances that shape the emission pattern in an unprecedented way. By suitably positioning the feed with respect to the sphere at a λ/3 distance, this compact antenna is able to spectrally sort the electromagnetic emission either in the forward or in the backward direction, together with a high gain in directivity. Materials with ε = 6 can be found in the whole spectrum of frequencies promising Mie antennas to become an enabling technology in numbers of applications, ranging from quantum single photon sources to telecommunications.

Small Dielectric Spheres with High Refractive Index as New Multifunctional Elements for Optical Devices

The future of ultra-fast optical communication systems is inevitably connected with progress in optical circuits and nanoantennas. One of the key points of this progress is the creation of elementary components of optical devices with scattering diagrams tailored for redirecting the incident light in a desired manner. Here we demonstrate theoretically and experimentally that a small, simple, spatially homogeneous dielectric subwavelength sphere with a high refractive index and low losses (as some semiconductors in the visible or near infrared region) exhibits properties allowing to utilize it as a new multifunctional element for the mentioned devices. This can be achieved by taking advantage of the coherent effects between dipolar and multipolar modes, which produce anomalous scattering effects. The effects open a new way to control the directionality of the scattered light. The directional tuning can be obtained in a practical way just by a change in the frequency of the incident wave, and/or by a well-chosen diameter of the sphere. Dielectric nanoparticles with the required optical properties in the VIS-NIR may be now readily fabricated. These particles could be an efficient alternative to the widely discussed scattering units with a more complicated design.

On the Calibration of a Multistatic Scattering Matrix Measured by a Fixed Circular Array of Antennas

The calibration of the multistatic scattering matrix plays an important part in the construction of a quantitative microwave imaging system. For scattering measurement applications, the calibration must be performed on the amplitude and on the phase of the flelds of interest. When the antennas are not completely identical, as for example with a multiplexed antennas array, a speciflc calibration procedure must be constructed. In the present work, we explain how a complex calibration matrix can be deflned which takes advantage of the geometrical organization of the antennas. Indeed, for arrays of antennas positioned on a circle, the inherent symmetries of the conflguration can be fully exploited by means of an adequate reorganization of the multistatic scattering matrix. In addition, the reorganization permits to detect antenna pairs which are not properly functioning and to estimate the signal-to-noise ratio. Experimental results obtained within a cylindrical cavity enclosed by a metallic casing are provided to assess the performance of the proposed calibration procedure.This calibration protocol, which is described here in detail, has already been applied to provide quantitative images of dielectric targets (1,2).

Aperture Antenna Modeling by a Finite Number of Elemental Dipoles From Spherical Field Measurements

A method to determine a distribution of a finite number of elementary dipoles that reproduce the radiation behavior of the antenna under test (AUT) from truncated spherical field measurements is proposed. It is based on the substitution of the actual antenna by a finite number of equivalent infinitesimal dipoles (electric and magnetic), distributed over the antenna aperture. This equivalent set of elementary dipoles is optimized using the transmission coefficient involving the spherical wave expansion of the measured field and using an appropriate matching method. Once the current excitation of each dipole is known, the radiated field of the antenna at different distances can be rapidly determined. Moreover, using an iterative simplification procedure, the number of equivalent dipoles is reduced, which eases the implementation of the antenna equivalent model in any existing electromagnetic code. The feasibility, the reliability and the accuracy of the method are shown using experimental data issued from the measurement of an X-band horn antenna, in two different measurement setups.

3D-Aggregate Quantitative Imaging: Experimental Results and Polarization Effects

We present reconstructions of an aggregate of small spheres from experimental scattered fields using a working frequency of 18 GHz. This target presents at the same time a complex 3D shape and a low-contrast permittivity. Concerted experimental and numerical efforts have enabled to obtain accurate reconstructions. In particular, we took into account the real random noise via a Bayesian framework. Reconstructions have been realized with scattered fields measured in different polarization cases: the results are compared and discussed.

Measurement strategies for a confined microwave circular scanner

This article deals with the inverse scattering problem from scattered field data measured inside a closed microwave scanner. This system is presently being developed to demonstrate the potentiality of a non-invasive microwave imaging system for volumetric water content monitoring. The final goal is to retrieve soil moisture information as it is an important parameter for understanding fluid flow modelling, as well as water uptake by plants roots. Based on the actual state of the setup, we are proposing appropriate numerical tools, in particular a finite element formalism combined with a Lagrangian minimization scheme to provide a fast and accurate imaging tool. We will also show how we can improve the reconstruction algorithms by changing in a very simpler manner the measurement configuration, using either off-centred information or impedance boundary matching environment.

Microwave analog to light scattering measurements on a fully characterized complex aggregate

We present experimental measurements of three-dimensional electromagnetic wave scattering in the microwave frequency range, by a complex aggregate consisting of 74 primary spheres with fully known optical and geometrical properties. We measured the complete amplitude scattering matrix (or Jones matrix), from which the electric fields (amplitude and phase) with arbitrary polarization can be obtained. These results offer the opportunity to test approximate computational methods against experiments.

Complex Permittivity Determination From Far-Field Scattering Patterns

An accurate knowledge of the complex permittivity value of materials is compulsory when performing experimental electromagnetic applications. Unfortunately, these values are not so obvious to determine in practice. In this letter, we propose a novel approach for determining the complex dielectric constant of materials. This method combines free-space far-field scattering pattern measurements with a Bayesian procedure, which fully exploits the measurement uncertainties. Therefore, the measured values weighted according to their experimental accuracy are incorporated in the permittivity determination algorithm. In this letter, the samples are all shaped as spheres in order to benefit from efficient Mie scattered field computations. The dielectric properties of typical plastic samples are first determined and compared to values found in the literature in order to assess the validity and the accuracy of the proposed methodology. A more “exotic” sample extracted from a microwave absorber, which is a polyurethane foam charged with carbon particles, is also analyzed.

Electromagnetic polarization-controlled perfect switching effect with high-refractive-index dimers and the beam-splitter configuration.

Sub-wavelength particles made from high-index dielectrics, either individual or as ensembles, are ideal candidates for multifunctional elements in optical devices. Their directionality effects are traditionally analysed through forward and backward measurements, even if these directions are not convenient for in-plane scattering practical purposes. Here we present unambiguous experimental evidence in the microwave range that for a dimer of HRI spherical particles, a perfect switching effect is observed out of those directions as a consequence of the mutual particle electric/magnetic interaction. The binary state depends on the excitation polarization. Its analysis is performed through the linear polarization degree of scattered radiation at a detection direction perpendicular to the incident direction: the beam-splitter configuration. The scaling property of Maxwells equations allows the generalization of our results to other frequency ranges and dimension scales, for instance, the visible and the nanometric scale.