Thomas Lepetit

Experimental measurement of negative index in an all-dielectric metamaterial

We designed and fabricated an all-dielectric metamaterial which exhibits negative index of refraction. This metamaterial consists of one layer made up of two sets of high permittivity resonators. We experimentally showed that this metamaterial exhibits resonant effective permeability and permittivity issued from the first two modes of Mie resonances, respectively, and that matching both these parameters results in negative effective index.

Coevaporated KInSe 2 : A Fast Alternative to KF Postdeposition Treatment in High-Efficiency Cu(In,Ga)Se 2 Thin Film Solar Cells

Cu(In,Ga)Se2-based thin film solar cells have reached 22.3% energy conversion efficiency. This outstanding level of performance has been made possible by the use of a so-called potassium fluoride postdeposition treatment (KF-PDT) after the absorber synthesis. Such a treatment, consisting of evaporating KF under Se atmosphere, has been suggested to enhance the formation of a KInSe2 surface layer. In this paper, we propose an alternative to the standard KF-PDT, in which we simultaneously evaporate KF and In under Se atmosphere. We compare by X-ray photoemission spectroscopy the modified absorber surfaces in both cases and discuss the advantages of this alternative in terms of robustness and rapidity of the process.

Experimental evidence of resonant effective permittivity in a dielectric metamaterial

We designed and fabricated a dielectric metamaterial consisting of one set of high permittivity resonators. We experimentally showed that this metamaterial exhibits resonant effective permittivity, issued from the second mode of Mie resonances of these resonators. Therefore, metamaterials consisting of high permittivity resonators may provide either effective permeability or permittivity issued from the first two modes of Mie resonances, respectively. This ensures that all-dielectric metamaterials involving two sets of matched high permittivity resonators may exhibit negative index issued from these two modes, notably up to the terahertz domain.

Incidence dependence of negative index in asymmetric cut wire pairs metamaterials

Metamaterials made of asymmetric cut wire pairs have experimentally demonstrated a negative refractive index at microwave frequencies. In this letter, we begin by presenting the analogy between asymmetric cut wire pairs and S-shaped metamaterials by a simple unifying approach. Then, using simulations and experiments in the microwave domain, we investigate the dependence of resonances and retrieved effective index on the incident angle in asymmetric cut wire pairs. While it is found that resonances shift in frequency with increasing oblique incidence in the E-plane, it is shown that the structure is angle-independent in the H-plane.

High excitation photoluminescence effects as a probing tool for the growth of Cu(In,Ga)Se2

Copper Indium Gallium deSelenide (Cu(In,Ga)Se2, CIGS) is a promising material for cost-efficient solar cells. Efficiencies above 20% have already been demonstrated in laboratory, and large area CIGS solar panels are already on the market. However, it is still an interesting issue to find efficient characterization techniques that can be used to validate the quality of the different layers at any step of the process, without having to process a complete cell and measure its electrical properties. In this work, we have deposited CIGS onto Mo coated soda lime glass by co-evaporation, using the so-called three step deposition process. Then, photoluminescence (PL) measurements were made on the samples, in the range of 10K to the room temperature, and the excitation intensity was varied in a very large range, in order to reach non-linear regime. We report the first observation of stimulated emission in mechanisms are discussed. The threshold at which sample photoluminescence changes from spontaneous to stimulated is well known to be sensitive to overall sample quality, and we propose to use this measurement as a probing tool for sample quality. This opens an interesting perspective for characterization of CIGS during solar cell processing.

Sharp fano resonances in Bi-layered symmetric Z-structures

We introduce a novel class of dark modes for electromagnetically induced transparency. These dark modes depend on both near-field and far-field coupling between resonators. As an example, a planar metasurface composed of a bilayer of metallic Z-shaped meta-atoms is investigated. Performed simulations and measurements show the appearance of sharp Fano resonances.