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Opto-electronic metamaterials

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Introduction :
Metamaterials are artificial composite media for electromagnetic waves. They consist of assemblies of inclusions that are much smaller than the wavelength (or, equivalently, the photon size), so that a wave passing through these unit cells does not “see” their individual contribution, but experiences a macroscopic, averaged response from the whole structure. Such interactions are similar to those occurring with the atoms of natural substances—except that the properties of metamaterials are not solely defined by their chemical composition, but also in large part by the geometry of their subwavelength unit cells. It is this geometrical dependence that makes metamaterials so interesting because the shape and size of their inner structure can be tailored at will to produce properties that are very difficult (and even sometimes impossible) to achieve with standard materials.

Optoelectronic devices :
In our group, we are interested in extending the metamaterial approach to optoelectronic components and devices operating in the near-infrared and visible part of the spectrum. Among others, we develop electroluminescent metamaterials based on metal nano-inclusions hybridized with colloidal quantum dots and use this approach to weave intricate light-emitting surfaces [1,2].

a) Sketch of a metamaterial LED. The typical layer thicknesses are 90 nm for Al, 65 nm for TiO2, 25 nm for the Au nanoparticles, 15 nm ( 2 monolayers) for the PbS CQDs, 10 nm for MoOx and 90 nm for ITO. Light is emitted through the top ITO transparent electrode. b) Schematics illustrating the operation principle of the devices. The drawings show a top view of the active layer (the CQDs are the grey dots and the gold nanorods are the yellow rectangles). The light emission is schematically represented by the red patches. Each metal inclusion and the dots in its immediate proximity form an artificial luminescent pixel of nanoscale dimensions. These pixels can be independently tuned to have independent properties. Due to the subwavelength period of the nanoparticle arrays, light in the far field seems to come from a homogeneous active medium.

Physics of the artificial composites :
In our devices, the emitters are directly touching the metallic inclusions, which, according to conventional wisdom, should lead to overwhelming quenching. This is however not the case—in fact, two unintuitive features stand out :
1) The emitters are not quenched by the metal, even if the latter is a non-plasmonic and very lossy material such as Pt. In fact, the radiation efficiency is maximum when the emitters are touching the metal [1-3].
2) The emitters do not only emit light via the recombination of the band-edge exciton, but also through other transitions above the bandgap that can be triggered by the metal particle geometry [3].
These observations are a consequence of light-matter interactions different from the usual Purcell effect. In ref. [3], we have shown that the physics of our devices is governed by a local form of Kirchhoff law (a generalization of the standard Kirchhoff law to highly inhomogeneous media recently introduced by Prof. Jean-Jacques Greffet from Laboratoire Charles Fabry). This finding has important consequences not only for the design of future optoelectronic metamaterials, but also for the field of nano-optics in general for which the existence of another interaction regime besides the Purcell effect represents an unexpected avenue with vast potential.

Funding :
ANR Générique GYN, coordinator J.-P. Hermier, Professor at UVSQ (2018-2022) : development of new sources of visible light.
ERC CoG FORWARD (2018-2023) : development of optoelectronic metamaterials for emitting, manipulating and detecting complex forms of infrared light.

Recent Publications
[1] Q. Le-Van, X. Le Roux, A. Aassime and A. Degiron, "Electrically driven optical metamaterials," Nature Communications 7, 12017 (2016).
[2] H. Wang, Q. Le-Van, A. Aassime, X. Le Roux, F. Charra, N. Chauvin and A. Degiron, “Electroluminescence of colloidal quantum dots in electrical contact with metallic nanoparticles,” Adv. Opt. Mater. 6, 1700658, (2018).
[3] H. Wang, A. Aassime, X. Le Roux, N.J. Schilder, J.-J. Greffet and A. Degiron, “Revisiting the Role of Metallic Antennas to Control Light Emission by Lead Salt Nanocrystal Assemblies,” Phys. Rev. Applied 10, 034042 (2018).