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New Frontiers for Optoelectronics with Artificial Media

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European Research Council grant FORWARD (reference : 771688)

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.
In the FORWARD project, we are interested in extending the metamaterial approach to optoelectronic components and devices operating in the near-infrared 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. We preferentially work with PbS colloidal quantum dots synthetized by Prof. Emmanuel Lhuillier at Institut des Nanosciences de Paris (CNRS/Sorbonne Université).
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.
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.
These observations are a consequence of light-matter interactions different from the usual Purcell effect. In 2018, we have shown that the physics of our devices was governed by a quasi-equilibrium of the excited photocarriers, leading to 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). More recently, we have identified two regimes of quasi-equilibria in our colloidal quantum dot assemblies, one concerning the excitons only, and the other concerning all the photocarriers [2]. This finding has important consequences for the design of future optoelectronic metamaterials.

Spontaneous Emission of vector beams and vector vortex beams :
Because of Heisenberg’s uncertainty principle, quantum emitters are too small to emit light in well-defined directions. Over the years, overcoming this limitation has become a hallmark of nano-optics. By coupling the emitters to cavities or nano-antennas, it is possible to beam their light in controlled directions of space.
Recently, we have pushed this control to a new level : we have made a luminescent nanocrystal assembly emit vector vortex beams [3], which are swirls of light with a twisting vector structure. This finding represents a significant advance because the construction of vector vortex beams requires coherence properties that are possessed by lasers—but not by photons produced by luminescence. To obtain this result, we have leveraged the local Kirchhoff law described above, by coupling the nanocrystals to holograms capable of imparting the necessary coherence and twist to the emitted light. Vector vortex beams are behind some of the latest advances in bio-imaging, nanomanipulation and telecommunications. The demonstration that they can be generated directly from nanoscale sources, without lasers, expands their potential. We have also demonstrated other forms of complex light such as vector beams with radial polarization [1].
Scanning electron micrographs of plasmonic holograms hybridized with a thin layer of PbS nanocrystal assembly (top panels). These structures have been designed to emit interfering optical vortices. The resulting interference patterns (bottom panels) have distinctive star-shaped features that reveal the phase singularities of the beams (adapted from [3]).

Recent Publications
[1] D. Schanne, S. Suffit, P. Filloux, E. Lhuillier and A. Degiron, “Shaping the spontaneous emission of extended incoherent sources into composite radial vector beams”, Appl. Phys. Lett. 119, 181105 (2021).
[2] A. Caillas, S. Suffit, P. Filloux, E. Lhuillier and A. Degiron, “Identification of Two Regimes of Carrier Thermalization in PbS Nanocrystal Assemblies,” J. Phys. Chem. Lett. 12, 5123–5131 (2021).
[3] D. Schanne, S. Suffit, P. Filloux, E. Lhuillier and A. Degiron, “Spontaneous Emission of Vector Vortex Beams,” Phys. Rev. Applied 14, 064077 (2020).