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Exciton-polaritons, hybrid light-matter elementary excitations arising from the strong coupling regime between excitons in semiconductors and photons in photonic nanostructures, offer a fruitful playground to explore the physics of quantum fluids of light as well as to develop all-optical devices. However, achieving room temperature propagation of polaritons with a large excitonic fraction, which would be crucial, e.g., for nonlinear light transport in prospective devices, remains a significant challenge. } Here we report on experimental studies of exciton-polariton propagation at room temperature in resonant metasurfaces made from a sub-wavelength lattice of perovskite pillars. Thanks to the large Rabi splitting, an order of magnitude larger than the optical phonon energy, the lower polariton band is completely decoupled from the phonon bath of perovskite crystals. The long lifetime of these cooled polaritons, in combination with the high group velocity achieved through the metasurface design, enables long-range propagation regardless of the polariton excitonic fraction. Remarkably, we observed propagation distances exceeding hundreds of micrometers at room temperature, even when the polaritons possess a very high excitonic component, approximately {80}$\%$. Furthermore, the design of the metasurface introduces an original mechanism for directing uni-directional propagation through polarization control. This discovery of a ballistic propagation mode, leveraging high-speed cooled polaritons, heralds a promising avenue for the development of advanced polaritonic devices.

Exciton-polaritons represent a promising platform that combines the strengths of both photonic and electronic systems for future optoelectronic devices. However, their application is currently limited to laboratory research due to the high cost and complexity of fabrication methods, which are not compatible with the mature CMOS technology developed for microelectronics. In this work, we develop an innovative, low-cost, and CMOS-compatible method for fabricating large surface polaritonic devices. This is achieved by direct patterning of a halide-perovskite thin film via thermal nanoimprint. As a result, we observe highly homogeneous polaritonic modes of quality factor $Q\approx 300$ at room temperature across a centimetric scale. Impressively, the process provides high reproducibility and fidelity, as the same mold can be reused more than 10 times to imprint the perovskite layer on different types of substrates. Our results could pave the way for the production of low-cost integrated polaritonic devices operating at room temperature.

Friedrich-Wintgen (FW) interference is an atypical coupling mechanism that grants loss exchange between leaky resonances in non-Hermitian classical and quantum systems. Intriguingly, such an mechanism makes it possible for destructive interference scenario in which a radiating wave becomes a bound state in the continuum (BIC) by giving away all of its losses. Here we propose and demonstrate experimentally an original concept to tailor FW-BICs as polarization singularity at on-demand wavevectors in optical metasurface. As a proof-of-concept, using hybrid organic-inorganic halide perovskite as active material, we empower this novel polarization singularity to obtain lasing emission exhibiting both highly directional emission at oblique angles and polarization vortex in momentum space. Our results pave the way to steerable coherent emission with tailored polarization pattern for applications in optical communication/manipulation in free-space, high-resolution imaging /focusing and data storage.

Light Management (LM) is essential for metal-halide perovskite solar cells in their race for record performance. In this review, criteria on materials, processes and photonic engineering are established such as to enhance mainly the short circuit current density, towards high energy yields. These criteria are used to analyse a large panel of solutions envisaged in the literature for single junction cells. Moreover, a perspective based on rigorous electromagnetic simulations performed on various comparable structures is proposed in order to clarify the conclusions, and to pave the way to further performance enhancement in the case of all-perovskite, two-terminal tandem cells.

We investigate the physics of photonic band structures of the moir\'e patterns that emerged when overlapping two uni-dimensional (1D) photonic crystal slabs with mismatched periods. The band structure of our system is a result of the interplay between intra-layer and inter-layer coupling mechanisms, which can be fine-tuned via the distance separating the two layers. We derive an effective Hamiltonian that captures the essential physics of the system and reproduces all numerical simulations of electromagnetic solutions with high accuracy. Most interestingly, \textit{magic distances} corresponding to the emergence of photonic flatbands within the whole Brillouin zone of the moir\'e superlattice are observed. We demonstrate that these flatband modes are tightly localized within a moir\'e period. Moreover, we suggest a single-band tight-binding model that describes the moir\'e minibands, of which the tunnelling rate can be continuously tuned via the inter-layer strength. Our results show that the band structure of bilayer photonic moir\'e can be engineered in the same fashion as the electronic/excitonic counterparts. It would pave the way to study many-body physics at photonic moir\'e flatbands and novel optoelectronic devices., Comment: 6 pages + Supplement. Comments are welcome!

Exciton-polaritons, elementary excitations arising from the strong coupling regime between photons and excitons in insulators or semiconductors, represent a promising platform for studying quantum fluids of light and realizing prospective all-optical devices. Among different materials for room temperature polaritonic devices, two-dimensional (2D) layered perovskites have recently emerged as one of the promising candidates thanks to their prominent excitonic features at room temperature. Here we report on the experimental demonstration of exciton-polaritons at room temperature in resonant metasurfaces made from a subwavelength 2D lattice of perovskite pillars. These metasurfaces are obtained via spincoating, followed by crystallization of the perovskite solution in a pre-patterned glass backbone. The strong coupling regime is revealed by both angular-resolved reflectivity and photoluminescence measurements, showing anticrossing between photonic modes and the exciton resonance with a Rabi splitting in the 200 meV range. Moreover, we show that the polaritonic dispersion can be engineered by tailoring the photonic Bloch mode to which perovskite excitons are coupled. Linear, parabolic, and multi-valley polaritonic dispersions are experimentally demonstrated. All of our results are perfectly reproduced by both numerical simulations based on a rigorous coupled wave analysis and an elementary model based on a quantum theory of radiation-matter interaction. Our results suggest a new approach to study exciton-polaritons and pave the way towards large-scale and low-cost integrated polaritonic devices operating at room temperature.

We demonstrate experimentally a new type of Bound state In the Continuum (BIC) which is induced by the vertical symmetry breaking of a photonic crystal slab. Such BIC emerges from the hybridization between Bloch resonances of opposite parities once the vertical symmetry is broken, and is observed in the vicinity of the anticrossing point at the lower hybrid band of the energy-momentum diagram. All experimental results are nicely reproduced by numerical simulations, and in good agreement with an analytical model based on Friedrich-Wintgen framework. Together with the possibility of dispersion engineering by vertical symmetry breaking, our results open the way to tailor photonic states in the entire complex plane to study non-hermitian photonics.

The effects resulting from the introduction of a controlled perturbation in a single pattern membrane on its absorption are first studied and then analyzed on the basis of band folding considerations. The interest of this approach for photovoltaic applications is finally demonstrated by overcoming the integrated absorption of an optimized single pattern membrane through the introduction of a proper pseudo disordered perturbation., Comment: MRS Fall Meeting 2013, Boston : United States (2013)

We investigate the specific optical regime occurring at short wavelengths, in the high absorption regime, in silicon thin-films patterned by periodically arranged nano-holes. Near-field scanning optical microscopy indicates that the incoming light is coupled to vertically channelling modes. Optical modelling and simulations show that the light, travelling inside the low-index regions, is absorbed at the direct vicinity of the nano-holes sidewalls. This channelling regime should be taken into account for light management in optoelectronic devices.

The aim of the study is to develop ultra-compact structures enabling an efficient conversion of single high energy photon (UV) to two lower energy photons (IR). The proposed structure combines rare-earths doped thin layer allowing the down-conversion process with a photonic crystal (PhC), in order to control and enhance the down-conversion using optical resonances. On the top of the rare-earths doped layer, a silicon nitride (SiN) 2D planar PhC is synthesized. For that, SiN is first deposited by PECVD. After holographic lithography and reactive ion etching, a periodic square lattice of holes is generated on the SiN layer. The PhC topographical parameters as well as the layers thickness are optimized using Finite-Difference-Time-Domain simulations. The design and realization of such PhC-assisted down-converter structures is presented. Optical simulations demonstrate that the PhC leads to the establishment of resonant modes located in the underneath doped layer, allowing a drastic enhancement of the absorption of the rare-earth ions without disturbing the transmission in the visible and near-IR parts of the spectrum, hence demonstrating the relevance of such an approach., Comment: 28th European Photovoltaic Solar Energy Conference and Exhibition, Paris : France (2013)

The positive effects of various perturbations introduced in a bidimensional photonic-crystal patterned membrane on its integrated absorption are investigated numerically and theoretically. Two phenomena responsible for the enhanced absorption observed are identified: an increase of the spectral density of modes, obtained thanks to folding mechanisms in the reciprocal lattice, and a better coupling of the modes with the incident light. By introducing a proper pseudodisordered pattern, we show that those two effects can be exploited so as to overcome the integrated absorption obtained for an optimized and single pattern unit cell photonic crystal.

In this paper, we propose a method for tailoring the absorption in a photonic crystal membrane. For that purpose, we first applied Time Domain Coupled Mode Theory to such a subwavelength membrane and demonstrated that 100% resonant absorption can be reached even for a symmetric membrane, if degenerate modes are involved. Design rules were then derived from this model in order to tune the absorption. Subsequently, Finite Difference Time Domain simulations were used as a proof of concept and carried out on a low absorbing material (extinction coefficient=10-2) with a high refractive index corresponding to the optical indices of amorphous silicon at around 720 nm. In doing so, 85% resonant absorption was obtained, which is significantly higher than the commonly reported 50% maximum value. Those results were finally analyzed and confronted to theory so as to extend our method to other materials, configurations and applications.

We report on very high enhancement of thin layer's absorption through band-engineering of a photonic crystal structure. We realized amorphous silicon (aSi) photonic crystals, where slow light modes improve absorption efficiency. We show through simulation that an increase of the absorption by a factor of 1.5 is expected for a film of aSi. The proposal is then validated by an experimental demonstration, showing an important increase of the absorption of a layer of aSi over a spectral range of 0.32-0.76 microns., Comment: 4 pages, 5 figures