Propagation characteristics of single and multilayer Ga:ZnO in the epsilon near zero region.

We numerically investigated the propagation characteristics of Ga:ZnO (GZO) thin films embedded in a ZnWO4 background in the epsilon near zero (ENZ) region. We found that, for GZO layer thickness ranging between 2 - 100 nm (∼ 1/600 - 1/12 of ENZ wavelength), such structure supports a novel non-radiating mode with its real part of effective index lower than surrounding refractive index or even less than 1. Such a mode has its dispersion curve lying to the left of the light line in the background region. However, the calculated electromagnetic fields display non-radiating nature contrary to the Berreman mode, because the transverse component of the wave vector is complex, ensuring a decaying field. Furthermore, while the considered structure supports confined and highly lossy TM modes in the ENZ region, no TE mode is supported. Subsequently, we studied the propagation characteristics of a multilayer structure constituting an array of GZO layers in the ZnWO4 matrix considering the modal field's excitation using the end-fire coupling. Such a multilayer structure is analyzed using high-precision rigorous coupled-wave analysis and shows strong polarization selective and resonant absorption/emission, the spectral location and bandwidth of which can be tuned by judiciously selecting the thickness of the GZO layer and other geometrical parameters.

Formamidinium Incorporation into Compact Lead Iodide for Low Band Gap Perovskite Solar Cells with Open-Circuit Voltage Approaching the Radiative Limit.

To bring hybrid lead halide perovskite solar cells toward the Shockley-Queisser limit requires lowering the band gap while simultaneously increasing the open-circuit voltage. This, to some extent divergent objective, may demand the use of large cations to obtain a perovskite with larger lattice parameter together with a large crystal size to minimize interface nonradiative recombination. When applying the two-step method for a better crystal control, it is rather challenging to fabricate perovskites with FA+ cations, given the small penetration depth of such large ions into a compact PbI2 film. In here, to successfully incorporate such large cations, we used a high-concentration solution of the organic precursor containing small Cl- anions achieving, via a solvent annealing-controlled dissolution-recrystallization, larger than 1 µm perovskite crystals in a solar cell. This solar cell, with a largely increased fluorescence quantum yield, exhibited an open-circuit voltage equivalent to 93% of the corresponding radiative limit one. This, together with the low band gap achieved (1.53 eV), makes the fabricated perovskite cell one of the closest to the Shockley-Queisser optimum.

Optical management for efficiency enhancement in hybrid organic-inorganic lead halide perovskite solar cells.

In a few years only, solar cells using hybrid organic-inorganic lead halide perovskites as optical absorber have reached record photovoltaic energy conversion efficiencies above 20%. To reach and overcome such values, it is required to tailor both the electrical and optical properties of the device. For a given efficient device, optical optimization overtakes electrical one. Here, we provide a synthetic review of recent works reporting or proposing so-called optical management approaches for improving the efficiency of perovskite solar cells, including the use of anti-reflection coatings at the front substrate surface, the design of optical cavities integrated within the device, the incorporation of plasmonic or dielectric nanostructures into the different layers of the device and the structuration of its internal interfaces. We finally give as outlooks some insights into the less-explored management of the perovskite fluorescence and its potential for enhancing the cell efficiency.

Mid-to-far infrared tunable perfect absorption by a sub - λ/100 nanofilm in a fractal phasor resonant cavity.

Integrating an absorbing thin film into a resonant cavity is the most practical way to achieve perfect absorption of light at a selected wavelength in the mid-to-far infrared, as required to target blackbody radiation or molecular fingerprints. The cavity is designed to resonate and enable perfect absorption in the film at the chosen wavelength λ. However, in current state-of-the-art designs, a still large absorbing film thickness (∼λ/50) is needed and tuning the perfect absorption wavelength over a broad range requires changing the cavity materials. Here, we introduce a new resonant cavity concept to achieve perfect absorption of infrared light in much thinner and thus, really nanoscale films, with a broad wavelength tenability by using a single set of cavity materials. It requires a nanofilm with giant refractive index and small extinction coefficient (found in emerging semi-metals, semi-conductors and topological insulators) backed by a transparent spacer and a metal mirror. The nanofilm acts both as absorber and multiple reflector for the internal cavity waves, which after escaping follow a fractal phasor trajectory. This enables a totally destructive optical interference for a nanofilm thickness more than 2 orders of magnitude smaller than λ. With this remarkable effect, we demonstrate angle-insensitive perfect absorption in sub - λ/100 bismuth nanofilms, at a wavelength tunable from 3 to 20 µm.

Temperature and atmosphere tunability of the nanoplasmonic resonance of a volumetric eutectic-based Bi2O3-Ag metamaterial.

Nanoplasmonic materials are intensively studied due to the advantages they bring in various applied fields such as photonics, optoelectronics, photovoltaics and medicine. However, their large-scale fabrication and tunability are still a challenge. One of the promising ways of combining these two is to use the self-organization mechanism and after-growth engineering as annealing for tuning the properties. This paper reports the development of a bulk nanoplasmonic, Bi2O3-Ag eutectic-based metamaterial with a tunable plasmonic resonance between orange and green wavelengths. The material, obtained by a simple growth technique, exhibits a silver nanoparticle-related localized surface plasmon resonance (LSPR) in the visible wavelength range. We demonstrate the tunability of the LSPR (spectral position, width and intensity) as a function of the annealing temperature, time and the atmosphere. The critical role of the annealing atmosphere is underlined, annealing in vacuum being the most effective option for a broad control of the LSPR. The various potential mechanisms responsible for tuning the localized surface plasmon resonance upon annealing are discussed in relation to the nanostructures of the obtained materials.

Annealing Effect on the Structural and Optical Properties of Sputter-Grown Bismuth Titanium Oxide Thin Films.

The aim of this work is to assess the evolution of the structural and optical properties of BixTiyOz films grown by rf magnetron sputtering upon post-deposition annealing treatments in order to obtain good quality films with large grain size, low defect density and high refractive index similar to that of single crystals. Films with thickness in the range of 220-250 nm have been successfully grown. After annealing treatment at 600 °C the films show excellent transparency and full crystallization. It is shown that to achieve larger crystallite sizes, up to 17 nm, it is better to carry the annealing under dry air than under oxygen atmosphere, probably because the nucleation rate is reduced. The refractive index of the films is similar under both atmospheres and it is very high (n =2.5 at 589 nm). However it is still slightly lower than that of the single crystal value due to the polycrystalline morphology of the thin films.

Excitation transfer mechanism along the visible to the Near-IR in rhodamine J-heteroaggregates.

An enhanced fluorescent emission of the dye Rhodamine 800 in the Near-IR is observed in the presence of other xanthene dye molecules (RhX) when they are hosted in different matrices due to the formation of a new type of fluorescent J-heteroaggregates. This enhanced emission of the acceptor occurs despite the low spectral overlapping and the low quantum yield of Rh800.

Optical and structural properties of Ag-Si3N4 nanocermets prepared by means of ion-beam sputtering in alternate and codeposition modes.

Ion-beam sputtering deposition has been used in two ways, as granular multilayers and as cosputtered film, to elaborate Ag-Si3N4 nanocermets. Multilayer deposition creates slightly oblate clusters, and cosputtering produces two cluster families: elongated clusters within the Si3N4 matrix and larger ones at the film surface. The transmittance spectra of these nanocermets are characterized by a surface plasmon resonance. In the reported research the position of this resonance is related to the morphological properties of silver nanoclusters, which are studied by transmission-electron microscopy, grazing-incidence small-angle x-ray scattering, and atomic-force microscopy.