Enhanced Light Trapping in GaAs/TiO2-Based Photocathodes for Hydrogen Production.

Photoelectrochemical cells (PEC) are appealing devices for the production of renewable energy carriers. In this context, III-V semiconductors such as GaAs are very promising materials due to their tunable band gaps, which can be appropriately adjusted for sunlight harvesting. Because of the high cost of these semiconductors, the nanostructuring of the photoactive layer can help to improve the device efficiency as well as drastically reduce the amount of material needed. III-V nanowire-based photoelectrodes benefit from the intrinsically high aspect ratio of nanowires, their enhanced ability to trap light, and their improved charge separation and collection abilities and thus are particularly attractive for PECs. However, III-V semiconductors often suffer from corrosion in aqueous electrolytes, preventing their utilization over long periods under relevant working conditions. Here, photocathodes of GaAs nanowires protected with thin TiO2 shells were prepared and studied under simulated sunlight irradiation to assess their photoelectrochemical performances in correlation with their structural degradation, highlighting the advantageous nanowire geometry compared to its thin-film counterpart. Morphological and electronic parameters, such as the aspect ratio of the nanowires and their doping pattern, were found to strongly influence the photocatalytic performances of the system. This work highlights the advantageous combination of nanowires featuring a buried radial p-n junction with Co nanoparticles used as a hydrogen evolution catalyst. The nanostructured photocathodes exhibit significant photocatalytic activities comparable with previous noble-metal-based systems. This study demonstrates the potential of a GaAs nanostructured semiconductor and its reliable use for photodriven hydrogen production.

Taming Friedrich-Wintgen Interference in a Resonant Metasurface: Vortex Laser Emitting at an On-Demand Tilted Angle.

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 a mechanism makes destructive interference possible for scenarios 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 with polarization singularity at on-demand wavevectors in an optical metasurface. As a proof-of-concept, using hybrid organic-inorganic halide perovskite as an active material, we empower this novel polarization singularity to obtain lasing emission, exhibiting both highly directional emission at oblique angles and a polarization vortex in momentum space. Our results pave the way to steerable coherent emission with a tailored polarization pattern for applications in optical communication/manipulation in free space, high-resolution imaging/focusing, and data storage.

Photonic crystal backbone for light trapping inside an ultrathin, low absorbing layer.

A few tens of nanometre thick ultrathin materials may suffer from a very low absorption at their band edges. In this work, we investigate a photonic crystal (PC) made of a lowcost, transparent patterned silicon nitride (SiNx) layer, conformally covered with an ultrathin active layer (e.g., 20 nm TiO2) in view of its use in various applications such as photocatalysis. A fair estimation of the absorption enhancement, considering the volume of the active material, is calculated using RCWA. A remarkable enhancement (more than ten-folds) in absorptance in the near UV range and a very high transmittance over the visible range are observed. A detailed modal analysis of the structures-of-interest unravels the Light Trapping (LT) mechanisms and allows the derivation of key design guidelines. Optical measurements on a patterned sample provide a first proof-of-concept of such possible photonic backbone structures suitable for highly efficient depollution and artificial photosynthesis for solar fuels production.

Enhanced light-extraction efficiency and emission directivity in compact photonic-crystal based AlGaInP thin-films for color conversion applications.

We investigated the use of photonic crystals with different opto-geometrical parameters for light extraction from AlGaInP/InGaP MQW color converters. Blue-to-red and green-to-red color conversions were demonstrated using room-temperature photoluminescence with excitation wavelengths at 405nm and 514nm. Complete, compact and highly directional light extraction was demonstrated. 3D-FDTD and a herein-developed phenomenological model derived from the standard coupled-mode theory were used to analyze the results. The highest light extraction gains were ∼8 times better than unpatterned reference structures, which were paired with short extraction lengths (between 2µm and 6µm depending on the acceptance angle) and directional light emission for square lattice of nanopillars with a lattice period of 400nm. The design guidelines set in this work could pave the way for the use of inorganic MQW epi-layer color converters to achieve full color microdisplays on a single wafer.

Tailoring Dispersion of Room-Temperature Exciton-Polaritons with Perovskite-Based Subwavelength Metasurfaces.

Exciton-polaritons represent a promising platform for studying quantum fluids of light and realizing prospective all-optical devices. Here we report on the experimental demonstration of exciton-polaritons at room temperature in resonant metasurfaces made from a sub-wavelength two-dimensional lattice of perovskite pillars. 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, by tailoring the photonic Bloch mode to which perovskite excitons are coupled, polaritonic dispersions are engineered exhibiting linear, parabolic, and multivalley dispersions. 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 toward large-scale and low-cost integrated polaritonic devices operating at room temperature.

Ultrathin Epitaxial Silicon Solar Cells with Inverted Nanopyramid Arrays for Efficient Light Trapping.

Ultrathin c-Si solar cells have the potential to drastically reduce costs by saving raw material while maintaining good efficiencies thanks to the excellent quality of monocrystalline silicon. However, efficient light trapping strategies must be implemented to achieve high short-circuit currents. We report on the fabrication of both planar and patterned ultrathin c-Si solar cells on glass using low temperature (T < 275 °C), low-cost, and scalable techniques. Epitaxial c-Si layers are grown by PECVD at 160 °C and transferred on a glass substrate by anodic bonding and mechanical cleavage. A silver back mirror is combined with a front texturation based on an inverted nanopyramid array fabricated by nanoimprint lithography and wet etching. We demonstrate a short-circuit current density of 25.3 mA/cm(2) for an equivalent thickness of only 2.75 µm. External quantum efficiency (EQE) measurements are in very good agreement with FDTD simulations. We infer an optical path enhancement of 10 in the long wavelength range. A simple propagation model reveals that the low photon escape probability of 25% is the key factor in the light trapping mechanism. The main limitations of our current technology and the potential efficiencies achievable with contact optimization are discussed.

Design rules for net absorption enhancement in pseudo-disordered photonic crystal for thin film solar cells.

The role of pseudo-disordered photonic crystals on the absorption efficiency of simplified thin film crystalline silicon solar cells is presented and discussed. The expected short circuit current can thus be further increased compared to a fully optimized square lattice of holes, thanks to carefully controlled positions of the nanoholes in the considered realistic simplified solar cell stack. In addition, the pseudo-disordered structures are less sensitive to the angle of incidence, especially in the long wavelength range.

Binary coded patterns for photon control using necklace problem concept.

Pseudo-disordered structures enable additional design freedom for photon management. However, the optimization and interpretation is challenging when the large number of degrees of freedom encounters computationally intensive electromagnetic simulation method. Here we propose a novel one-dimensional multi-periodic pattern generation method to help us squeeze the disorder design space before performing rigorous calculation, by making use of the periodic attribute of the pattern. Consequently, thanks to the pre-filtered design space, it typically relieves us from computational burden and enables us to 'globally' optimize and study pseudo-disordered patterns. As an example, we show how this approach can be used to comprehensively optimize and systematically analyze generated disorder for broadband light trapping in thin film.

Photonic crystals and optical mode engineering for thin film photovoltaics.

In this paper, we present the design, analysis, and experimental results on the integration of 2D photonic crystals in thin film photovoltaic solar cells based on hydrogenated amorphous silicon. We introduce an analytical approach based on time domain coupled mode theory to investigate the impact of the photon lifetime and anisotropy of the optical resonances on the absorption efficiency. Specific design rules are derived from this analysis. We also show that, due to the specific properties of the photonic crystal resonances, the angular acceptance of such solar cells is particularly high. Rigorous Coupled Wave Analysis simulations show that the absorption in the a-Si:H active layers, integrated from 300 to 750 nm, is only decreased from 65.7% to 60% while the incidence angle is increased from 0 to 55°. Experimental results confirm the stability of the incident light absorption in the patterned stack, for angles of incidence up to 50°.

Combined front and back diffraction gratings for broad band light trapping in thin film solar cell.

In this paper, we present the integration of combined front and back 1D and 2D diffraction gratings with different periods, within thin film photovoltaic solar cells based on crystalline silicon layers. The grating structures have been designed considering both the need for incident light absorption enhancement and the technological feasibility. Long wavelength absorption is increased thanks to the long period (750 nm) back grating, while the incident light reflection is reduced by using a short period (250 nm) front grating. The simulated short circuit current in a solar cell combining a front and a back grating structures with a 1.2 µm thick c-Si layer, together with the back electrode and TCO layers, is increased up to 30.3 mA/cm2, compared to 18.4 mA/cm2 for a reference stack, as simulated using the AM1.5G solar spectrum intensity distribution from 300 nm to 1100 nm, and under normal incidence.

Design, fabrication and optical characterization of photonic crystal assisted thin film monocrystalline-silicon solar cells.

In this paper, we present the integration of an absorbing photonic crystal within a monocrystalline silicon thin film photovoltaic stack fabricated without epitaxy. Finite difference time domain optical simulations are performed in order to design one- and two-dimensional photonic crystals to assist crystalline silicon solar cells. The simulations show that the 1D and 2D patterned solar cell stacks would have an increased integrated absorption in the crystalline silicon layer would increase of respectively 38% and 50%, when compared to a similar but unpatterned stack, in the whole wavelength range between 300 nm and 1100 nm. In order to fabricate such patterned stacks, we developed an effective set of processes based on laser holographic lithography, reactive ion etching and inductively coupled plasma etching. Optical measurements performed on the patterned stacks highlight the significant absorption increase achieved in the whole wavelength range of interest, as expected by simulation. Moreover, we show that with this design, the angle of incidence has almost no influence on the absorption for angles as high as around 60°.

Absorbing one-dimensional planar photonic crystal for amorphous silicon solar cell.

We report on the absorption of a 100nm thick hydrogenated amorphous silicon layer patterned as a planar photonic crystal (PPC), using laser holography and reactive ion etching. Compared to an unpatterned layer, electromagnetic simulation and optical measurements both show a 50% increase of the absorption over the 0.38-0.75micron spectral range, in the case of a one-dimensional PPC. Such absorbing photonic crystals, combined with transparent and conductive layers, may be at the basis of new photovoltaic solar cells.

Absorption enhancement using photonic crystals for silicon thin film solar cells.

We propose a design that increases significantly the absorption of a thin layer of absorbing material such as amorphous silicon. This is achieved by patterning a one-dimensional photonic crystal (1DPC) in this layer. Indeed, by coupling the incident light into slow Bloch modes of the 1DPC, we can control the photon lifetime and then, enhance the absorption integrated over the whole solar spectrum. Optimal parameters of the 1DPC maximize the integrated absorption in the wavelength range of interest, up to 45% in both S and P polarization states instead of 33% for the unpatterned, 100 nm thick amorphous silicon layer. Moreover, the absorption is tolerant with respect to fabrication errors, and remains relatively stable if the angle of incidence is changed.

Directional channel-drop filter based on a slow Bloch mode photonic crystal waveguide section.

A two-dimensional photonic crystal channel-drop filter is proposed. This device has two high group velocity waveguides that are selectively coupled by a single, low group velocity intermediate waveguide section. It exhibits computed quality factors as high as 1300, and directional dropping efficiencies as high as 90%.

Theoretical study of amplitude and phase filtering of guided waves.

The design of integrated optics filters by use of refinement software based on the Abelès thin-film computation method and the film mode matching method is studied. The results obtained with the two computation methods are compared. Good agreement is obtained provided that the fill factor of the guided mode in the component is high and that modal losses between waveguide sections are simulated by absorption with the Abelès computation method. Integrated optics devices that manage either the amplitude of guided waves such as a dense wavelength division multiplexing narrow-bandpass filter and a gain-flattening filter or the phase of guided waves such as a broadband dispersion compensator are

Deltan/deltaT measurements performed with guided waves and their application to the temperature sensitivity of wavelength-division multiplexing filters.

Measurements of deltan/deltaT of thin films by the m-lines technique are presented. The importance of the substrate material is shown. An example of the wavelength shift of an optical thin-film filter with temperature is studied both theoretically and experimentally. The theoretical wavelength shift of a dense wavelength-division multiplexing filter is discussed.