Ultra-thin sputter-deposited infrared rugate mirror for enhancing solar-to-thermal energy conversion.

A dielectric mirror with high infrared reflection and high visible transmission, based on an easily fabricated stepped index rugate filter structure, is presented. Its fabrication involves sputtering depositions, using only two targets, to make five different material compositions. The ultra-wide reflection band is tunable in both position and width, adapting the thickness of the layers and eventually introducing chirped layers.

3D Hybrid Trilayer Heterostructure: Tunable Au Nanorods and Optical Properties.

Engineering plasmonic nanostructures from three dimensions (3D) is very attractive toward controllable and tunable nanophotonic components and devices. Herein, Au-based trilayer heterostructures composed of a dielectric spacer sandwiched by hybrid Au-TiN vertically aligned nanocomposite (VAN) nanoplasmonic claddings are demonstrated with a broad range of geometries and property tuning. Two types of spacer layers, that is, a pure dielectric BaTiO layer and a hybrid plasmonic Au-BaTiO VAN layer, contribute to the tuning of the Au nanorod dimension.

Feature issue introduction: Optical Devices and Materials for Solar Energy and Solid-state Lighting (PVLED) 2019.

This special feature issue of Optics Express highlights contributions from authors who presented their latest research in the Optical Devices and Materials for Solar Energy and Solid-state Lighting (PVLED) topical meeting of the OSA Advanced Photonics Congress, held in Burlingame, California, from 29 July - August 1, 2019. This feature issue is comprised of nine contributed papers, expanding upon their respective conference proceedings to cover timely research topics applying optics and photonics to solar energy and solid-state lighting.

Light, Energy and the Environment, 2018: introduction to the joint feature issue.

This joint feature issue of Optics Express and Applied Optics highlights contributions from authors who presented their latest research at the OSA Light, Energy and the Environment Congress, held in Sentosa Island, Singapore from 5 to 8 November 2018. The joint feature issue comprises 11 contributed papers, which expand upon their respective conference proceedings. The published papers introduced here cover a broad range of timely research topics in optics and photonics for lighting and illumination, solar energy, hyperspectral imaging, and environmental sensing.

Light, Energy and the Environment, 2018: introduction to the joint feature issue.

This joint feature issue of Optics Express and Applied Optics highlights contributions from authors who presented their latest research at the OSA Light, Energy and the Environment Congress, held in Sentosa Island, Singapore from 5-8 November 2018. The joint feature issue comprises 11 contributed papers, which expand upon their respective conference proceedings. The published papers introduced here cover a broad range of timely research topics in optics and photonics for lighting and illumination, solar energy, hyperspectral imaging, and environmental sensing.

Radiative cooling for low-bandgap photovoltaics under concentrated sunlight.

Radiative cooling is a uniquely compact and passive cooling mechanism. Significant applications can be found in energy generation, particularly concentrating photovoltaics (CPV) and thermophotovoltaics (TPV). Both rely on low-bandgap PV cells that experience significant reductions in performance and lifetime when operating at elevated temperatures.

Scalable Nanoshaping of Hierarchical Metallic Patterns with Multiplex Laser Shock Imprinting Using Soft Optical Disks.

Large-area patterning of metals in nanoscale has always been a challenge. Traditional microfabrication processes involve many high-cost steps, including etching and high-vacuum deposit, which limit the development of functional nanostructures, especially multiscale metallic patterns. Here, multiplex laser shock imprinting (MLSI) process is introduced to directly manufacture hierarchical micro/nanopatterns at a high strain rate on metallic surfaces using soft optical disks with 1D periodic trenches as molds.

Sesame: a 2-dimensional solar cell modeling tool.

This work introduces a new software package "Sesame" for the numerical computation of classical semiconductor equations. It supports 1 and 2-dimensional systems and provides tools to easily implement extended defects such as grain boundaries or sample surfaces. Sesame is designed to facilitate fast exploration of the system parameter space and to visualize local charge transport properties.

Thermophotovoltaics with spectral and angular selective doped-oxide thermal emitters.

Deliberate control of thermal emission properties using nanophotonics has improved a number of applications including thermophotovoltaics (TPV), radiative cooling and infrared spectroscopy. In this work, we study the effect of simultaneous control of angular and spectral properties of thermal emitters on the efficiencies of TPV systems. While spectral selectivity reduces sub-bandgap losses, angular selectivity is expected to enhance view factors at larger separation distances and hence to provide flexibilities in cooling the photovoltaic converter.

Directing solar photons to sustainably meet food, energy, and water needs.

As we approach a "Full Earth" of over ten billion people within the next century, unprecedented demands will be placed on food, energy and water (FEW) supplies. The grand challenge before us is to sustainably meet humanity's FEW needs using scarcer resources. To overcome this challenge, we propose the utilization of the entire solar spectrum by redirecting solar photons to maximize FEW production from a given land area.

Modeling and designing multilayer 2D perovskite / silicon bifacial tandem photovoltaics for high efficiencies and long-term stability.

A key challenge in photovoltaics today is to develop cell technologies with both higher efficiencies and lower fabrication costs than incumbent crystalline silicon (c-Si) single-junction cells. While tandem cells have higher efficiencies than c-Si alone, it is generally challenging to find a low-cost, high-performance material to pair with c-Si. However, the recent emergence of 22% efficient perovskite photovoltaics has created a tremendous opportunity for high-performance, low-cost perovskite / crystalline silicon tandem photovoltaic cells.

Tailoring high-temperature radiation and the resurrection of the incandescent source.

In solar cells, the mismatch between the Sun's emission spectrum and the cells' absorption profile limits the efficiency of such devices, while in incandescent light bulbs, most of the energy is lost as heat. One way to avoid the waste of a large fraction of the radiation emitted from hot objects is to tailor the thermal emission spectrum according to the desired application. This strategy has been successfully applied to photonic-crystal emitters at moderate temperatures, but is exceedingly difficult for hot emitters (>1,000 K).

Control of radiative processes for energy conversion and harvesting.

We review recent advances in the fundamental understanding and technological applications of radiative processes for energy harvesting, conversion, efficiency, and sustainability. State-of-the-art and remaining challenges are discussed, together with the latest developments outlined in the papers comprising this focus issue. The topics range from the fundamentals of the thermal emission manipulation in the far and near field, to applications in radiative cooling, thermophotovoltaics, thermal rectification, and novel approaches to photon detection and conversion.

High-efficiency cascaded up and down conversion in nonlinear Kerr cavities.

In this work, we derive general conditions to achieve high efficiency cascaded third harmonic generation and three photon parametric down conversion in Kerr nonlinear resonant cavities. We employ the general yet rapid temporal coupled-mode method, previously shown to accurately predict electromagnetic conversion processes in the time domain. In our study, we find that high-efficiency cascaded third harmonic generation can be achieved in a triply resonant cavity.

Time-domain simulations of nonlinear interaction in microring resonators using finite-difference and coupled mode techniques.

Nonlinear interactions within compact, on-chip microring resonant cavities is a topic of increasing interest in current silicon photonics research. Frequency combs, one of the emerging nonlinear applications in microring optics, offers great potential from both scientific and practical perspectives. However, the mechanisms of comb formation appear to differ from traditional frequency combs formed by pulsed femtosecond lasers, and thus require detailed elucidation through theory and simulation.

Enhanced light trapping in solar cells with a meta-mirror following generalized Snell's law.

As the performance of photovoltaic cells approaches the Shockley-Queisser limit, appropriate schemes are needed to minimize the losses without compromising the current performance. In this paper we propose a planar absorber-mirror light trapping structure where a conventional mirror is replaced by a meta-mirror with asymmetric light scattering properties. The meta-mirror is tailored to have reflection in asymmetric modes that stay outside the escape cone of the dielectric, hence trapping light with unit probability.

Performance-limiting factors for GaAs-based single nanowire photovoltaics.

GaAs nanowires (NWs) offer the possibility of decoupling light absorption from charge transport for high-performance photovoltaic (PV) devices. However, it is still an open question as to whether these devices can exceed the Shockley-Queisser efficiency limit for single-junction PV. In this work, single standing GaAs-based nanowire solar cells in both radial and vertical junction configurations is analyzed and compared to a planar thin-film design.

Performance-limiting factors for GaAs-based single nanowire photovoltaics.

GaAs nanowires (NWs) offer the possibility of decoupling light absorption from charge transport for high-performance photovoltaic (PV) devices. However, it is still an open question as to whether these devices can exceed the Shockley-Queisser efficiency limit for single-junction PV. In this work, single standing GaAs-based nanowire solar cells in both radial and vertical junction configurations is analyzed and compared to a planar thin-film design.

Toward high-energy-density, high-efficiency, and moderate-temperature chip-scale thermophotovoltaics.

The challenging problem of ultra-high-energy-density, high-efficiency, and small-scale portable power generation is addressed here using a distinctive thermophotovoltaic energy conversion mechanism and chip-based system design, which we name the microthermophotovoltaic (μTPV) generator. The approach is predicted to be capable of up to 32% efficient heat-to-electricity conversion within a millimeter-scale form factor. Although considerable technological barriers need to be overcome to reach full performance, we have performed a robust experimental demonstration that validates the theoretical framework and the key system components.

Enabling high-temperature nanophotonics for energy applications.

The nascent field of high-temperature nanophotonics could potentially enable many important solid-state energy conversion applications, such as thermophotovoltaic energy generation, selective solar absorption, and selective emission of light. However, special challenges arise when trying to design nanophotonic materials with precisely tailored optical properties that can operate at high-temperatures (> 1,100 K). These include proper material selection and purity to prevent melting, evaporation, or chemical reactions; severe minimization of any material interfaces to prevent thermomechanical problems such as delamination; robust performance in the presence of surface diffusion; and long-range geometric precision over large areas with severe minimization of very small feature sizes to maintain structural stability.

Frequency-selective near-field radiative heat transfer between photonic crystal slabs: a computational approach for arbitrary geometries and materials.

We demonstrate the possibility of achieving enhanced frequency-selective near-field radiative heat transfer between patterned (photonic-crystal) slabs at designable frequencies and separations, exploiting a general numerical approach for computing heat transfer in arbitrary geometries and materials based on the finite-difference time-domain method. Our simulations reveal a tradeoff between selectivity and near-field enhancement as the slab-slab separation decreases, with the patterned heat transfer eventually reducing to the unpatterned result multiplied by a fill factor (described by a standard proximity approximation). We also find that heat transfer can be further enhanced at selective frequencies when the slabs are brought into a glide-symmetric configuration, a consequence of the degeneracies associated with the nonsymmorphic symmetry group.

Tailoring photonic metamaterial resonances for thermal radiation.

Selective solar absorbers generally have limited effectiveness in unconcentrated sunlight, because of reradiation losses over a broad range of wavelengths and angles. However, metamaterials offer the potential to limit radiation exchange to a proscribed range of angles and wavelengths, which has the potential to dramatically boost performance. After globally optimizing one particular class of such designs, we find thermal transfer efficiencies of 78% at temperatures over 1,000°C, with overall system energy conversion efficiencies of 37%, exceeding the Shockley-Quiesser efficiency limit of 31% for photovoltaic conversion under unconcentrated sunlight.

Design and global optimization of high-efficiency solar thermal systems with tungsten cermets.

Solar thermal, thermoelectric, and thermophotovoltaic (TPV) systems have high maximum theoretical efficiencies; experimental systems fall short because of losses by selective solar absorbers and TPV selective emitters. To improve these critical components, we study a class of materials known as cermets. While our approach is completely general, the most promising cermet candidate combines nanoparticles of silica and tungsten.

Design and global optimization of high-efficiency thermophotovoltaic systems.

Despite their great promise, small experimental thermophotovoltaic (TPV) systems at 1000 K generally exhibit extremely low power conversion efficiencies (approximately 1%), due to heat losses such as thermal emission of undesirable mid-wavelength infrared radiation. Photonic crystals (PhC) have the potential to strongly suppress such losses. However, PhC-based designs present a set of non-convex optimization problems requiring efficient objective function evaluation and global optimization algorithms.

Global optimization of silicon photovoltaic cell front coatings.

The front-coating (FC) of a solar cell controls its efficiency, determining admission of light into the absorbing material and potentially trapping light to enhance thin absorbers. Single-layer FC designs are well known, especially for thick absorbers where their only purpose is to reduce reflections. Multilayer FCs could improve performance, but require global optimization to design.