Perfect selective metamaterial solar absorbers

Uncooled High Detectivity Mid-Infrared Photoconductor Using HgTe Quantum Dots and Nanoantennas

Using a metal/insulator/metal (MIM) structure with a gold nanoantenna array made by electron beam lithography, the responsivity of a HgTe colloidal quantum dot film is enhanced in the mid-infrared. Simulations indicate that the spatially averaged peak spectral absorption of an 80 nm film is 60%, enhanced 23-fold compared to that of the same film on a bare sapphire substrate. The field intensity enhancement is focused near the antenna tips, being 20-fold 100 nm away, which represents only 1% of the total area and up to 1000-fold at the tips. The simulated polarized absorption spectra are in good agreement with the experiments, with a strong resonance around 4 μm. A responsivity of 0.6 A/W is obtained at a 1 V bias. Noise measurements separate the 1/f noise from the generation-recombination white noise and give a spatially averaged photoconductive gain of 0.3 at 1 V bias. The spatially averaged peak detectivity is improved 15-fold compared to the same film on a sapphire substrate without an MIM structure. The experimental peak detectivity reaches 9 × 109 Jones at 2650 cm-1 and 80 kHz, decreasing at lower frequencies. The MIM structure also enhances the spatially averaged peak photoluminescence of the CQD film by 16-fold, which is a potential Purcell enhancement. The good agreement between simulations and measurements confirms the viability of lithographically designed nanoantenna structures for vastly improving the performance of mid-IR colloidal quantum dot photoconductors. Further improvements will be possible by matching the optically enhanced and current collection areas.

Metasurface absorber based single junction thin film solar cell exceeding 30% efficiency

In this article, we report, as per our knowledge, for the first time, a thin film single junction solar cell with a metasurface absorber layer directly incorporated. We have used an interconnected dual inverted split ring resonator pattern in the InAsP absorber layer. The structure eliminated patterns of conventional metals, such as silver, aluminum, and gold, from the active layer, a common drawback in conventional solar absorbers, hindering their direct integration into solar cells. Optical simulation results show a peak ideal short circuit current density of 76.23mA/cm2 for the meta-absorber structure under solar illumination. This current is the highest among previously reported absorbers based on Group IV materials and III-V compounds, overcoming the low solar absorption of such metasurfaces. The final proposed solar cell structure combines this meta-absorber layer with traditional efficiency enhancement methods namely anti-reflecting coating, textured back reflector, and transparent top electrode. This novel single junction structure shows a solar absorption efficiency of 97.86% and a power conversion efficiency of 30.87%, the highest for III-V solar cells. Our device proves the ability of metasurface absorber layers to produce high-efficiency solar cells and is expected to pave the way for integrating novel meta-devices into state-of-the-art photovoltaic devices, aiding the global transition towards clean energy sources.

Phase-transition materials derived photonic metamaterials for passively dynamic solar thermal and coldness harvesting

The rising need for energy to actively heat and cool human-made structures is contributing to the growing energy crisis and intensifying global warming. Consequently, there's a pressing need for a sustainable approach to temperature management that minimizes energy consumption and carbon emissions. The substantial temperature differences between the Sun (approximately 5800 K), Earth (around 300 K), and outer space (about 3 K) offer a unique opportunity for passive thermal regulation on a global scale. Recent research indicates the possibility of addressing this issue through various low-carbon, passive technologies such as solar heating and radiative cooling. However, their practical application is often limited to certain seasons and climatic regions due to their static and single-function nature in managing temperature. In this context, we introduce a concept of phase-change metamaterials that provide passive, dynamic, and adjustable radiative thermal control, suitable for widespread engineering applications. Our designed metafilm comprises a Polydimethylsiloxane (PDMS) layer infused with vanadium dioxide (VO2) nanoparticles, backed by a layer of broadband-reflective silver (Ag). This metafilm exhibits a self-adjusting solar absorptance, shifting from 0.96 to 0.25 at a pivotal temperature while maintaining a nearly constant thermal emittance. We can finely tune the metafilm's optical characteristics by altering the VO2 nanoparticle concentration and PDMS layer thickness. To demonstrate its efficacy in solar thermal management and radiative cooling, we simulate its temperature behavior under various weather conditions.

Broadband near unity absorption meta-structure for solar thermophotovoltaic systems and optical window applications

Developing a meta-structure with near-unity absorbance in the visible and infrared spectra for solar energy harvesting, photodetection, thermal imaging, photo-trapping, and optical communications is a long-term research challenge. This research presents a four-layered (insulator-metal-insulator-metal) meta-structure unit cell that showed a peak absorbance of 99.99% at 288-300 nm and the average absorbance of 99.18% over the 250-2000 nm wavelength range in TE and TM modes, respectively. The symmetric pattern of the resonator layer shows polarization insensitivity with an average absorption of 99.18% in both TE and TM modes. Furthermore, the proposed design shows a wide incident angle stability up to ≤60 degrees in both TE and TM modes. The proposed structure also exhibits negative index properties at 288-300 nm and 1000-2000 nm, respectively. The negative index properties of the proposed design generate an anti-parallel surface current flow in the ground and resonator layers, which induces magnetic and electric field resonance and increases absorption. The performance of the proposed design is further validated by the interference theory model and a zero value for the polarization conversion ratio (PCR). The electric field E, magnetic field H, and current distribution are analyzed to explain the absorption mechanism of the proposed meta-structure unit cell. It also exhibits the highest photo-thermal conversion efficiency of 99.11%, demonstrating the viability of the proposed design as a solar absorber. The proposed design promises potentially valuable applications such as solar energy harvesting, photodetection, thermal imaging, photo-trapping, and optical communications because of its decent performance.

Polarization- and angle-insensitive broadband long wavelength infrared absorber based on coplanar four-sized resonators

In many potential applications, there is a high demand for long wavelength infrared (LWIR) absorbers characterized by a compact configuration, broad operational bandwidth, high absorption efficiency, and polarization- and angle-insensitive characteristics. In this study, we design and demonstrate a high-performance broadband LWIR absorber based on coplanar four-sized resonators, consisting of arrays of titanium (Ti) disks with different diameters supported by a continuous zinc selenide (ZnSe) layer and by a Ti film acting as a back-reflector. Particle swarm optimization (PSO) is employed to optimize the complicated geometry parameters, and the final optimized device exhibits near-unity absorption (∼96.7%) across the entire operational bandwidth (8 µm∼14 µm) under unpolarized normal incidence, benefiting from the impedance-matching condition and the multiple surface plasmon resonances of this configuration. Furthermore, the proposed absorber is insensitive to the angle of incidence due to the localized surface plasmon resonances supported by these four-sized resonators, and is insensitive to the state of polarization thanks to the highly symmetric feature of the circular pattern. The measured absorption of the fabricated sample exhibits a relatively high coincidence with the simulation, with an average absorption of 88.9% ranging from 8 µm to 14 µm. The proposed absorber, which can be easily integrated into a standardized micro/nano manufacture process for cost-effective large-scale production, provides a feasible solution for improving optical performance in thermal emitter, infrared detection, and imaging applications. Furthermore, the generalized design principle employing the optimized method opens up new avenues for realizing target absorption, reflection, and transmission based on more complicated structure configurations.

Metamaterials for light extraction and shaping of micro-scale light-emitting diodes: from the perspective of one-dimensional and two-dimensional photonic crystals

Metamaterials have attracted broad attention owing to their unique versatile micro- and nano-structures. As a kind of typical metamaterial, photonic crystals (PhCs) are capable of controlling light propagation and constraining spatial light distribution from the chip level. However, introducing metamaterial into micro-scale light-emitting diodes (µLED) still exists many unknowns to explore. This paper, from the perspective of one-dimensional and two-dimensional PhCs, studies the influence of metamaterials on the light extraction and shaping of µLEDs. The µLEDs with six different kinds of PhCs and the sidewall treatment are analyzed based on finite difference time domain (FDTD) method, in which the optimal match between the PhCs type and the sidewall profile is recommended respectively. The simulation results show that the light extraction efficiency (LEE) of the µLEDs with 1D PhCs increases to 85.3% after optimizing the PhCs, and is further improved to reach 99.8% by the sidewall treatment, which is the highest design record so far. It is also found that the 2D air ring PhCs, as a kind of left-handed metamaterials, can highly concentrate the light distribution into 30° with the LEE of 65.4%, without help of any light shaping device. The surprising light extraction and shaping capability of metamaterials provides a new direction and strategy for the future design and application of µLED devices.

Deep learning-based inverse design of microstructured materials for optical optimization and thermal radiation control

Microstructures with engineered properties are critical to thermal management in aerospace and space applications. Due to the overwhelming number of microstructure design variables, traditional approaches to material optimization can have time-consuming processes and limited use cases. Here, we combine a surrogate optical neural network with an inverse neural network and dynamic post-processing to form an aggregated neural network inverse design process. Our surrogate network emulates finite-difference time-domain simulations (FDTD) by developing a relationship between the microstructure's geometry, wavelength, discrete material properties, and the output optical properties. The surrogate optical solver works in tandem with an inverse neural network to predict a microstructure's design properties that will match an input optical spectrum. As opposed to conventional approaches that are constrained by material selection, our network can identify new material properties that best optimize the input spectrum and match the output to an existing material. The output is evaluated using critical design constraints, simulated in FDTD, and used to retrain the surrogate-forming a self-learning loop. The presented framework is applicable to the inverse design of various optical microstructures, and the deep learning-derived approach will allow complex and user-constrained optimization for thermal radiation control in future aerospace and space systems.

Metamaterials-based broadband absorption in long-wave infrared frequency enabled by multilayered ENZ films on metal-coated patterned silicon

Large-scale, and high-throughput produced devices with strong ultrabroadband absorption and high angular tolerance are in demand for applications such as thermal imaging, energy harvesting, and radiative cooling. Despite long-standing efforts in design and fabrication, it has been challenging to achieve all these desired properties simultaneously. Here, we create a metamaterial-based infrared absorber fabricated from thin films of epsilon-near-zero (ENZ) materials grown on metal-coated patterned silicon substrates that exhibit ultrabroadband infrared absorption in both p- and s-polarization at angles ranging from 0° to 40°. The results show that the structured multilayered ENZ films exhibit high absorption (> 0.9) covering the entire 8∼14 µm wavelengths. In addition, the structured surface can be realized via scalable, low-cost methods on large-area substrates. Overcoming the limitations on angular and polarized response improves performance for applications such as thermal camouflage, radiative cooling for solar cell, thermal image and et., al.

A Highly Efficient Infinity-Shaped Large Angular- and Polarization-Independent Metamaterial Absorber

An efficient diagonally symmetric infinity-shaped broadband solar absorber has been demonstrated in this research paper. The structure was developed with an infinity-shaped resonator made of titanium (Ti) and gallium arsenide (GaAs) at the base substrate layer to achieve absorption in a wideband spectrum under solar energy radiation, and absorption efficiencies were calculated employing the finite element method. The average solar energy absorption spectrum ranges from the ultraviolet to the mid-infrared regions, and 93.93% average absorption in this band is achieved. Moreover, bandwidths of 2800 and 1110 nm were observed, and, in these bands, we attained continuous absorption above 90% and 95%, respectively, with average absorption rates of 93.93% and 96.25%, respectively. Furthermore, based on this solar energy absorber, which was optimized after varying many design parameters, it is also observed that the developed design is angle-insensitive from 0° to 50° and polarization-insensitive from the results of the transverse electric (TE) and transverse magnetic (TM) modes. The developed infinity-shaped broadband solar absorber design is highly efficient and provides broadband absorptance that can be used as an absorber layer in solar cells.

Design of multilayer planar film structures for near-perfect absorption in the visible to near-infrared

In this work, a near-perfect broadband absorber, consisting of Fe, MgF₂, Fe, TiO₂ and MgF₂ planar film, is proposed and investigated through simulations and experiments. The Fe material is first applied in the multilayer film structure, and it is proved to be more favorable for achieving broadband absorption. MgF₂ and TiO₂ are chosen as anti-reflection coatings to decrease unwanted reflections. The proposed absorber is optimized by employing a hybrid numerical method combining the transfer matrix method (TMM) and the genetic algorithm (GA). Under normal incidence conditions, the average absorption of the absorber is 97.6% in the range of 400 to 1400 nm. The finite difference time domain (FDTD) method and phase analysis reveal that the anti-reflection property and the Fabry-Perot resonance result in broadband absorption performance. Furthermore, when an additional Fe-MgF₂ layer is inserted on the bottom Fe layer, an average absorption of 97.9% in the range of 400 to 2000 nm can be achieved. Our approach could be of vital significance for numerous applications involving solar energy.

Nanoparticle-on-Mirror Metamaterials for Full-Spectrum Selective Solar Energy Harvesting

Most broadband metamaterial absorbers are realized by patterning periodic arrays of plasmonic nanoparticles (>100 nm) on dielectric/metallic substrates to enable both electric and magnetic resonances. These metamaterials, however, require costly nanolithographic top-down techniques for fabrication. Here, we demonstrate new-concept nanoparticle-on-mirror (NoM) metamaterial absorbers by densely packing plasmonic nanoparticles of much smaller size (∼30 nm) on metal films directly. Such a simple but rational design enables the use of all-solution-based bottom-up processes. Because of the decoupling of electric and magnetic polarizations in these ultrasmall nanoparticles, excellent impedance match and near-perfect light absorption can be achieved in a broad band over the solar spectrum with weak thermal emission. Proof-of-concept large-area NoM metamaterial absorbers that offer a solar absorptance of 94% but a low IR emittance of 2% are experimentally demonstrated. The outstanding performance, bottom-up process, and great compatibility render the design promising for efficient and large-scale solar energy harvesting.

Thermoplasmonics in Solar Energy Conversion: Materials, Nanostructured Designs, and Applications

The indispensable requirement for sustainable development of human society has forced almost all countries to seek highly efficient and cost-effective ways to harvest and convert solar energy. Though continuous progress has advanced, it remains a daunting challenge to achieve full-spectrum solar absorption and maximize the conversion efficiency of sunlight. Recently, thermoplasmonics has emerged as a promising solution, which involves several beneficial effects including enhanced light absorption and scattering, generation and relaxation of hot carriers, as well as localized/collective heating, offering tremendous opportunities for optimized energy conversion. Besides, all these functionalities can be tailored via elaborated designs of materials and nanostructures. Here, first the fundamental physics governing thermoplasmonics is presented and then the strategies for both material selection and nanostructured designs toward more efficient energy conversion are summarized. Based on this, recent progress in thermoplasmonic applications including solar evaporation, photothermal chemistry, and thermophotovoltaic is reviewed. Finally, the corresponding challenges and prospects are discussed.

Deep learning based analysis of microstructured materials for thermal radiation control

Microstructured materials that can selectively control the optical properties are crucial for the development of thermal management systems in aerospace and space applications. However, due to the vast design space available for microstructures with varying material, wavelength, and temperature conditions relevant to thermal radiation, the microstructure design optimization becomes a very time-intensive process and with results for specific and limited conditions. Here, we develop a deep neural network to emulate the outputs of finite-difference time-domain simulations (FDTD). The network we show is the foundation of a machine learning based approach to microstructure design optimization for thermal radiation control. Our neural network differentiates materials using discrete inputs derived from the materials’ complex refractive index, enabling the model to build relationships between the microtexture’s geometry, wavelength, and material. Thus, material selection does not constrain our network and it is capable of accurately extrapolating optical properties for microstructures of materials not included in the training process. Our surrogate deep neural network can synthetically simulate over 1,000,000 distinct combinations of geometry, wavelength, temperature, and material in less than a minute, representing a speed increase of over 8 orders of magnitude compared to typical FDTD simulations. This speed enables us to perform sweeping thermal-optical optimizations rapidly to design advanced passive cooling or heating systems. The deep learning-based approach enables complex thermal and optical studies that would be impossible with conventional simulations and our network design can be used to effectively replace optical simulations for other microstructures.

Selective broadband absorption by mode splitting for radiative cooling

A plasmonic-photonic structure based on colloidal lithography was designed for a scalable radiative cooling system and its absorption properties were theoretically investigated. The structure comprises a SiO₂ core, which is on top of an Au reflector and partially covered by an indium tin oxide (ITO) shell. This simple and scalable structure possesses a strong selective absorption in the primary atmospheric transparency window (8-13 µm). The strong selective absorption is attributed to a mode splitting of the localized surface plasmon (LSP) of the ITO shell. To understand the mechanisms of the mode splitting, a quantitative analysis was conducted using a coupled-oscillator model and a coupled-dipole method. The analysis revealed that the mode splitting is induced by a strong coupling between the LSP of the ITO shell and a magnetic dipole Mie resonance of the SiO₂ core.

Nanostructured multilayer hyperbolic metamaterials for high efficiency and selective solar absorption

Highly efficient solar-to-thermal conversion is desired for the renewable energy technologies, such as solar thermo-photovoltaics and solar thermo-electric systems. In order to maximize the energy conversion efficiency, solar-selective absorbers are essential with its absorption characteristics specially tailored for solar applications. Here, we propose a wideband spectral-selective absorber based on three-dimensional (3D) nanostructured hyperbolic metamaterial (HMM), which can realize near-unity absorption across the UV and NIR spectral ranges. Moreover, the optical topological transition (OTT) of iso-frequency surface (IFS) is manipulated to selectively enhance light absorption in the entire solar spectrum, crucial for improved energy utilization. Impressive solar-to-thermal conversion efficiency of 95.5% has been achieved. Particularly, such superior properties can be retained well even over a wide range of incident angles. These findings open new avenues for designing high-performance solar thermal devices, especially in the fields related to solar energy harvesting.

Metamaterial perfect absorber using elliptical nanoparticles in a multilayer metasurface structure with polarization independence

A metamaterial perfect absorber (MPA) using elliptical silver nanoparticles is proposed and investigated to provide 100% absorption for both transverse electric and transverse magnetic polarizations with a wide range of incident angles and polarization independence. Metamaterial absorbers with narrow absorption performance over a wide frequency range are significantly desired in sensing applications. Incident angle insensitivity and polarization angle independence are key features of MPAs. The output characteristics are examined using the three-dimensional finite difference time domain method. The effective medium theory and transmission line theory are applied to investigate the simulation results. Here, the 100% absorption occurs at resonance wavelength of λ_res= 2290 nm, and maximum sensitivity and figure of merit become 200 nm/RIU and 720 RIU^-1, respectively. The results show that an absorption spectrum is insensitive to the incident angle of 0°-60°. The proposed device can be used as a high-performance biosensor and photodetector.

Ultrabroadband and ultrathin absorber based on an encapsulated T-shaped metasurface

Ultrabroadband absorbers are vital for applications such as solar energy harvesting and integrated optoelectronic devices. Herein, we design, fabricate and characterize a novel ultrabroadband and ultrathin absorber based on the encapsulated T-shaped metasurface (ETM). The ETM consists of a 20 nm Cr film and a Cr substrate sandwiched by the T-shaped polymethyl methacrylate (PMMA) arrays. The Cr film provides a robust absorptive surface with improved impedance matching, and ultrabroadband absorption can be achieved via the excitation of the localized surface plasmon (LSP) of this ultrathin film. The average absorption of simulated and experimental results of the ETM in the visible range of 400-800 nm for the TM (TE) polarization are 96.4% (96.3%) and 90.6% (89.4%), respectively. Three-dimensional (3D) power dissipation density distributions of the proposed structure have been investigated, which indicates that the synergistic absorption effect of different parts of the T-shaped ultrathin Cr film contributes to the major absorption enhancement. The absorption of the ETM is very robust to the changes of geometrical parameters and the symmetry of the structure, and it can be maintained almost the same even if T-shaped profiles are changed to L-shaped profiles. Moreover, the absorption performance of the ETM exhibits polarization-insensitive and wide-angle features, which has advantages for many potential applications.

Numerical Study of Ultra-Broadband Metamaterial Perfect Absorber Based on Four-Corner Star Array

In recent years, research on solar absorbers provides a significant breakthrough to solve the energy crisis. A perfect solar absorber based on a four-corner star array is designed and the absorption performance is analyzed numerically. The results show that the absorber reaches more than 90% of the full band in the range of 400-2000 nm. In particular, the absorption efficiency of the continuous more than 95% of the bandwidth reached 1391 nm, and the average absorption efficiency of the whole study band is more than 98%, and the loss of the solar spectrum only accounted for 2.7%. At the same time, the absorption efficiency can be adjusted by changing the geometric structure of the absorber. In addition, due to the perfect symmetry of the structure, it has an excellent insensitivity of the incident angle and polarization angle. In general, the proposed solar absorber has exciting prospects in solar energy collection and utilization, photothermal conversion and other related fields.

Ultra-narrow-band metamaterial perfect absorber based on surface lattice resonance in a WS2 nanodisk array

Metamaterial perfect absorbers (MPAs) are attractive platforms for the unique manipulation of electromagnetic waves from near-field to far-field. Narrow-band MPAs are particularly intriguing for their potential applications as thermal emitters or biosensors. In this work, we proposed ultra-narrow-band MPAs based on surface lattice resonance (SLR) modes of WS₂ nanodisk arrays on gold films. The SLR modes stem from the coupling between the magnetic dipole modes of individual nanodisks and the Rayleigh anomaly of the array giving rise to high quality-factor resonances. With proper design of the nanodisk array, an ultra-narrow-band of 15 nm is achieved in the near infrared wavelength range. The underneath gold film provides the loss channel converting the incident light within the narrow band into heat in the gold film, effectively creating a perfect absorber. Systematic numerical simulations were performed to investigate the effects of the geometrical parameters on their optical properties, demonstrating the great tunability of this type of MPAs as well as their potential for engineering light-matter interactions.

Tungsten Based Spectrally Selective Absorbers with Anisotropic Rough Surface Texture

Spectrally selective absorbers have received considerable interest due to their applications in thermophotovoltaic devices and as solar absorbers. Due to extreme operating conditions in these applications, such as high temperatures, thermo-mechanically stable and broadband spectrally selective absorbers are of interest. This paper demonstrates anisotropic random rough surfaces that provide broadband spectrally selective absorption for the thermo-mechanically stable Tungsten surfaces. Anisotropic random rough surface has different correlation lengths in the x- and y-directions, which means their topography parameters have directional dependence. In particular, we demonstrate that spectral absorptance of Tungsten random rough surfaces at visible (VIS) and near-infrared (NIR) spectral regions are sensitive to correlation length and RMS height variations. Our results indicate that by optimizing random rough surface parameters, absorption values exceeding 95% can be obtained. Moreover, our results indicate that anisotropic random rough surfaces broaden the bandwidth of the high absorption region. It is shown that in VIS and NIR regions, the absorption enhancements of up to 47% and 52% are achieved for the isotropic and anisotropic rough surfaces, respectively.

Ultra-Broadband Refractory All-Metal Metamaterial Selective Absorber for Solar Thermal Energy Conversion

A full-spectrum near-unity solar absorber has attracted substantial attention in recent years, and exhibited broad application prospects in solar thermal energy conversion. In this paper, an all-metal titanium (Ti) pyramid structured metamaterial absorber (MMA) is proposed to achieve broadband absorption from the near-infrared to ultraviolet, exhibiting efficient solar-selective absorption. The simulation results show that the average absorption rate in the wavelength range of 200-2620 nm reached more than 98.68%, and the solar irradiation absorption efficiency in the entire solar spectrum reached 98.27%. The photothermal conversion efficiency (PTCE) reached 95.88% in the entire solar spectrum at a temperature of 700 °C. In addition, the strong and broadband absorption of the MMA are due to the strong absorption of local surface plasmon polariton (LSPP), coupled results of multiple plasmons and the strong loss of the refractory titanium material itself. Additionally, the analysis of the results show that the MMA has wide-angle incidence and polarization insensitivity, and has a great processing accuracy tolerance. This broadband MMA paves the way for selective high-temperature photothermal conversion devices for solar energy harvesting and seawater desalination applications.

Metamaterial perfect absorber with morphology-engineered meta-atoms using deep learning

Metamaterial perfect absorbers (MPAs) typically have regularly-shaped unit structures owing to constraints on conventional analysis methods, limiting their absorption properties. We propose an MPA structure with a general polygon-shaped meta-atom. Its irregular unit structure provides multiple degrees-of-freedom, enabling flexible properties, such as dual-band absorption. We constructed a deep neural network to predict the parameters of the corresponding MPA structure with a given absorptivity as input, and vice versa. The mean-square error was as low as 0.0017 on the validation set. This study provides a basis for the design of complicated artificial electromagnetic structures for application in metamaterials and metasurfaces.

Broadening Bandwidths of Few-Layer Absorbers by Superimposing Two High-Loss Resonators

Efficient broadband absorption of solar radiation is desired for sea water desalination, icephobicity and other renewable energy applications. We propose an idea of superimposing two high-loss resonances to broaden bandwidths of a few-layer absorber, which is made of dielectric/ metal/dielectric/ metal layers. Both the simulation and experiment show that the structure has an averaged absorption efficiency higher than 97% at wavelengths ranging from 350 to 1200 nm. The bandwidth of the absorption larger than 90% is up to 1000 nm (410-1410 nm), which is greater than that (≤ 750 nm) of previous MIM planar absorbers. Especially, the average absorption from 350 to 1000 nm is kept above 90% at an incidence angle as high as 65°, meanwhile still maintained above 80% even at an incident angle of 75°. The performance of angular insensitivity is much better than that of previous few-layer solar absorbers. The flexible 1D nonoble metasurface absorbers are fabricated in a single evaporation step. Under the illumination of a halogen lamp of P = 1.2 kW/m², the flexible metasurface increases its surface temperature by 25.1 K from room temperature. Further experiments demonstrate that the heat localization rapidly melts the accumulated ice. Our illumination intensity (P = 1.2 kW/m²) is only half of that (P = 2.4 kW/m²) in previous solar anti-ice studies based on gold/TiO₂ particle metasurfaces, indicating that our metasurface is more advantageous topractical applications. Our results illustrate an effective pathway toward the broadband metasurface absorbers with the attractive properties of mechanical flexibility, low cost of the no-noble metals, and large-area fabrications, which have promising prospects in the applications of solar heat utilization.

Refractory materials and plasmonics based perfect absorbers

In the past decades, metamaterial light absorbers have attracted tremendous attention due to their impressive absorption efficiency and significant potential for multiple kinds of applications. However, the conventional noble metals based metamaterial and nanomaterial absorbers always suffer from the structural damage by the local high temperature resulting from the strong plasmonic photo-thermal effects. To address this challenge, intensive research has been conducted to develop the absorbers which can realize efficient light absorption and simultaneously keep the structural stability under high temperatures. In this review, we present detail discussion on the refractory materials which can provide robust thermal stability and high performance for light absorption. Moreover, promising theoretical designs and experimental demonstrations that possess excellent features are also reviewed, including broadband strong light absorption, high temperature durability, and even the easy-to-fabricate configuration. Some applications challenges and prospects of refractory materials based plasmonic perfect absorbers are also introduced and discussed.

Modeling of microsphere photolithography

Microsphere photolithography (MPL) is a fabrication technique that combines the ability to self-assemble arrays of microspheres with the ability of a microsphere to focus light to a photonic jet, in order to create highly ordered nanoscale features in photoresist. This paper presents a model of photoresist exposure with the photonic jet, combining a full-wave electromagnetic model of the microsphere/photoresist interaction with the sequential removal of exposed photoresist by the developer. The model is used to predict the dose curves for the MPL process based on the photoresist thickness, illumination conditions, and development time. After experimental validation, the model provides insight into the process including the resolution, sensitivity, and effects of off-normal illumination. This guides the fabrication of sub-100 nm hole/disk arrays using lift-off, and superposition is shown to predict the geometry for split-ring resonators created using multiple exposures. This model will assist synthesizing fabrication parameters to create large area scalable metasurfaces with sensing and energy management applications.

Enhancing solar-thermal energy conversion with silicon-cored tungsten nanowire selective metamaterial absorbers

This work experimentally studies a silicon-cored tungsten nanowire selective metamaterial absorber to enhance solar-thermal energy harvesting. After conformally coating a thin tungsten layer about 40 nm thick, the metamaterial absorber exhibits almost the same total solar absorptance of 0.85 as the bare silicon nanowire stamp but with greatly reduced total emittance down to 0.18 for suppressing the infrared emission heat loss. The silicon-cored tungsten nanowire absorber achieves an experimental solar-thermal efficiency of 41% at 203°C during the laboratory-scale test with a stagnation temperature of 273°C under 6.3 suns. Without parasitic radiative losses from side and bottom surfaces, it is projected to reach 74% efficiency at the same temperature of 203°C with a stagnation temperature of 430°C for practical application, greatly outperforming the silicon nanowire and black absorbers. The results would facilitate the development of metamaterial selective absorbers at low cost for highly efficient solar-thermal energy systems.

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Si-cored tungsten nanowires are fabricated by conformal coating of 40-nm tungsten

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Total solar absorptance of 0.85 with greatly reduced total emittance is achieved

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Solar-thermal efficiency of 41% at 203°C under 6.3 suns is measured

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74% efficiency at 203°C with a stagnation temperature of 430°C is projected

Broadband absorption using all-graphene grating-coupled nanoparticles on a reflector

In this paper, the hybridized localized surface plasmon resonances (LSPRs) of a periodic assembly of graphene-wrapped nanoparticles are used to design a nanoparticle assisted optical absorber. Bandwidth enhancement of this structure via providing multiple types of plasmonic resonances in the associated unit cell using two densely packed crossly stacked graphene strips is proposed. The designed graphene strips support fundamental propagating surface plasmons on the ribbons, and gap plasmons in the cavity constructed by the adjacent sections. Graphene strips exhibit a hyperbolic dispersion region in the operating spectrum and assist in the bandwidth enhancement. Moreover, since the nanoparticles are deposited on the top strips, real-time biasing of them can be easily conducted by exciting the surface plasmons of the strip without the necessity to electrically connect the adjacent nanoparticles. The overall dynamic bandwidth of the structure, using a two-state biasing scheme, covers the frequencies of 18.16–40.47 THz with 90% efficiency. Due to the symmetry of the structure, the device performs similarly for both transverse electric (TE) and transverse magnetic (TM) waves and it has a high broadband absorption rate regarding different incident angles up to 40°. Due to the presence of 2D graphene material and also using hollow spherical particles, our proposed absorber is also lightweight and it is suitable for novel compact optoelectronic devices due to its sub-wavelength dimensions.

Numerical investigation of an ultra-broadband, wide-angle, and polarization-independent metasurface light absorber

In this paper, we propose and numerically investigate an ultra-broadband, wide-angle, and polarization-independent metasurface absorber based on periodic hexagon-latticed titanium (Ti) nanoring arrays over a continuous Ti film. The proposed absorber can achieve more than 90% absorptivity under normal incidence, ranging from 350 to 1453 nm, and the average absorption is up to 95.6%. Additionally, the absorptivity still remained beyond 70% when the incident angles varied from 0° to 60°. The simulations of electric field distributions indicate that the broadband absorption performance can be ascribed to the superposition of the localized surface plasmon resonance (LSPR) originated from the nanopillars and nanoholes, respectively. The proposed approach is simple and inexpensive, and the metal material is optional. Therefore, we believe that the proposed absorber will be a candidate for many potential applications, such as thermophotovoltaic cells, thermal emitters, and optoelectronic devices.

Multiband metamaterial selective absorber for infrared stealth

Nanostructured selective absorbers have widespread applications ranging from artificial color to thermophotovoltaics and radiative cooling. In this paper, we propose a metamaterial selective absorber with a metal-insulator-metal structure for infrared stealth. It can realize multiband absorption, and one sharp peak is at 1.54 µm, which can be used to reduce the scattering signals in laser-guided missiles. The other two relatively broad absorption peaks are at 2.83 µm and 6.11 µm, which can match the atmospheric absorption band. It can reduce up to 90 % of the detected infrared signals while maintaining a relatively high level of thermal emission capability. The dependence of the spectral characteristics on the incident angle is studied. The infrared signatures of the structure could be suppressed across a wide temperature range.

Ultrabroadband Absorption Enhancement via Hybridization of Localized and Propagating Surface Plasmons

Broadband metamaterial absorbers (MAs) are critical for applications of photonic and optoelectronic devices. Despite long-standing efforts on broadband MAs, it has been challenging to achieve ultrabroadband absorption with high absorptivity and omnidirectional characteristics within a comparatively simple and low-cost architecture. Here we design, fabricate, and characterize a novel compact Cr-based MA to achieve ultrabroadband absorption in the visible to near-infrared wavelength region. The Cr-based MA consists of Cr nanorods and Cr substrate sandwiched by three pairs of SiO₂/Cr stacks. Both simulated and experimental results show that an average absorption over 93.7% can be achieved in the range of 400–1000 nm. Specifically, the ultrabroadband features result from the co-excitations of localized surface plasmon (LSP) and propagating surface plasmon (PSP) and their synergistic absorption effects, where absorption in the shorter and longer wavelengths are mainly contributed bythe LSP and PSP modes, respectively. The Cr-based MA is very robust to variations of the geometrical parameters, and angle-and polarization-insensitive absorption can be operated well over a large range of anglesunder both transverse magnetic(TM)- and transverse electric (TE)-polarized light illumination.

Deep Q-network to produce polarization-independent perfect solar absorbers: a statistical report

Using reinforcement learning, a deep Q-network was used to design polarization-independent, perfect solar absorbers. The deep Q-network selected the geometrical properties and materials of a symmetric three-layer metamaterial made up of circular rods on top of two films. The combination of all the possible permutations gives around 500 billion possible designs. In around 30,000 steps, the deep Q-network was able to produce 1250 structures that have an integrated absorption of higher than 90% in the visible region, with a maximum of 97.6% and an integrated absorption of less than 10% in the 8-13 µm wavelength region, with a minimum of 1.37%. A statistical analysis of the distribution of materials and geometrical parameters that make up the solar absorbers is presented.

Cascaded Nanorod Arrays for Ultrabroadband, Omnidirectional and Polarization-Insensitive Absorption

An ultrabroadband, omnidirectional, and polarization-insensitive absorber based on cascaded nanorod arrays (CNAs) is numerically demonstrated, and an average absorptivity of 98.2% with a relative absorption bandwidth (RAB) of 149.8% can be achieved in the 0.38–2.65 μm wavelength range. The proposed CNA-based absorber requires only several pairs of multilayers to achieve excellent absorption performance. More significantly, the physical mechanism for this intriguing ultrabroadband absorption results from the synergistic effect of localized surface plasmon (LSP) and plasmonic resonant cavity (PRC) modes, which is fundamentally different from the tapered metal/dielectric multilayer-based absorbers associated with the slow-light mode. We investigated the absorption properties of the CNA-based metasurface by using the impedance theory, which indicates that the impedance of the structure matches well with the impedance of the free space from the visible to near-infrared wavelength range. In addition, the absorption properties of the CNA-based metasurface are robust to the variation of the structural parameters and the metal/dielectric materials, and ultrabroadband absorption performance can be maintained within 0–60° for both TM and TE modes.

Design of an ultra-broadband near-perfect bilayer grating metamaterial absorber based on genetic algorithm

An ultra-broadband metamaterial absorber, consisting of 2D SiO₂-Ti square bilayer grating on SiO₂ film and Ti substrate, is proposed and designed by rigorous coupled wave analysis (RCWA) and genetic algorithm (GA) methods. The optimized structure shows an average absorption of 98.3% in the wavelength range of 300 nm to 2100 nm. Moreover, the metamaterial absorber is polarization-independent and also insensitive to incidence angle for both TM- and TE-polarized waves. The physical mechanisms responsible for nearly perfect broadband absorption, including the Wood's anomaly (WA), cavity resonance (CR), surface plasmon polaritons (SPPs) and the resonance of magnetic polaritons (MPs), have been analyzed clearly by finite-difference time-domain (FDTD) method and the inductor-capacitor (LC) circuit model. Overall, the proposed metamaterial absorber is a promising candidate in solar applications.

Absorption characteristics of a metal-insulator-metal nanodisk for solar thermal applications

Due to their ability to confine light in a sub-wavelength scale and achieve coherent absorption, plasmonic nanostructures have been intensively studied for solar energy harvesting. Although nanoparticles generating localized surface plasmon resonance (LSPR) have been thoroughly studied for application in a direct absorption solar collector (DASC), nanoparticles exciting magnetic polaritons (MP) for use in a DASC have not drawn much attention. In this work, we report a metal-insulator-metal (MIM) nanodisk that can excite MP peaks apart from the LSPR in the solar spectrum. It was found that the MIM nanodisk generates a broader and relatively more uniform absorption band compared to a pure metallic nanodisk. The MP peaks were also found to cause less significant scattering compared to those associated with the LSPR. We finally showed that the peaks induced by the MIM nanodisk are highly tunable by varying the particle dimensions, making the proposed MIM nanodisk a potential candidate for solar thermal applications.

A Perfect Absorber Based on Similar Fabry-Perot Four-Band in the Visible Range

A simple metamaterial absorber is proposed to achieve near-perfect absorption in visible and near-infrared wavelengths. The absorber is composed of metal-dielectric-metal (MIM) three-layer structure. The materials of these three-layer structures are Au, SiO₂, and Au. The top metal structure of the absorber is composed of hollow three-dimensional metal rings regularly arranged periodically. The results show that the high absorption efficiency at a specific wavelength is mainly due to the resonance of the Fabry-Perot effect (FP) in the intermediate layer of the dielectric medium, resulting in the resonance light being trapped in the middle layer, thus improving the absorption efficiency. The almost perfect multiband absorption, which is independent of polarization angle and insensitivity of incident angle, lends the absorber great application prospects for filtering and optoelectronics.

Large-scale, low-cost, broadband and tunable perfect optical absorber based on phase-change material

Metamaterial-based electromagnetic absorbers have attracted much attention recently, but most previous realizations suffer from issues of narrow bandwidth, time-consuming and high-cost fabrication methods, and/or fixed functionalities, and so are unfavorable for practical applications. Here, we demonstrate experimentally a large-scale, broadband, polarization-independent, and tunable metamaterial absorber, which works for both visible and near-infrared light. A lithography-free and low-cost method was utilized to fabricate a centimeter-sized metamaterial sample in a metal-insulator-metal (MIM) configuration with nano-scale precision, in which a phase-change material, Ge₂Sb₂Te₅ (GST), was adopted as the insulating spacer of the MIM structure. With two different resonance mechanisms working together, the proposed device was shown to exhibit high absorptivity (>80%) within a broad wavelength band (480-1020 nm). By thermally tuning the phase state of the GST layer, we can dramatically enlarge the working bandwidth of the metamaterial absorber by shifting one absorption peak by about 470 nm. These findings may stimulate many potential applications in, for example, solar cells, energy harvesting, smart sensing/imaging, and color printing.

Bismuth plasmonics for extraordinary light absorption in deep sub-wavelength geometries

In this Letter, we demonstrate an ultra-broadband metamaterial absorber of unrivaled bandwidth (BW) using extraordinary optical response of bismuth (Bi), which is the material selected through our novel analysis. Based on our theoretical model, we investigate the maximum metal-insulator-metal (MIM) cavity BW, achievable by any metal with known n-k data. We show that an ideal metal in such structures should have a positive real permittivity part in the near-infrared (NIR) regime. Contrary to noble and lossy metals utilized by most research groups in the field, this requirement is satisfied only by Bi, whose data greatly adhere to the ideal material properties predicted by our analysis. A Bi nanodisc-based MIM resonator with an absorption above 0.9 in an ultra-broadband range of 800 nm-2390 nm is designed, fabricated, and characterized. To the best of our knowledge, this is the broadest absorption BW reported for a MIM cavity in the NIR with its upper-to-lower absorption edge ratio exceeding best contenders by more than 150%. According to the findings in this Letter, the use of proper materials and dimensions will lead to realization of deep sub-wavelength efficient optical devices.

Symmetry-broken square silicon patches for ultra-narrowband light absorption

The effect of ultra-narrowband light absorption enhancement is presented by using metamaterials with symmetry-broken square silicon patches (SSPs). The symmetry of the SSP can be broken by introducing a narrow slit deviating from its center. By breaking the symmetry of the SSPs, slit resonance mode with standing wave patterns can be excited, and the locations of the absorption peaks can be well estimated by using the Fabry-Pérot (F-P) cavity model. Although there is no excitation of surface plasmon resonance, ultra-narrowband light absorption can be achieved by minimizing the reflectance through perfect impedance matching and simultaneously eliminating the transmittance by the metallic substrate. Good ultra-narrowband absorption features can be maintained as the parameters of the buffer layer and the SSPs are altered. When this type of symmetry-broken SSPs-based metamaterial is used in refractive-index sensors, it shows excellent sensing properties due to its stable ultra-narrowband absorption enhancement.

Broadband Near-Infrared Absorber Based on All Metallic Metasurface

Perfect broadband absorbers have increasingly been considered as important components for controllable thermal emission, energy harvesting, modulators, etc. However, perfect absorbers which can operate over a wide optical regime is still a big challenge to achieve. Here, we propose and numerically investigate a perfect broadband near-infrared absorber based on periodic array of four isosceles trapezoid prism (FITP) unit cell made of titanium (Ti) over a continuous silver film. The structure operates with low quality (Q) factor of the localized surface plasmon resonance (LSPR) because of the intrinsic high loss, which is the foundation of the broadband absorption. The high absorption of metal nanostructures mainly comes from the power loss caused by the continuous electron transition excited by the incident light inside the metal, and the resistance loss depends on the enhanced localized electric field caused by the FITP structure. Under normal incidence, the simulated absorption is over 90% in the spectrum ranging from 895 nm to 2269 nm. The absorber is polarization-independent at normal incidence, and has more than 80% high absorption persisting up to the incident angle of ~45° at TM polarization.

Broadband Optical Absorber Based on Nanopatterned Metallic Glass Thin Films

Applications of metallic glass thin films (MGTFs) in the optical area have seldom been reported. We successfully design and fabricate a broadband and durable absorber with MGTFs. It is found that the absorber, with a total thickness of only ∼230 nm smaller than the light wavelength, exhibits excellent performance, that is, an average absorption of >90% from the visible to the near-infrared range (500-1300 nm). This is of significance for the optical application of MGTFs with the potential for large-scale fabrication using superplastic deformation in the supercooled liquid region of MGTFs.

An On-Chip Quad-Wavelength Pyroelectric Sensor for Spectroscopic Infrared Sensing

Merging photonic structures and optoelectronic sensors into a single chip may yield a sensor-on-chip spectroscopic device that can measure the spectrum of matter. In this work, an on-chip concurrent multiwavelength infrared (IR) sensor, which consists of a set of narrowband wavelength-selective plasmonic perfect absorbers combined with pyroelectric sensors, where the response of each pyroelectric sensor is boosted only at the resonance of the nanostructured absorber, is proposed and realized. The proposed absorber, which is based on Wood's anomaly absorption from a 2D plasmonic square lattice, shows a narrowband polarization-independent resonance (quality factor - Q of 73) with a nearly perfect absorptivity as high as 0.99 at normal incidence. The fabricated quad-wavelength IR sensors exhibit four different narrowband spectral responses at normal incidence following the predesigned resonances in the mid-wavelength infrared region that corresponds to the atmospheric window. The device can be applied for practical spectroscopic applications such as nondispersive IR sensors, IR chemical imaging devices, pyrometers, and spectroscopic thermography imaging.

Tunable wavelength selectivity of photonic metamaterials-based thermal devices

Wavelength-selective thermal devices have great applications in concentrating solar power systems, high-temperature thermoelectric systems, and solar thermophotovoltaics (STPVs). Lack of high-temperature stability and spectrally selective emissivity in different wavelength regions limits their efficiency. We propose a one-dimensional HfO₂/Al₂O₃-W nanocomposites/W/Al₂O₃/W multilayered photonic structure as potential wavelength selective thermal devices, and theoretically investigate the emission properties of the proposed Mie-resonance metamaterials from visible (VIS) to midinfrared (MIR) region. HfO₂ thin layer is introduced to serve as an antireflection coating film and W layer acts as an IR reflection layer that enhances the absorptivity/emissivity in VIS and near-infrared (NIR) region while reducing the MIR emission simultaneously. Effects of geometric parameters are discussed, such as different radii and volume fractions of W nanoparticles, the thickness of Al₂O₃-W nanocomposites, and HfO₂ thin film. The proposed thermal absorber and emitter exhibit nearly unity absorptance in both VIS and NIR regions, while the emittance approaches zero in the MIR region. The selective absorption/emission window is tunable by varying geometric parameters. The proposed solar thermal devices have great potentials in engineering applications such as STPVs and solar thermoelectric generator due to flexibility of geometric parameters and ease of fabrication.

High-performance and cost-effective absorber for visible and near-infrared spectrum based on a spherical multilayered dielectric-metal structure

A broadband, polarization-insensitive, wide-angle absorber based on a spherical multilayered dielectric-metal structure is numerically designed and experimentally demonstrated in this paper. This absorber has average absorbance of 0.98 between 380 and 1910 nm, indicating a spectral width of 1530 nm with absorbance exceeding 0.9, and covering the entire visible and near-infrared spectrum. The physical mechanism leading to this broadband absorption is discussed along with the effect of structural parameters on the absorber performance. Importantly, the absorbance is hardly affected by incident angle below 45° and it still stays at a high level with incident angle up to 60°, for both transverse magnetic and transverse electric plane waves.

Angle-Insensitive Broadband Absorption Enhancement of Graphene Using a Multi-Grooved Metasurface

An angle-insensitive broadband absorber of graphene covering the whole visible spectrum is numerically demonstrated, which is resulted from multiple couplings of the electric and magnetic dipole resonances in the narrow metallic grooves. This is achieved by integrating the graphene sheet with a multi-grooved metasurface separated by a polymethyl methacrylate (PMMA) spacer, and an average absorption efficiency of 71.1% can be realized in the spectral range from 450 to 800 nm. The location of the absorption peak of graphene can be tuned by the groove depth, and the bandwidth of absorption can be flexibly controlled by tailoring both the number and the depth of the groove. In addition, broadband light absorption enhancement of graphene is robust to the variations of the structure parameters, and good absorption properties can be maintained even the incident angle is increased to 60°.

Facile design of an ultra-thin broadband metamaterial absorber for C-band applications

We report a facile design of an ultra-thin broadband metamaterial absorber (MA) for C-band applications by utilizing a single layer of a metal-dielectric-metal structure of FR-4 substrate. The absorption performances are characterized using a numerical method. The proposed MA exhibits the broadband absorption response over the entire C-band spectrum range from 4.0 GHz to 8.0 GHz with absorptivity above 90% and the high absorptivity is remained over 80% for a large incident angle up to 40° under both transverse electric (TE) and transverse magnetic (TM) polarizations over the band. The origin of absorption mechanism is explained by the electric and surface current distributions, which is also supported by the retrieved constitutive electromagnetic parameters, significantly affected by magnetic resonance. In addition, compared with the previous reports, the proposed MA presents a greater practical feasibility in term of low-profile and wide incident angle insensitivity, suggesting that the proposed absorber is a promising candidate for C-band applications.

Implementing infrared metamaterial perfect absorbers using dispersive dielectric spacers

A typical metamaterial perfect absorber (MPA) is comprised of a metamaterial layer, a dielectric spacer, and a ground plane. The conventional spacer material is usually a lossy dielectric with little-dispersion for the purpose of easing the design and optimization procedure of the MPA. In this paper, we present the design, fabrication, and characterization of metamaterial perfect absorbers with a highly dispersive spacer, which is compatible with functional microelectromechanical systems. The measured dispersive permittivity of a silicon nitride thin film is used in modeling the absorption response of MPAs with rigorous coupled wave analysis. Different designs of MPA structures are fabricated and characterized. Spectroscopy data shows two perfect absorption peaks in wavelengths ranging from 8 μm to 20 μm, which supports the theoretical calculation and numerical simulation. The dispersion of silicon nitride enables the shared resonant modes of the two peak wavelengths and decreases the wavelength shift led by variations in structural parameters. We demonstrate that the use of dispersive dielectric materials in MPAs potentiates various functional devices.

Broadband infrared binary-pattern metasurface absorbers with micro-genetic algorithm optimization

Broadband binary-pattern metasurface absorbers are designed and demonstrated in the mid-infrared wavelength range through the micro-genetic algorithm. The tungsten-based metasurface absorbers with the optimized binary-pattern nanostructures exhibit broadband near-perfect absorption due to the multiple plasmonic resonances supported within the unit cell. Furthermore, the influence of minor pixel modifications in the optimized binary-pattern nanostructures on the absorption performance is investigated in the experiment. This Letter presents a promising approach to design and optimize complex optical nanostructures with the desired properties for metamaterial and metasurface applications.

Plasmonic Metasurface Absorber Based on Electro-Optic Substrate for Energy Harvesting

A highly efficient and broad light absorber capable of wide-angle absorption in the visible and near infrared range is presented and numerically investigated for energy harvesting in a simple geometry. According to the calculated results, the proposed device has a peak absorption level of about 99.95%. The actual absorption efficiency is 76.35%, which is approaching that of complex multilayer absorbers with 88 layers working in the wavelength range of 300 nm to 2000 nm. The electro-optic material has the potential of shifting the absorption peak position, compensating fabrication errors and thus reducing the fabrication technique difficulties. Also, the high electro-optic tunability can be used for filters, infrared detection, and imaging applications. More directly, the proposed absorber can be potentially deployed in solar cells and solar thermals.