Nanostructured chromium-based broadband absorbers and emitters to realize thermally stable solar thermophotovoltaic systems

A tungsten-based metamaterial emitter for solar thermophotovoltaic systems

Solar thermophotovoltaic systems are capable of showing efficient photoelectric conversion and are expected to surpass the Shockley-Queisser limit, owing to the spectrum-selective functionality of metamaterial selective emitters. Generally, metamaterial emitters are manufactured from multifarious materials, which also makes their manufacturing process complicated. Here, we propose a tungsten-only emitter composed of two rectangular bars with different widths and heights arranged in a cruciform structure, featuring a rectangular cavity at the top. Results from the simulations reveal that the emissivity of the metamaterial emitter exceeds 90% at the wavelength of 950-1590 nm and drops below 20% for wavelengths exceeding 2025 nm, which can effectively match GaSb photovoltaic cells. The outstanding emission performance is attributed to the coupling effect of surface plasmon resonance, cavity resonance and guided mode resonance, as evidenced by the analysis of electric and magnetic fields. We also explored the radiation spectrum in the 500-2500 K temperature range and found that it performed best at 1400 K. It is concluded that the emission performance is slightly affected by structural parameters and angles. This study presents a meaningful exploration of efficient solar utilization.

An elliptical nanoantenna array plasmonic metasurface for efficient solar energy harvesting

Plasmonic metasurfaces with subwavelength nanoantenna arrays have attracted much attention for their ability to control and manage optical properties. Solar absorbers are potential candidates for effectively converting photons into heat and electricity. This study introduces a novel ultrathin metasurface solar absorber employing elliptical-shaped nanoantenna arrays. We theoretically and numerically demonstrate a near-perfect broadband absorber with over 90% absorption efficiency in a wide range of wavelengths of 300-2500 nm, using finite element (FEM) and finite-difference time-domain (FDTD) methods. The proposed nanostructure configuration enhances light absorption by exciting localized surface plasmon resonances (LSPRs) between elliptical-shaped nanoantenna gaps at many wavelengths, maintaining stability at wide incident angles and insensitivity to light polarization. Compared to other state-of-the-art absorbers with a thickness of less than 300 nm, the designed nanostructure with 260 nm thickness achieves over 90% optical absorption across a broad range of wavelengths of 300-1116 nm in air (or vacuum) environments and performs effectively under water conditions for solar energy harvesting in a range of wavelengths of 300-1436 nm, and therefore can serve as a solar evaporator. Combining refractory plasmonic titanium nitride (TiN) and semiconductor gallium nitride (GaN) nanostructures holds great potential for efficient optoelectronic and photocatalytic applications, especially in harsh and high-temperature environments like thermophotovoltaic systems. The TiN-based metasurface absorber, with its ultrathin nanostructure and suitable spectral absorption in ultraviolet-visible-infrared spectra, offers scalability and cost-effectiveness. The findings in this work will deepen our understanding of LSPRs and pave a novel path for efficient solar energy conversion.

Perovskite Lanthanum-Doped Barium Stannate: A Refractory Near-Zero-Index Material for High-Temperature Energy Harvesting Systems

The recent interests in bridging intriguing optical phenomena and thermal energy management has led to the demonstration of controlling thermal radiation with epsilon-near-zero (ENZ) and the related near-zero-index (NZI) optical media. In particular, the manipulation of thermal emission using phononic ENZ and NZI materials has shown promise in mid-infrared radiative cooling systems operating under low-temperature environments (below 100 °C). However, the absence of NZI materials capable of withstanding high temperatures has limited the spectral extension of these advanced technologies to the near-infrared (NIR) regime. Herein, a perovskite conducting oxide, lanthanum-doped barium stannate (La:BaSnO3 [LBSO]), as a refractory NZI material well suited for engineering NIR thermal emission is proposed. This work focuses on the experimental demonstration of superior high-temperature stability (of at least 1000 °C) of LBSO films in air and its durability under intense UV-pulsed laser irradiation below peak power of 9 MW cm-2 . Based on the low optical-loss in LBSO, a selective narrow-band thermal emission utilizing a metal-insulator-metal (MIM) Fabry-Pérot nanocavity consisting of LBSO films as metallic component is demonstrated. This study shows that LBSO is an ideal candidate as a refractory NZI component for thermal energy conversion operating at high temperatures in air and under strong light irradiations.

Spin-isolated ultraviolet-visible dynamic meta-holographic displays with liquid crystal modulators

Wearable displays or head-mounted displays (HMDs) have the ability to create a virtual image in the field of view of one or both eyes. Such displays constitute the main platform for numerous virtual reality (VR)- and augmented reality (AR)-based applications. Meta-holographic displays integrated with AR technology have potential applications in the advertising, media, and healthcare sectors. In the previous decade, dielectric metasurfaces emerged as a suitable choice for designing compact devices for highly efficient displays. However, the small conversion efficiency, narrow bandwidth, and costly fabrication procedures limit the device's functionalities. Here, we proposed a spin-isolated dielectric multi-functional metasurface operating at broadband optical wavelengths with high transmission efficiency in the ultraviolet (UV) and visible (Vis) regimes. The proposed metasurface comprised silicon nitride (Si₃N₄)-based meta-atoms with high bandgap, i.e., ∼ 5.9 eV, and encoded two holographic phase profiles. Previously, the multiple pieces of holographic information incorporated in the metasurfaces using interleaved and layer stacking techniques resulted in noisy and low-efficiency outputs. A single planar metasurface integrated with a liquid crystal was demonstrated numerically and experimentally in the current work to validate the spin-isolated dynamic UV-Vis holographic information at broadband wavelengths. In our opinion, the proposed metasurface can have promising applications in healthcare, optical security encryption, anti-counterfeiting, and UV-Vis nanophotonics.

A unique physics-inspired deep-learning-based platform introducing a generalized tool for rapid optical-response prediction and parametric-optimization for all-dielectric metasurfaces

Metasurfaces are composed of a two-dimensional array of carefully engineered subwavelength structures. They provide a novel compact alternative to conventional voluminous optical components. However, their design involves a time-consuming hit and trial procedure, requiring many iterative electromagnetic simulations through expensive commercial solvers. To overcome this non-practical design strategy, recently, various deep-learning-based fast and low computational cost networks have been proposed to design and optimize individual meta-atoms and complete metasurfaces. Most of them focus on optimizing the amplitude response of nanostructures, whereas mapping the phase response is a much more challenging problem that needs to be addressed. Since the metaatom's optical response is entirely reliant on and vulnerable to its geometrical structure, underlying material, and operating wavelength, changing any of these parameters changes the entire physics of the problem in hand. Here, we propose novel deep-learning-based generalized forward and inverse design approaches to optimize all-dielectric transmissive metasurfaces. The proposed forward predicting neural networks take all the geometrical parameters and the physical properties of the bar-shaped dielectric nano-resonators as the input and predict the cross-polarized transmission amplitude and modulated phase at eight distinct rotation angles of the nano-bar. These networks are generalized to predict the electromagnetic (EM) response of different dielectric materials at different operating wavelengths. An inverse design neural network is also proposed that takes the target transmission amplitude and phase at eight discrete orientation angles of the nano-bar as the primary input. The underlying physics of the problem is also incorporated by feeding the intrinsic material properties and the operating wavelength of the nano-bar as a second input to the inverse neural network. It predicts the optimum set of geometrical parameters to achieve maximum cross-polarized transmission and complete Pancharatnam-Berry (PB) phase coverage from 0 to 2π. The average test data mean square error (MSE) achieved for the forward predicting neural network is 1.8 × 10^-3 and that of the inverse design neural network is 2.8 × 10^-1. The average MSEs for different material's test samples are demonstrated to validate the generalizability of the proposed models in terms of seen and unseen materials. A comparative analysis of the proposed approach with conventional EM software optimization tools is performed to prove that the proposed inverse design works much faster than the conventional methods, also it can handle a comparatively larger range of parameters and predicts the results in a single run with high accuracy.

Electrically addressable integrated intelligent terahertz metasurface

Reconfigurable intelligent surfaces (RISs) play an essential role in various applications, such as next-generation communication, uncrewed vehicles, and vital sign recognizers. However, in the terahertz (THz) region, the development of RISs is limited because of lacking tunable phase shifters and low-cost sensors. Here, we developed an integrated self-adaptive metasurface (SAM) with THz wave detection and modulation capabilities based on the phase change material. By applying various coding sequences, the metasurface could deflect THz beams over an angle range of 42.8°. We established a software-defined sensing reaction system for intelligent THz wave manipulation. In the system, the SAM self-adaptively adjusted the THz beam deflection angle and stabilized the reflected power in response to the detected signal without human intervention, showing vast potential in eliminating coverage dead zones and other applications in THz communication. Our programmable controlled SAM creates a platform for intelligent electromagnetic information processing in the THz regime.

Nickel-Based High-Bandwidth Nanostructured Metamaterial Absorber for Visible and Infrared Spectrum

The efficient control of optical light at the nanoscale level attracts marvelous applications, including thermal imaging, energy harvesting, thermal photovoltaics, etc. These applications demand a high-bandwidth, thermally robust, angularly stable, and miniaturized absorber, which is a key challenge to be addressed. So, in this study, the simple and cost-effective solution to attain a high-bandwidth nanostructured absorber is demonstrated. The designed nanoscale absorber is composed of a simple and plain circular ring of nickel metal, which possesses many interesting features, including a miniaturized geometry, easily fabricable design, large operational bandwidth, and polarization insensitivity, over the previously presented absorbers. The proposed nanoscale absorber manifests an average absorption of 93% over a broad optical window from 400 to 2800 nm. Moreover, the detailed analysis of the absorption characteristics is also performed by exciting the optical light's various incident and polarization angles. From the examined outcome, it is concluded that the nanostructured absorber maintains its average absorption of 80% at oblique incident angles in a broad wavelength range from 400 to 2800 nm. Owing to its appealing functionalities, such as the large bandwidth, simple geometry, low cost, polarization insensitivity, and thermal robustness of the constituting metal, nickel (Ni), this nano-absorber is made as an alternative for the applications of energy harvesting, thermal photovoltaics, and emission.

Highly Efficient Perfect Vortex Beams Generation Based on All-Dielectric Metasurface for Ultraviolet Light

Featuring shorter wavelengths and high photon energy, ultraviolet (UV) light enables many exciting applications including photolithography, sensing, high-resolution imaging, and optical communication. The conventional methods of UV light manipulation through bulky optical components limit their integration in fast-growing on-chip systems. The advent of metasurfaces promised unprecedented control of electromagnetic waves from microwaves to visible spectrums. However, the availability of suitable and lossless dielectric material for the UV domain hindered the realization of highly efficient UV metasurfaces. Here, a bandgap-engineered silicon nitride (Si3N4) material is used as a best-suited candidate for all-dielectric highly efficient UV metasurfaces. To demonstrate the wavefront manipulation capability of the Si3N4 for the UV spectrum, we design and numerically simulate multiple all-dielectric metasurfaces for the perfect vortex beam generation by combing multiple phase profiles into a single device. For different numerical apertures (NA =0.3 and 0.7), it is concluded that the diffracted light from the metasurfaces with different topological charges results in an annular intensity profile with the same ring radius. It is believed that the presented Si3N4 materials and proposed design methodology for PV beam-generating metasurfaces will be applicable in various integrated optical and nanophotonic applications such as information processing, high-resolution spectroscopy, and on-chip optical communication.

Silicon-based spectrally selective emitters with good high-temperature stability on stepped metasurfaces

Solar thermophotovoltaic (STPV) systems have attracted increasing attention due to their great prospects for breaking the Shockley-Queisser limit. As a critical component of high-performance STPV systems, fabrication of a spectrally selective emitter with good stability at high temperature is one of the main research challenges. In this study, we developed a hybrid silicon-based metasurface emitter with spectral selectivity and high temperature stability using a simple fabrication process by introducing a controlled silicon nitride (SiN_x) layer on a silicon stepped nanopillar substrate coated with molybdenum (Mo). Owing to the cooperative effect of cavity mode resonance and the interference effect of the SiN_x dielectric layer, our proposed silicon-based metasurface emitter achieves a broadband optical absorption of ∼95% in the wavelength range of 220-2000 nm, while effectively suppressing the heat radiation to ∼19% in the long wavelength range (>5 μm). Moreover, polarization-independence and angle-insensitivity behaviors are demonstrated in the emitters. Additionally, due to the presence of a SiN_x protection layer, this silicon-based metasurface emitter is experimentally proved to sustain its excellent spectral properties after ultra-high temperature treatments, including annealing at 1273 K under an Ar atmosphere for 6 h, even at 1073 K in air for 1 h, which makes it an alternative candidate for application in actual STPV systems.