Ultrabroadband light absorption based on photonic topological transitions in hyperbolic metamaterials

All-day uninterrupted thermoelectric generator by simultaneous harvesting of solar heating and radiative cooling

Passive power generation has recently stimulated interest in thermoelectric generators (TEGs) using the radiative cooling mechanism. However, the limited and unstable temperature difference across the TEGs significantly degrades the output performance. In this study, an ultra-broadband solar absorber with a planar film structure is introduced as the hot side of the TEG to increase the temperature difference by utilizing solar heating. This device not only enhances the generation of electrical power but also realizes all-day uninterrupted electrical output due to the stable temperature difference between the cold and hot sides of the TEG. Outdoor experiments show the self-powered TEG obtains maximum temperature differences of 12.67 °C, 1.06 °C, and 5.08 °C during sunny daytime, clear nighttime, and cloudy daytime, respectively, and generates output voltages of 166.2 mV, 14.7 mV, and 95 mV, respectively. Simultaneously, the corresponding output powers of 879.25 mW/m², 3.85 mW/m², and 287.27 mW/m² are produced, achieving 24-hour uninterrupted passive power generation. These findings propose a novel strategy to combine solar heating and outer space cooling by a selective absorber/emitter to generate all-day continuous electricity for unsupervised small devices.

Multifunctional Nanostructures with Controllable Band Gap Giving Highly Stable Infrared Emissivity for Smart Thermal Management

Thermal control is essential to guarantee the optimal performance of most advanced electronic devices or systems. In space, orbital satellites face the issues of high thermal gradients, heating, and different thermal loads mediated by differential illumination from the Sun. Todaýs state-of-the-art thermal control systems provide protection; however, they are bulky and restrict the mass and power budgets for payloads. Here, we develop a lightweight optical superlattice nanobarrier structure to provide a smart thermal control solution. The structure consists of a moisture and outgassing physical barrier (MOB) coupled with atomic oxygen (AO)-UV protection functionality. The nanobarrier exhibits transmission and reflection of light by controlling the optical gap of individual layers to enable high infrared emissivity and variable solar absorptivity (minimum ΔαS = 0.30) across other wavelengths. The multifunctional coating can be applied to heat-sensitive substrates by means of a bespoke room-temperature process. We demonstrate enhanced stability, energy-harvesting capability, and power savings by facilitating the radiation cooling and facility for active self-reconfiguration in orbit. In this way, the reduction of the operating temperature from ∼120 to ∼60 °C on space-qualified and nonmechanically controlled composite structures is also demonstrated.

Multi-octave metasurface-based refractory superabsorber enhanced by a tapered unit-cell structure

An ultra-broadband metasurface-based perfect absorber is proposed based on a periodic array of truncated cone-shaped \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\text {TiO}_2$$\end{document}TiO2 surrounded by TiN/\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\text {TiO}_2$$\end{document}TiO2 conical rings. Due to the refractory materials involved in the metasurface, the given structure can keep its structural stability at high temperatures. The proposed structure can achieve a broadband spectrum of 4.3 µm at normal incidence spanning in the range of 0.2–4.5 µm with the absorption higher than \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$90\%$$\end{document}90% and the average absorption around \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$94.71\%$$\end{document}94.71%. The absorption can be tuned through the angle of the cone. By optimizing geometrical parameters, a super absorption is triggered in the range of 0.2–3.25 µm with the absorption higher than 97.40\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\%$$\end{document}% and substantially average absorption over 99\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\%$$\end{document}%. In this regard, the proposed structure can gather more than \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$99\%$$\end{document}99% of the full spectrum of solar radiation. Furthermore, the absorption of the designed structure is almost insensitive to the launching angle up to \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$50^\circ $$\end{document}50∘ for TE polarization, while it has a weak dependence on the incident angle for TM polarization. The proposed structure can be a promising candidate for thermal energy harvesting and solar absorption applications.

Wafer-scale ultra-broadband perfect absorber based on ultrathin Al-SiO2 stack metasurfaces

Broadband absorbers with high absorption, ultrathin thickness, and lithography-free planar structure have a wide range of potential applications, such as clocking and solar energy harvesting. For plasmonic metal materials, achieving perfect ultra-broadband absorption remains a challenge owing to the intrinsically narrow bandwidth. In this study, wafer-scale Al-SiO₂ stack metasurfaces were experimentally fabricated to realize perfect ultra-broadband absorption. The experimental results show that the absorption for Al-SiO₂ stack metasurfaces can reach up to 98% for the wavelength range from the ultraviolet to the near-infrared (350-1400 nm). It was experimentally verified that the absorption performance of Al-SiO₂ stack metasurfaces is dependent on the layer number and is superior to that of other metal-based stack metasurfaces. This study will pave the way for development of plasmonic metal-based ultra-broadband absorbers as in low cost and high performance robust solar energy devices.

Visible and Near-Infrared Broadband Absorber Based on Ti3C2Tx MXene-Wu

A high absorption broadband absorber based on MXene and tungsten nanospheres in visible and near-infrared bands is proposed. The absorber has a maximum absorption of 100% and an average absorption of 95% in the wavelength range of 400-2500 nm. The theoretical mechanism and parameter adjustability of the absorber are analyzed by FDTD solutions. The results show that the structural parameters can effectively adjust the absorption performance. The good absorption performance is due to the action of the local surface plasmon resonance coupling with the gap surface plasmon resonance and Fabry-Perot resonance. The simulation results show that the absorber is insensitive to the polarization and oblique incidence angle of incident light, and that high absorption and broadband can be maintained when the oblique incidence angle is up to 60°.

Tunable angle-selective optical transparency induced by photonic topological transition in Dirac semimetals-based hyperbolic metamaterials

The tunable angle-selective transparency of hyperbolic metamaterials consisting of various multilayers of Dirac semimetal and dielectric materials are theoretically and numerically studied in the terahertz range. Three stack configurations are considered: alternating, sandwiched, and disordered. It is found that the proposed structures exhibit strong optical angular selectivity induced by photonic topological transition for transverse magnetic waves. Interestingly, the topological transition frequency can be flexibly modulated by changing the Fermi energy, temperature, and the releasing time of the Dirac semimetal, as well as the thickness ratio of the dielectric and semimetal layers. It is also noticed that the angular optical transparency properties are independent of the order of the proposed structure even in alternating/disordered/random configurations if the total thickness ratio of the semimetal to dielectric are the same, which makes the properties particularly easy to realize experimentally. The proposed hyperbolic metamaterial structures present a promising opportunity for wavefront engineering, offering crucial properties for applications in private screens, optical detectors, and light manipulation.

Lithography-free wide-angle polarization-independent ultra-broadband absorber based on anti-reflection effect

We propose a lithography-free wide-angle polarization-insensitive ultra-broadband absorber by using three pairs of tungsten (W) and calcium fluoride (CaF₂) films. The simulation results show that the absorptivity is larger than 0.9 with normal incidence in the wavelength range from 400 nm to 1529 nm. By adding a pair of CaF₂-W films, we can get a broader absorption bandwidth with absorptivity larger than 0.9 over the wavelength of 400-1639 nm. In addition, the absorption performance is insensitive to the polarization and angle of incidence. The electric field distributions at the absorption peaks show that the absorption is originated from the destructive interference between the reflection waves from the top and bottom interfaces of the multilayer CaF₂-W films. Furthermore, the ultra-broad bandwidth is attributed to the anti-reflection effect from the increased effective refractive index from top to down of the proposed absorber. Such physical mechanism of broadening bandwidth based on anti-reflection effect provides a new idea for the design of broadband absorber. Meanwhile, this broadband absorber is a good candidate for potential applications such as detection and energy harvesting.

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.

Triangular metallic ring-shaped broadband polarization-insensitive and wide-angle metamaterial absorber for visible regime

The inherent bandwidth limitations make it quite challenging to achieve the wideband response of metamaterial absorbers. In this paper, a metamaterial absorber based on triangular metallic rings has been proposed to attain wideband absorption (>90%) in the wavelength span of 400-750 nm. The absorber is constituted of periodically placed unit cells, where each unit cell contains three concentric triangular chromium metal rings. The absorption of the design remains stable (above 70%) over a wide range of incidence obliquity (0°-60°) under transverse electric (TE) and transverse magnetic (TM) polarization. Further, the absorber shows polarization-insensitive behavior over different polarization states. The low-cost and thermally endurable chromium metal, wide absorption, and wide-angle stability make the proposed absorber a suitable candidate for applications like solar energy harvesting, solar detectors, solar thermal photovoltaics, and photonic devices.

Broadband wide-angle multilayer absorber based on a broadband omnidirectional optical Tamm state

Recently, broadband optical Tamm states (OTSs) in heterostructures composed of highly lossy metal layers and all-dielectric one-dimensional (1D) photonic crystals (PhCs) have been utilized to realize broadband absorption. However, as the incident angle increases, the broadband OTSs in such heterostructures shift towards shorter wavelengths along the PBGs in all-dielectric 1D PhCs, which strongly limits the bandwidths of wide-angle absorption. In this paper, we realize a broadband omnidirectional OTS in a heterostructure composed of a Cr layer and a 1D PhC containing layered hyperbolic metamaterials with an angle-insensitive photonic band gap. Assisted by the broadband omnidirectional OTS, broadband wide-angle absorption can be achieved. High absorptance (A > 0.85) can be remained when the wavelength ranges from 1612 nm to 2335 nm and the incident angle ranges from 0° to 70°. The bandwidth of wide-angle absorption (0°-70°) reaches 723 nm. The designed absorber is a lithography-free 1D structure, which can be easily fabricated under the current magnetron sputtering or electron-beam vacuum deposition technique. This broadband, wide-angle, and lithography-free absorber would possess potential applications in the design of photodetectors, solar thermophotovoltaic devices, gas analyzers, and cloaking devices.

Ultra-broadband metamaterial absorbers from long to very long infrared regime

Broadband metamaterials absorbers with high absorption, ultrathin thickness and easy configurations are in great demand for many potential applications. In this paper, we first analyse the coupling resonances in a Ti/Ge/Ti three-layer absorber, which can realise broadband absorption from 8 to 12 μm. Then we experimentally demonstrate two types of absorbers based on the Ti/Ge/Si3N4/Ti configuration. By taking advantage of coupling surface plasmon resonances and intrinsic absorption of lossy material Si3N4, the average absorptions of two types of absorbers achieve almost 95% from 8 to 14 μm (experiment result: 78% from 6.5 to 13.5 μm). In order to expand the absorption bandwidth, we further propose two Ti/Si/SiO2/Ti absorbers which can absorb 92% and 87% of ultra-broadband light in the 14-30 μm and 8-30 μm spectral range, respectively. Our findings establish general and systematic strategies for guiding the design of metamaterial absorbers with excellent broadband absorption and pave the way for enhancing the optical performance in applications of infrared thermal emitters, imaging and photodetectors.

Efficient inverse design and spectrum prediction for nanophotonic devices based on deep recurrent neural networks

The development of nanophotonic devices has presented a revolutionary means to manipulate light at nanoscale. How to efficiently design these devices is an active area of research. Recently, artificial neural networks (ANNs) have displayed powerful ability in the inverse design of nanophotonic devices. However, there is limited research on the inverse design for modeling and learning the sequence characteristics of a spectrum. In this work, we propose a deep learning method based on an improved recurrent neural network to extract the sequence characteristics of a spectrum and achieve inverse design and spectrum prediction. A key feature of the network is that the memory or feedback loops it comprises allow it to effectively recognize time series data. In the context of nanorods hyperbolic metamaterials, we demonstrated the high consistency between the target spectrum and the predicted spectrum, and the network learned the deep physical relationship concerning the structural parameter changes reflected on the spectrum. The effectiveness of our approach is also tested by user-drawn spectra. Moreover, the proposed model is capable of predicting an unknown spectrum based on a known spectrum with only 0.32% mean relative error. The prediction model may be helpful to predict data beyond the detection limit. We propose this versatile method as an effective and accurate alternative to the application of ANNs in nanophotonics, paving way for fast and accurate design of desired devices.

Conductive metal-oxide-based tunable, wideband, and wide-angle metamaterial absorbers operating in the near-infrared and short-wavelength infrared regions

This paper investigates the absorption features of the gallium-doped zinc oxide (GZO) nanowire-based metamaterial absorber. The absorption spectrum was analyzed in the near-infrared regime (NIR) and short-wavelength infrared regime (SWIR). The wideband absorption was observed in the NIR and SWIR. The effect of the volume fraction of the nanorod and thickness of the metamaterial was depicted on the absorption spectrum. Observations reveal that at a lower volume fraction of GZO, absorption is noticed at SWIR of the spectrum; however, for a large volume fraction, blueshift in the absorption band is noticed, and absorption is found in the NIR of the spectrum. Further, a larger volume fraction of GZO attributes to the property of hyperbolic metamaterial. Moreover, the figure of merit and operational bandwidth are analyzed to optimize the absorption results. Such an absorber would be useful for energy harvesting, SWIR photodetectors, and imaging applications.

Broadband polarization-insensitive and wide-angle solar energy absorber based on tungsten ring-disc array

Nowadays, solar energy is considered one of the most clean energy sources. In addition, the data from the literature tell us that its main radiation bandwidth is approximately 295-2500 nm. In this work, we proposed a novel kind of broadband solar energy absorber based on tungsten (W) to achieve broadband absorption of solar energy. A four-layer ring-disk structure (SiO2-SiO2-W) is employed in our design. A finite-difference time-domain (FDTD) simulation was used to ascertain the absorption performance of the absorber. The results demonstrate that a broadband solar energy absorption was realized, the bandwidth is of 1530 nm with an absorption efficiency of more than 90%, and an absorption efficiency of 97% was achieved in this region. The absorption spectra can be tuned through changing the structural and geometric parameters. Moreover, the absorber has excellent polarization independence and can be used under incident angles from 0° to 60°. The proposed solar energy absorber is simple to fabricate, and can be used for photothermal conversion, solar energy harvesting and utilization.

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.

Anisotropic Photonics Topological Transition in Hyperbolic Metamaterials Based on Black Phosphorus

Based on in-plane anisotropy of black phosphorus (BP), anisotropic photonics topological transition (PTT) can be achieved by the proposed hyperbolic metamaterials structure, which is composed of alternating BP/SiO₂ multilayer. Through effective medium theory and calculated iso-frequency contour, PTT can be found by carefully choosing the incident plane and other parameters. With the finite element method and transfer matrix method, a narrow angular optical transparency window with angular full width at half maximum of 1.32° exists at PTT. By changing the working wavelength, thickness of SiO₂, or electron doping of black phosphorus, the incident plane of realizing PTT can be modulated, and anisotropic PTT is achieved.

A near-ideal solar selective absorber with strong broadband optical absorption from UV to NIR

A near-ideal solar absorber, which is composed of a metal nanowire array and a planar multilayer system, is proposed and investigated. Both numerical and analytical results show that the proposed nanostructure can achieve over 90% optical absorption throughout the wavelength range of 300-1909 nm due to the coupled effect of multiple resonance modes, and can maintain a good absorption stability over a wide incident angle regardless of the polarization states. Meanwhile, for practical applications, the total photothermal conversion efficiency can reach 95.57% at operating temperature of 373.15 K, which is particularly useful in solar energy harvesting. The absorption performance is also strongly dependent on the diameter and height of nanowire as well as the thicknesses of dielectric layers, enabling the further improvement of both the operating bandwidth and absorption efficiency. Moreover, by adjusting the period of the multilayer or nanowire materials, the selective absorption properties of this nanostructure can be flexibly controlled to satisfy more spectral requirements. These features make the presented designs hold promise for a series of solar-dependent optical applications, such as photothermal energy generation and thermal emitters.