Gradient index effect assisted anisotropic broadband absorption in α-MoO3 metamaterial

As an excellent natural hyperbolic material (HM), α-M o O ₃ has a larger hyperbolic bandwidth and longer polariton lifetime than other HMs, which makes it an ideal candidate for broadband absorbers. In this work, we theoretically and numerically investigated the spectral absorption of an α-M o O ₃ metamaterial using the gradient index effect. The results show that the absorber has an average spectral absorbance of 99.99% at 12.5-18 µm at transverse electric polarization. When the incident light is transverse magnetic polarization, the broadband absorption region of the absorber is blueshifted, and a similar strong absorption is achieved at 10.6-12.2 µm. By simplifying the geometric model of the absorber using equivalent medium theory, we find that the broadband absorption is caused by the refractive index matching of the metamaterial to the surrounding medium. The electric field and power dissipation density distributions of the metamaterial were calculated to clarify the location of the absorption. Moreover, the influence of geometric parameters of pyramid structure on broadband absorption performance was discussed. Finally, we investigated the effect of polarization angle on the spectral absorption of the α-M o O ₃ metamaterial. This research contributes to developing broadband absorbers and related devices based on anisotropic materials, especially in solar thermal utilization and radiation cooling.

Switchable ultra-broadband absorption and polarization conversion metastructure controlled by light

This article proposed a metastructure device that can realize polarization conversion (PC) and absorption function switching in the terahertz (THz) range based on the photoconductivity effect. The photoconductance is formed by exposing silicon to different intensities of light, then the PC and absorption function can be switched. At the same time, the absorption bandwidth is expanded by inserting air resonant cavities into the dielectric substrate, changing the thickness of the dielectric locally, and cutting rectangular slots at the metal bottom plate. When the device works as a polarization converter, linear-to-linear PC with a polarization conversion rate of over 90% at 0.96-1.47 THz can be achieved, and its relative bandwidth is 42%. And when the silicon conductivity is fixed at 3500 S/m through illuminating, the device switches to an ultra-broadband absorber with over 90% absorption at 0.75-1.73 THz and a relative bandwidth of 79%. The designed device can be applied efficiently in many fields, such as electromagnetic cloaking and communication.

Absorption and scattering in perfect thermal radiation absorber-emitter metasurfaces

Detailed spectral analysis of radiation absorption and scattering behaviors of metasurfaces was carried out via finite-difference time-domain (FDTD) photonic simulations. It revealed that, for typical metal-insulator-metal (MIM) nanodisc metasurfaces, absorbance and scattering cross-sections exhibit a ratio of σ_abs/σ_sca = 1 at the absorption peak spectral position. This relationship was likewise found to limit the attainable photo-thermal conversion efficiency in experimental and application contexts. By increasing the absorption due to optical materials, such as Cr metal nano-films typically used as an adhesion layer, it is possible to control the total absorption efficiency η = σ_abs/σ_sca and to make it the dominant extinction mechanism. This guided the design of MIM metasurfaces tailored for near-perfect-absorption and emission of thermal radiation. We present the fabrication as well as the numerical and experimental spectral characterisation of such optical surfaces.

Kirchhoff's Thermal Radiation from Lithography-Free Black Metals

Lithography-free black metals composed of a nano-layered stack of materials are attractive not only due to their optical properties but also by virtue of fabrication simplicity and the cost reduction of devices based on such structures. We demonstrate multi-layer black metal layered structures with engineered electromagnetic absorption in the mid-infrared (MIR) wavelength range. Characterization of thin SiO2 and Si films sandwiched between two Au layers by way of experimental electromagnetic radiation absorption and thermal radiation emission measurements as well as finite difference time domain (FDTD) numerical simulations is presented. Comparison of experimental and simulation data derived optical properties of multi-layer black metals provide guidelines for absorber/emitter structure design and potential applications. In addition, relatively simple lithography-free multi-layer structures are shown to exhibit absorber/emitter performance that is on par with what is reported in the literature for considerably more elaborate nano/micro-scale patterned metasurfaces.

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.

Tunable Perfect Narrow-Band Absorber Based on a Metal-Dielectric-Metal Structure

In this paper, a metal-dielectric-metal structure based on a Fabry–Perot cavity was proposed, which can provide near 100% perfect narrow-band absorption. The lossy ultrathin silver film was used as the top layer spaced by a lossless silicon oxide layer from the bottom silver mirror. We demonstrated a narrow bandwidth of 20 nm with 99.37% maximum absorption and the absorption peaks can be tuned by altering the thickness of the middle SiO2 layer. In addition, we established a deep understanding of the physics mechanism, which provides a new perspective in designing such a narrow-band perfect absorber. The proposed absorber can be easily fabricated by the mature thin film technology independent of any nano structure, which make it an appropriate candidate for photodetectors, sensing, and spectroscopy.

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.

Quad-Band Plasmonic Perfect Absorber for Visible Light with a Patchwork of Silicon Nanorod Resonators

In this paper, a plasmonic perfect absorber (PPA) based on a silicon nanorod resonator (SNRR) for visible light is proposed and investigated numerically. The proposed PPA is only a two-layer nanostructure consisting of a SNRR periodic array and metal substrate. The perfect absorption mainly originates from excitation of the localized surface plasmon resonance (LSPR) mode in the SNRR structure. The absorption properties of this design can be adjusted by varying the radius (r) and height (h) of the SNRR structure. What is more, the stronger quad-band absorption can be achieved by combing four different radius of the SNRR in one period as a super unit-cell. Numerical simulation indicates that the designed quad-band PPA can achieve the absorbance of 99.99%, 99.8%, 99.8%, and 92.2% at 433.5 THz, 456 THz, 482 THz, and 504.5 THz, respectively. Further simulations show that the proposed PPA is polarization-insensitive for both transverse electric (TE) and transverse magnetic (TM) modes. The proposed PPA can be a desirable candidate for some potential applications in detecting, sensing, and visible spectroscopy.

97 percent light absorption in an ultrabroadband frequency range utilizing an ultrathin metal layer: randomly oriented, densely packed dielectric nanowires as an excellent light trapping scaffold

In this paper, we propose a facile and large scale compatible design to obtain perfect ultrabroadband light absorption using metal-dielectric core-shell nanowires. The design consists of atomic layer deposited (ALD) Pt metal uniformly wrapped around hydrothermally grown titanium dioxide (TiO₂) nanowires. It is found that the randomly oriented dense TiO₂ nanowires can impose excellent light trapping properties where the existence of an ultrathin Pt layer (with a thickness of 10 nm) can absorb the light in an ultrabroadband frequency range with an amount near unity. Throughout this study, we first investigate the formation of resonant modes in the metallic nanowires. Our findings prove that a nanowire structure can support multiple longitudinal localized surface plasmons (LSPs) along its axis together with transverse resonance modes. Our investigations showed that the spectral position of these resonance peaks can be tuned with the length, radius, and orientation of the nanowire. Therefore, TiO₂ random nanowires can contain all of these features simultaneously in which the superposition of responses for these different geometries leads to a flat perfect light absorption. The obtained results demonstrate that taking unique advantages of the ALD method, together with excellent light trapping of chemically synthesized nanowires, a perfect, bifacial, wide angle, and large scale compatible absorber can be made where an excellent performance is achieved while using less materials.

Disordered Nanohole Patterns in Metal-Insulator Multilayer for Ultra-broadband Light Absorption: Atomic Layer Deposition for Lithography Free Highly repeatable Large Scale Multilayer Growth

In this paper, we demonstrate a facile, lithography free, and large scale compatible fabrication route to synthesize an ultra-broadband wide angle perfect absorber based on metal-insulator-metal-insulator (MIMI) stack design. We first conduct a simulation and theoretical modeling approach to study the impact of different geometries in overall stack absorption. Then, a Pt-Al₂O₃ multilayer is fabricated using a single atomic layer deposition (ALD) step that offers high repeatability and simplicity in the fabrication step. In the best case, we get an absorption bandwidth (BW) of 600 nm covering a range of 400 nm–1000 nm. A substantial improvement in the absorption BW is attained by incorporating a plasmonic design into the middle Pt layer. Our characterization results demonstrate that the best configuration can have absorption over 0.9 covering a wavelength span of 400 nm–1490 nm with a BW that is 1.8 times broader compared to that of planar design. On the other side, the proposed structure retains its absorption high at angles as wide as 70°. The results presented here can serve as a beacon for future performance enhanced multilayer designs where a simple fabrication step can boost the overall device response without changing its overall thickness and fabrication simplicity.

Ultra-broadband, wide angle absorber utilizing metal insulator multilayers stack with a multi-thickness metal surface texture

In this paper, we propose a facile route to fabricate a metal insulator multilayer stack to obtain ultra-broadband, wide angle behavior from the structure. The absorber, which covers near infrared (NIR) and visible (Vis) ranges, consists of a metal-insulator-metal-insulator (MIMI) multilayer where the middle metal layer has a variant thickness. It is found that this non-uniform thickness of the metal provides us with an absorption that is much broader compared to planar architecture. In the non-uniform case, each thickness is responsible for a specific wavelength range where the overall absorption is the superposition of these resonant responses and consequently a broad, perfect light absorption is attained. We first numerically examine the impact of different geometries on the overall light absorption property of the multilayer design. Afterward, we fabricate the designs and characterize them to experimentally verify our numerical findings. Characterizations show a good agreement with numerical results where the optimum absorption bandwidth for planar design is found to be 620 nm (380 nm-1000 nm) and it is significantly boosted to an amount of 1060 nm (350 nm-1410 nm) for multi-thickness case.

Percolative cobalt/silicon nitride composites with tunable negative electromagnetic parameters

A 3D fishnet structure is formed in the Co/Si₃N₄ composites to obtain tunable negative electromagnetic parameters from 550 MHz to 1 GHz, which have potential applications in antennas, electromagnetic shielding and band-pass filter.

Tunable negative permittivity behavior of random carbon/alumina composites in the radio frequency band

As carbon content increases, the composites undergo a capacitive–inductive transition, and a weakly negative permittivity behavior is observed.