Coupled Tamm Phonon and Plasmon Polaritons for Designer Planar Multiresonance Absorbers

Wavelength-selective absorbers (WS-absorbers) are of interest for various applications, including chemical sensing and light sources. Lithography-free fabrication of WS-absorbers can be realized via Tamm plasmon polaritons (TPPs) supported by distributed Bragg reflectors (DBRs) on plasmonic materials. While multifrequency and nearly arbitrary spectra can be realized with TPPs via inverse design algorithms, demanding and thick DBRs are required for high quality-factors (Q-factors) and/or multiband TPP-absorbers, increasing the cost and reducing fabrication error tolerance. Here, high Q-factor multiband absorption with limited DBR layers (3 layers) is experimentally demonstrated by Tamm hybrid polaritons (THPs) formed by coupling TPPs and Tamm phonon polaritons when modal frequencies are overlapped. Compared to the TPP component, the Q-factors of THPs are improved twofold, and the angular broadening is also reduced twofold, facilitating applications where narrow-band and nondispersive WS-absorbers are needed. Moreover, an open-source algorithm is developed to inversely design THP-absorbers consisting of anisotropic media and exemplify that the modal frequencies can be assigned to desirable positions. Furthermore, it is demonstrated that inversely designed THP-absorbers can realize same spectral resonances with fewer DBR layers than a TPP-absorber, thus reducing the fabrication complexity and enabling more cost-effective, lithography-free, wafer-scale WS-absorberss for applications such as free-space communications and gas sensing.

Broadband Solar Metamaterial Absorbers Empowered by Transformer-Based Deep Learning

The research of metamaterial shows great potential in the field of solar energy harvesting. In the past decade, the design of broadband solar metamaterial absorber (SMA) has attracted a surge of interest. The conventional design typically requires brute-force optimizations with a huge sampling space of structure parameters. Very recently, deep learning (DL) has provided a promising way in metamaterial design, but its application on SMA development is barely reported due to the complicated features of broadband spectrum. Here, this work develops the DL model based on metamaterial spectrum transformer (MST) for the powerful design of high-performance SMAs. The MST divides the optical spectrum of metamaterial into N patches, which overcomes the severe problem of overfitting in traditional DL and boosts the learning capability significantly. A flexible design tool based on free customer definition is developed to facilitate the real-time on-demand design of metamaterials with various optical functions. The scheme is applied to the design and fabrication of SMAs with graded-refractive-index nanostructures. They demonstrate the high average absorptance of 94% in a broad solar spectrum and exhibit exceptional advantages over many state-of-the-art counterparts. The outdoor testing implies the high-efficiency energy collection of about 1061 kW h m^-2 from solar radiation annually. This work paves a way for the rapid smart design of SMA, and will also provide a real-time developing tool for many other metamaterials and metadevices.

Tunable Optimal Dual Band Metamaterial Absorber for High Sensitivity THz Refractive Index Sensing

We present a simple dual band absorber design and investigate it in the terahertz (THz) region. The proposed absorber works in dual operating bands at 5.1 THz and 11.7 THz. By adjusting the graphene chemical potential, the proposed absorber has the controllability of the resonance frequency to have perfect absorption at various frequencies. The graphene surface plasmon resonance results in sharp and narrow resonance absorption peaks. For incident angles up to 8°, the structure possesses near-unity absorption. The proposed sensor absorber's functionality is evaluated using sensing medium with various refractive indices. The proposed sensor is simulated for glucose detection and a maximum sensitivity of 4.72 THz/RIU is observed. It has a maximum figure of merit (FOM) and Quality factor (Q) value of 14 and 32.49, respectively. The proposed optimal absorber can be used to identify malaria virus and cancer cells in blood. Hence, the proposed plasmonic sensor is a serious contender for biomedical uses in the diagnosis of bacterial infections, cancer, malaria, and other diseases.

Predictable infrared dual-band narrow-band absorber for infrared detection

Dual-band infrared absorbers have received a great deal of attention for their potential applications in the field of sensing and detection. In this paper, we proposed a composite model consisting of Platinum nano-cylinder and micro-ring column stacked on top of Si₃N₄and Platinum films. The effect of geometrical parameters on spectral absorption was explored by finite difference in time domain methods, and the results revealed that there were narrow perfect absorption peaks in each of the two atmospheric window bands due to the magnetic polaritons. Meanwhile, the quantitative relationship of resonance wavelength and geometrical parameters were predicted by LC equivalent circuits. In addition, graphene was added to the structure to dynamically adjust the resonance wavelength by varying the Fermi level. The combination of graphene and microstructure achieved full coverage detection of wavelengths in the atmospheric window range. This dual-band absorber has potential applications in infrared detection because of its good absorption properties and its tunability.

Electroplating Deposition of Bismuth Absorbers for X-ray Superconducting Transition Edge Sensors

An absorber with a high absorbing efficiency is crucial for X-ray transition edge sensors (TESs) to realize high quantum efficiency and the best energy resolution. Semimetal Bismuth (Bi) has shown greater superiority than gold (Au) as the absorber due to the low specific heat capacity, which is two orders of magnitude smaller. The electroplating process of Bi films is investigated. The Bi grains show a polycrystalline rhombohedral structure, and the X-ray diffraction (XRD) patterns show a typical crystal orientation of (012). The average grain size becomes larger as the electroplating current density and the thickness increase, and the orientation of Bi grains changes as the temperature increases. The residual resistance ratio (RRR) (R300 K/R4.2 K) is 1.37 for the Bi film (862 nm) deposited with 9 mA/cm2 at 40 °C for 2 min. The absorptivity of the 5 μm thick Bi films is 40.3% and 30.7% for 10 keV and 15.6 keV X-ray radiation respectively, which shows that Bi films are a good candidate as the absorber of X-ray TESs.

Double-Sided Metasurface Array for a Dual-Band and Polarization-Independent Microwave-Energy-Harvesting System

In this article, we present a simple and novel design of a double-sided metasurface for a dual-band and polarization-independent microwave-energy-harvesting system. The proposed metasurface is constructed from the dual-sided design of 8 × 8 unit cells. Different from the regular dual-band unit cells that contain two loops or multiple shapes of resonators printed in the same layer, the proposed metasurface is based on designing double loops, each combined with two arms of a dipole printed on the top and bottom sides of a single substrate. Thus, the bottom layer is utilized to generate the second frequency band of interest. Three main numerical simulations were conducted to investigate the performance of a single unit cell, a 2 × 2 supercell, and an array of an 8 × 8 metasurface structure. The numerical simulation demonstrated that 98% and 95% of the incident energy is collected at two bands of 1.8 and 6.5 GHz for the proposed harvester.

A Review on Lead-Free Hybrid Halide Perovskites as Light Absorbers for Photovoltaic Applications Based on Their Structural, Optical, and Morphological Properties

Despite the advancement made by the scientific community in the evolving photovoltaic technologies, including the achievement of a 29.1% power conversion efficiency of perovskite solar cells over the past two decades, there are still numerous challenges facing the advancement of lead-based halide perovskite absorbers for perovskite photovoltaic applications. Among the numerous challenges, the major concern is centered around the toxicity of the emerging lead-based halide perovskite absorbers, thereby leading to drawbacks for their pragmatic application and commercialization. Hence, the replacement of lead in the perovskite material with non-hazardous metal has become the central focus for the actualization of hybrid perovskite technology. This review focuses on lead-free hybrid halide perovskites as light absorbers with emphasis on how their chemical compositions influence optical properties, morphological properties, and to a certain extent, the stability of these perovskite materials.

Deep Learning: A Rapid and Efficient Route to Automatic Metasurface Design

Metasurfaces provide unprecedented routes to manipulations on electromagnetic waves, which can realize many exotic functionalities. Despite the rapid development of metasurfaces in recent years, the design process of metasurface is still time-consuming and computational resource-consuming. Moreover, it is quite complicated for layman users to design metasurfaces as plenty of specialized knowledge is required. In this work, a metasurface design method named REACTIVE is proposed on the basis of deep learning, as deep learning method has shown its natural advantages and superiorities in mining undefined rules automatically in many fields. REACTIVE is capable of calculating metasurface structure directly through a given design target; meanwhile, it also shows the advantage in making the design process automatic, more efficient, less time-consuming, and less computational resource-consuming. Besides, it asks for less professional knowledge, so that engineers are required only to pay attention to the design target. Herein, a triple-band absorber is designed using the REACTIVE method, where a deep learning model computes the metasurface structure automatically through inputting the desired absorption rate. The whole design process is achieved 200 times faster than the conventional one, which convincingly demonstrates the superiority of this design method. REACTIVE is an effective design tool for designers, especially for laymen users and engineers.

Design of Polarization-Independent and Wide-Angle Broadband Absorbers for Highly Efficient Reflective Structural Color Filters

We propose a design of angle-insensitive and polarization-independent reflective color filters with high efficiency (>80%) based on broad resonance in a Fabry–Pérot cavity where asymmetric metal-dielectric-metal planar structures are employed. Broadband absorption properties allow the resonance in the visible range to remain nearly constant over a broad range of incident angles of up to 40° for both s- and p-polarizations. Effects of the angles of incidence and polarization state of incident light on the purity of the resulting colors are examined on the CIE 1931 chromaticity diagram. In addition, higher-order resonances of the proposed color filters and their electric field distributions are investigated for improved color purity. Lastly, the spectral properties of the proposed structures with different metallic layers are studied. The simple strategy described in this work could be adopted in a variety of research areas, such as color decoration devices, microscopy, and colorimetric sensors.

Hybrid Metamaterial Absorber Platform for Sensing of CO2 Gas at Mid-IR

Application of two major classes of CO2 gas sensors, i.e., electrochemical and nondispersive infrared is predominantly impeded by the poor selectivity and large optical interaction length, respectively. Here, a novel "hybrid metamaterial" absorber platform is presented by integrating the state-of-the-art complementary metal-oxide-semiconductor compatible metamaterial with a smart, gas-selective-trapping polymer for highly selective and miniaturized optical sensing of CO2 gas in the 5-8 µm mid-IR spectral window. The sensor offers a minimum of 40 ppm detection limit at ambient temperature on a small footprint (20 µm by 20 µm), fast response time (≈2 min), and low hysteresis. As a proof-of-concept, net absorption enhancement of 0.0282%/ppm and wavelength shift of 0.5319 nm ppm-1 are reported. Furthermore, the gas- selective smart polymer is found to enable dual-mode multiplexed sensing for crosschecking and validation of gas concentration on a single platform. Additionally, unique sensing characteristics as determined by the operating wavelength and bandwidth are demonstrated. Also, large differential response of the metamaterial absorber platform for all-optical monitoring is explored. The results will pave the way for a physical understanding of metamaterial-based sensing when integrated with the mid-IR detector for readout and extending the mid-IR functionalities of selective polymers for the detection of technologically relevant gases.

Metal-Insulator-Metal-Based Plasmonic Metamaterial Absorbers at Visible and Infrared Wavelengths: A Review

Electromagnetic wave absorbers have been investigated for many years with the aim of achieving high absorbance and tunability of both the absorption wavelength and the operation mode by geometrical control, small and thin absorber volume, and simple fabrication. There is particular interest in metal-insulator-metal-based plasmonic metamaterial absorbers (MIM-PMAs) due to their complete fulfillment of these demands. MIM-PMAs consist of top periodic micropatches, a middle dielectric layer, and a bottom reflector layer to generate strong localized surface plasmon resonance at absorption wavelengths. In particular, in the visible and infrared (IR) wavelength regions, a wide range of applications is expected, such as solar cells, refractive index sensors, optical camouflage, cloaking, optical switches, color pixels, thermal IR sensors, IR microscopy and gas sensing. The promising properties of MIM-PMAs are attributed to the simple plasmonic resonance localized at the top micropatch resonators formed by the MIMs. Here, various types of MIM-PMAs are reviewed in terms of their historical background, basic physics, operation mode design, and future challenges to clarify their underlying basic design principles and introduce various applications. The principles presented in this review paper can be applied to other wavelength regions such as the ultraviolet, terahertz, and microwave regions.

Carbon-Based Sunlight Absorbers in Solar-Driven Steam Generation Devices

Carbon-based sunlight absorbers in solar-driven steam generation have recently attracted much attention due to the possibility of huge applications of low-cost steam for medical sterilization or sanitization, seawater desalination, chemical distillation, and water purification. In this minireview, recent developments in carbon-based sunlight absorbers in solar-driven steam generation systems are reviewed, including graphene, graphite, carbon nanotubes, other carbon materials, and carbon-based composite materials, highlighting important contributions worldwide that promise low-cost, efficient, robust, reusable, chemically stable, and excellent broadband solar absorption. Furthermore, the crucial challenges associated with employing carbon materials in this field are emphasized.

Structured metal film as a perfect absorber

A new type of absorber, a four-tined fish-spear-like resonator (FFR), constructed by the two-photon polymerization process, is reported. An absorbance of more than 90% is experimentally realized and the resonance occurs in the space between the tines. Since a continuous layer of metallic thin film covers the structure, it is perfectly thermo- and electroconductive, which is the mostly desired feature for many applications.

Structured metal film as a perfect absorber

A new type of absorber, a four-tined fish-spear-like resonator (FFR), constructed by the two-photon polymerization process, is reported. An absorbance of more than 90% is experimentally realized and the resonance occurs in the space between the tines. Since a continuous layer of metallic thin film covers the structure, it is perfectly thermo- and electroconductive, which is the mostly desired feature for many applications.