Metallic Photonic Crystal Absorber‐Emitter for Efficient Spectral Control in High‐Temperature Solar Thermophotovoltaics

A high‐temperature stable solar absorber based on a metallic 2D photonic crystal (PhC) with high and tunable spectral selectivity is demonstrated and optimized for a range of operating temperatures and irradiances. In particular, a PhC absorber with solar absorptance 0.86 and thermal emittance = 0.26 at 1000 K, using high‐temperature material properties, is achieved resulting in a thermal transfer efficiency more than 50% higher than that of a blackbody absorber. Furthermore, an integrated double‐sided 2D PhC absorber/emitter pair is demonstrated for a high‐performance solar thermophotovoltaic (STPV) system. The 2D PhC absorber/emitter is fabricated on a double‐side polished tantalum substrate, characterized, and tested in an experimental STPV setup along with a flat Ta absorber and a nearly blackbody absorber composed of an array of multiwalled carbon nanotubes (MWNTs). At an irradiance of 130 kW m^?2 the PhC absorber enables more than a two‐fold improvement in measured STPV system efficiency (3.74%) relative to the nearly blackbody absorber (1.60%) and higher efficiencies are expected with increasing operating temperature. These experimental results show unprecedented high efficiency, demonstrating the importance of the high selectivity of the 2D PhC absorber and emitter for high‐temperature energy conversion.     

Gold Nanowires-Based Hyperbolic Metamaterial Multiband Absorber Operating in the Visible and Near-Infrared Regimes

Plasmonics - Spectral feature of gold nanowires-based hyperbolic metamaterial (NWHMM) absorber was investigated. The absorber has NWHMM surface as the top layer, which is composed of periodically...     

Miniaturized Five-Band Perfect Metamaterial THz Absorber with Small Frequency Ratio

A five-band polarization-insensitive perfect metamaterial absorber (PMA) is reported in this paper for THz detection and sensing applications. The proposed absorber is constructed using interconnected circular ring elements enclosed by a square loop. The ring elements are interconnected using short strip lines which increases the electrical length to offer resonance at the lower frequencies of the THz regime without increasing the electrical length. The proposed absorber has a footprint of 0.12 λ_eff?×?0.12 λ_eff where λ_eff is the effective wavelength calculated at the lowest operating frequency. The absorber provides 92%, 84%, 90%, 100%, and 100% absorption at 0.24, 0.56, 0.65, 0.82, and 0.95 THz, respectively. The proposed structure offers structural symmetry, and hence, it is polarization-insensitive. The proposed five-band absorber has good angular stability consistent with many research works reported in the literature and has a small frequency ratio of 1:2.3:2.7:3.4:3.9. The proposed absorber can be used as a permittivity sensor and its sensitivity is estimated to vary from 5.8 GHz/permittivity unit (PU) to 23.56 GHz/PU.

     

A Compact Design of Multiband Terahertz Metamaterial Absorber with Frequency and Polarization Tunability

A new and simple design of quad-band metamaterial absorber for terahertz frequency has been proposed. The unit cell of the absorber is composed of a top metallic patch having H-shaped slot and a ground metallic plane, both separated by a dielectric layer. The proposed design is capable of providing four distinct absorption peaks over at 0.81, 1.98, 3.25, and 3.50 THz. Our design is a step ahead of the previously proposed terahertz absorbers for its simplistic design approach which removes the fabrication difficulty. Interestingly, rather placing multiple resonators in a single unit cell, we able to accommodate multiple orders of resonances in the proposed design using only a single metallic structure to achieve multiband absorbance. The sensing performance of the absorber in terms of surrounding index is also analyzed. Moreover, we have shown how the proposed structure can be easily converted into a frequency tunable absorber using a simple stub without changing the overall geometry of the absorber. This fast and easy frequency tunability feature is an additional advantage over the simple design of the structure. Also, we lead our work to its upgradation into a polarization tunable absorber where the absorption frequencies are controllable by the polarization of the incident light. The vibrant design of the proposed absorber is expected to find application in detection, imaging, radar cross-section (RCS) reduction, and sensing-related activities.     

Numerical Study of an Ultra-Broadband and Polarization Independence Metamaterial Cross-Shaped Fractal Absorber

A three-dimensional cross-shaped fractal metamaterial absorber with ultra-wide wavelength band, polarization-independence and wide-angle, is numerically investigated by the finite-difference time-domain method. In this absorber, the solar energy is trapped by the cross-shaped fractal of the upper layer, and the Si-ring filled with iron in the middle layer and the wavelength band can be broadened by the self-similarity of fractal structure. The absorber exhibits absorptivity higher than 91% for the wavelengths from 400 to 2000 nm and an absorption bandwidth of about 133%. Furthermore, the proposed absorber realizes polarization independence, and the maximum incident angle is 76°. However, as the iron material applied in the nano-metamaterial absorber (NMA) can be easily oxidized and rusted, it is replaced by nickel with characteristics such as corrosion resistance and high-temperature resistance; thus, an improved NMA is obtained. The improved absorber not only eliminates the corrosion-prone defects of the above proposed structure but also maintains polarization independence and high absorption and widens the angle of incidence up to 79° and thereby can be applied in many areas, such as solar energy harvesting.

     

High-Quality Plasmon Sensing with Excellent Intensity Contrast by Dual Narrow-Band Light Perfect absorbers

A new strategy for realizing ultra-narrowband plasmonic absorber has been theoretically demonstrated. Dual-band perfect light absorber with the bandwidth down to single digit level and the maximal absorption exceeding 99.2 % is achieved. Moreover, novel absorber-based sensor platform with high-quality factors (S?>?420 nm/RIU, FOM?>?84, and FOM*?>?5600) are obtained. These features hold the proposed absorber to be a feasible candidate for applications in the sensing detection and notch filtering.     

pH regulation of alkaline wastewater with carbon dioxide: a case study of treatment of brewery wastewater in UASB reactor coupled with absorber

Studies were carried out with carbon dioxide absorber (CA) to evaluate the usage of carbon dioxide (CO(2)) in the biogas as an acidifying agent by Up-flow Anaerobic Sludge Blanket (UASB) reactor. Investigation on the 5l absorber revealed that ratio of brewery wastewater (BW) flow rate to biogas flow rate of 4.6-5.2 was optimum for minimum consumption of CO(2) for acidification. The acidified BW after the absorber was treated in UASB reactor with optimum organic loading rate (OLR) of 23.1 kg COD/m(3)/day and hydraulic retention time (HRT) of 2h. UASB reactor exhibited good performance with respect to reduction of chemical oxygen demand (COD) and methane yield. The implications of the present study on the full scale anaerobic reactor of medium scale brewery revealed that sufficient cost savings could be made if CO(2) in the biogas or CO(2) that was being wasted (let out to the atmosphere) can be used instead of sulfuric acid (H(2)SO(4)) for pH control.     

Tunable Salisbury Screen Absorber Using Square Lattice of Plasmonic Nanodisk

We propose a novel polarization independent Salisbury screen absorber to provide tunable resonant absorption at terahertz (THz) frequencies. The Salisbury screen absorber is designed by using a planar array of thin gold nanodisks arranged in a square lattice. Certain configurations of Salisbury screen have multiple distinctive absorption bands that support near-unity/FWHM absorption bandwidth reaching 36 THz/169 THz, respectively. Moreover, the absorption bandwidth depends upon the optical thickness of the dielectric spacer between the metasurface and the metallic ground plane. The proposed tunable Salisbury screen absorber can find practical applications in photonic detection, imaging, sensing, and solar cells at optical frequencies.     

High-Performance Tunable Multichannel Absorbers Coupled with Graphene-Based Grating and Dual-Tamm Plasmonic Structures

Plasmonics - We present a hybrid Tamm system targeting the tunable multichannel absorber. The proposed optical absorber is analyzed and investigated by using the transfer matrix method (TMM). The...     

A Design for Band Enhanced Dielectric Absorber Based on Fractal-Like Structure

A dielectric metamaterial absorber has been proposed, which consists of fractal-like structure and conductive sheet. The fractal-like structure is made by the high permittivity dielectric and also is covered by the conductive sheet. Absorptivity of such a dielectric metamaterial absorber is 99.1%, which can be found at 10.196 GHz; meanwhile, the absorber is polarization insensitive. To enhance the bandwidth of absorber, a novel absorber also is proposed, whose bandwidth is 0.566 GHz, which ranges from 9.752 to 10.318 GHz, and relative bandwidth is 5.64%. The maximum absorptivity can reach to 99.8%, and the proposed absorber also is polarization insensitive. In the meantime, the absorber shows excellent performance which is incident angle insensitive; when the incident angle is increased to 70°, the absorptivity is larger than 75%.

     

Surface Cleaning and Passivation Using (NH4)2S Treatment for Cu(In,Ga)Se2 Solar Cells: A Safe Alternative to KCN

With the aim of developing a safe alternative to the KCN etchant for the removal of Cu_xSe secondary phases at the surface of Cu(In,Ga)Se₂ (CIGSe) absorber, a method based on ammonium sulfide (AS) chemical treatment is proposed. Although lower etching rates are observed compared with the KCN reference solution, the AS solution is found to selectively etch Cu_xSe phases. In addition, it allows modifying the surface chemical state of the CIGSe absorber by incorporation of sulfur. As a consequence, the minority carrier lifetime located close to the surface of the absorber is found to be improved. Furthermore, it is demonstrated that optimizing the AS treatment time induces a remarkable enhancement in the electrical performances of the CIGSe‐based solar cells.     

Significant Efficiency Enhancement in Ultrathin CZTS Solar Cells by Combining Al Plasmonic Nanostructures Array and Antireflective Coatings

In the few past years, the economic and eco-friendly Cu₂ZnSnS₄ (CZTS) solar cells have caught lots of attentions. However, due to rather poor efficiency, identifying deficiencies and making improvements is necessary. In the present study, the performance improvement of ultrathin CZTS solar cells was achieved through (1) incorporation of anti-reflective coating (ARC) on the surface of cell and (2) embedding Al plasmonic nanostructures with different radius, periods, and vertical positions in the absorber layer. Various thicknesses of CZTS absorber layer were simulated optically and electrically using FDTD and DEVICE solver of Lumerical software. The reference solar cell consists of a 1.5-nm-thick CZTS absorber and exhibit an efficiency of up to 5.67%, short-circuit current density (Jsc) of 18.48 mA cm⁻² and open circuit voltage of 0.58 V. Result showed a remarkable performance enhancement of the solar cell in spite of a very thin absorber layer. For a 500-μm-thick CZTS solar cell with the assistance of ARC and embedding Al plasmonic nanostructures, the efficiency is increased to 7.45% due to an increase in Jsc to 22.62 mA cm⁻² with an open circuit voltage of 0.62 V.

     

A Newfangled Terahertz Absorber Tuned Temper by Temperature Field Doped by the Liquid Metal

In this article, a terahertz absorber tuned by temperature field with a newfangled structure is presented, which comprises the mercury resonators. In this scheme, temperature (T) build-up will lead the mercury stored in the bottom slot to expand through the columniform hole and be full of the upper central cross container, which can transform the absorption bands of such an absorber. The simulated results manifest that when T is increased from 0 to 25 °C, the dual-frequency absorption points (2.59 THz, 3.03 THz) and a narrow absorption region over 90% (6.54–7.10 THz), whose relative bandwidth (RB) is 7.9%, will be tailored to a single-frequency point absorption (3.12 THz) and a broadband absorption area (6.00–7.21 THz, and RB = 18.3%). For figuring out the property of the absorber mentioned above, the impacts of incident and polarization angles along with some relevant parameters of the structure on the absorption property are investigated. In addition, for plainly expounding the physical mechanism of absorption, the distributions of the surface current diagrams of the presented absorber are calculated, as well as the electric field diagrams, the magnetic field diagrams, the power loss density diagrams, and the power flow density diagrams. The proffered scheme in this article may offer a novel idea for realizing the reconfigurable absorbers.

     

Design of a Multi-functional Device Based on the Solid-state Plasma: Absorber and Splitter

Plasmonics - We propose a multi-functional device by using the solid-state plasma, which can be called a plasma metamaterial absorber (PMA). The absorber can get tunable absorption spectrum by...     

Tunable Terahertz Plasmonic Perfect Absorber Based on T-Shaped InSb Array

High absorption efficiency is particularly desirable for various microtechnological applications. In this paper, a nearly perfect terahertz absorber for transverse magnetic (TM) polarization based on T-shaped InSb array is proposed and numerically investigated. Incident wave at the Fabry-Perot resonant frequency can be totally absorbed into the narrow grooves between the two adjacent T-shaped InSb arms. The absorption mechanism is theoretically and numerically studied by using the Fabry-Perot model and the finite element method (FEM), respectively. It is found that the proposed absorber has large angle tolerance. Moreover, the absorption peak can be controlled by varying the temperature. Furthermore, a new absorption peak will emerge while breaking the symmetry of the T-shaped InSb array. This tunable and angle-independent THz perfect absorber may find important applications in THz devices such as microbolometers, coherent thermal emitters, solar cells, photo detectors, and sensors.     

Broadband Ultrathin Absorber and Sensing Application Based on Hybrid Materials in Infrared Region

A broadband and ultra-thin absorber in the infrared region is proposed. The structure is composed of three layers, and the most remarkable difference is that two hybrid materials (Sn and InSb) are used in the top layer. The numerical results show that a broadband perfect absorption from 85.2 to 114.3 THz can be achieved for either transverse electric or magnetic polarization waves due to the effect of using hybrid materials. Moreover, the power loss and surface current distribution in the absorber are investigated to explain the physical mechanism of high absorption. The metamaterial absorber is ultra-thin, having total thickness of 0.3 μm, i.e.,λ/10 with respect to the center frequency of the high absorption bands. The proposed hybrid materials which are used in the same layer provides a useful way to realize a broadband perfect absorber in the infrared region and it is important for a variety of applications, such as solar energy harvest, sensors, and integrated photodetectors .     

Solar Selective Absorbers for High-efficiency Photothermal Conversion via Core–Shell Nanocone Structured Surface

Nanostructured surface, a promising photon management strategy, enables to enhance photon-to-heat conversion efficiency by manipulating spectral radiative properties ranging from solar spectrum (0.3–2.5 μm) to mid-infrared spectrum (2.5–20 μm). Here, a core–shell nanocone structured surface made of silica core and tungsten shell as a solar selective absorber is introduced. The photothermal conversion efficiency (PTCE) is calculated in consideration of solar spectrum absorption and mid-infrared emission. It is obvious that high solar spectrum absorption and low mid-infrared emission are beneficial for high PTCE. The influence of structural parameters on the PTCE is studied, and then the absorption enhancement mechanism is elucidated in detail. Meanwhile, the influences of incident angle, polarized state, and lattice arrangement are also presented. The calculated results exhibit that our optimized solar absorber possesses the total solar absorption of 97.3% and total thermal emission of 7.6%, resulting in a maximum PTCE of 91.4% under one sun illumination conditions at normal incidence. Moreover, our solar selective absorber is independent to the incident angle and polarization state. The excellent photothermal conversion performance with wide-angle and polarization-insensitive properties for the solar selective absorber can serve as a good candidate for various solar thermal applications including seawater desalination, steam generation, thermophotovoltaic, and photocatalysis.

     

Adaptation of beta-ray gauge technique for measurement of water deficit of intact rice leaves

A linear relationship was obtained between the logarithm of count rate and absorber’s (Aluminum foil and leaf) effective thickness in the range of 4.25 to 18.00 mg cm^?2. Also relative water content (RWC) and absorber effective thickness was linearly related (Y = ?89.80 + + 15.84 X, and r = 0.99). From these relationships it was possible to determine the RWC of a rice leaf simply by measuring the radiation intensity of fresh, fully turgid and completely dry leaf by beta ray gauge technique without the added necessity of determining the mass of the leaf.     

1-D Metal-Dielectric-Metal Grating Structure as an Ultra-Narrowband Perfect Plasmonic Absorber in the Visible and Its Application in Glucose Detection

The need for an easy to fabricate perfect and narrowband light absorber in the visible range of electromagnetic (EM) spectrum has always been in demand for many scientific and device applications. Here, we propose a metal-dielectric-metal (MDM) 1-D grating plasmonic structure as a perfect narrow band light absorber in the visible and its application in glucose detection. The proposed structure consists of a 1- D grating of gold on the top of a dielectric layer on a gold film. Optimization for dielectric grating index (n), grating thickness (t), grating width (W), and grating period (P) has been done to improve the performance of plasmonic structure by calculating its quality factor and figure-of-merit (FOM). The optimized plasmonic structure behaves as a perfect narrowband light absorber. The flexibility to work at a specific wavelength is also offered by the proposed structure through an appropriate selection of the geometrical parameters and refractive index of the dielectric grating. The equivalent RC model is used to understand different components of the proposed structure on the optical response. The absorption response of the structure is invariant to the incident angle. Moreover, the calculated absorbance of the proposed plasmonic structure is ~ 100% with a narrow full-width half maxima (FWHM) of ~ 2.8 nm. We have numerically demonstrated a potential application of the proposed MDM absorber as a plasmonic glucose sensor in the visible range with detection sensitivity in the range of 140 to 195 nm/RIU.

     

Design of Tunable Multi-Band Metamaterial Perfect Absorbers Based on Magnetic Polaritons

A tunable multi-band metamaterial perfect absorber is designed in this paper. The absorber made of a composite array of gold elliptical and circular disks on a thick metallic substrate, separated by a thin dielectric spacer. The absorptivity and the field enhancement of proposed structures are numerically investigated by the finite difference time domain method. Three absorption peaks (1.15, 1.55, and 2.05 μm) with the maximal absorption of 99.2, 99.7, and 97.3% have been achieved, respectively. By altering the dimensions of associated geometric parameters in the structure, three resonance wavelengths can be tuned individually. Physical mechanism of the multi-band absorption is construed as the resonance of magnetic polaritons. And the absorber exhibits the characteristics that are insensitive to the polarization angle due to its symmetry. The research results can have access to selective control of thermal radiation and the design of multi-band photodetectors.