Omnidirectional Absorption Research Articles

Intensifying Solar Interfacial Heat Accumulation for Clean Water Generation Excluding Heavy Metal Ions and Oil Emulsions

Solar‐driven interfacial steam generation has emerged as an innovative technique for seawater desalination due to its high photothermal conversion efficiency and potential industrial applications. Herein, a superior interfacial heat accumulation structure composed of semiconductive in situ polymerization (polypyrrole) of nickel foam (IPNF) is reported. The IPNF photothermal layer is assembled with superhydrophilic polyurethane substrate for synchronous water transport and excellent thermal insulation. The 2D ultrablack mesh induces multiple incident rays within the diffused polymerized surface, which allows omnidirectional solar absorption (88.5 %) and intensifying heat localization (49.5 °C @ 1 sun). The state‐of‐the‐art evaporation performances reveal that the integrated IPNF solar evaporator exhibits an excellent evaporation rate (1.74 kg m−2 h−1) and solar‐to‐vapor conversion efficiency (90% excluding heat losses) under 1 kW m−2 solar intensity. Besides this, the long‐term evaporation experiments show negligible discrepancy under seawater conditions (13.27 kg m−2/8 h under 1 kW m−2) and engrain its functioning potential for multimedia and salt rejection (3.2 g × NaCl/240 min). More importantly, herein, insights into different water states in the polymeric network systems during solar‐driven evaporation are provided. This work shows a significant potential to generate freshwater excluding heavy metals and other oil emulsions for industrial applications.

Overcoming the Challenge of High Surface Recombination in Thin‐Film Photovoltaic Cells Based on Subwavelength Arrays for Elevated Light Trapping

Surface arrays of subwavelength photoactive structures have been demonstrated for photovoltaic (PV) applications with elevated omnidirectional and broadband absorption. However, this approach suffers from an increase in surface recombination and loss of photovoltage. Herein, surface decoration with subwavelength dielectric (SiO2) arrays that provide enhanced light trapping without compromising the photovoltage performance is proposed. An increase of ≈ 50% in broadband absorption is shown in an ultrathin film of 200 nm due to the presence of a top surface SiO2 nanopillar array in comparison with the same film decorated with an optimized antireflective coating of 50 nm of Si3N4. Interestingly, the broadband enhancement is not due to lower reflection, but rather the presence of the arrays forces a considerable lower transmission. The distribution of the optical power flux density suggests that the low transmission is due to appreciable refraction, which is induced by the presence of the dielectric arrays. Finally, the photovoltaic performance is examined for various array geometries and absorber acceptor concentrations and an overall increase of >60% in PV efficiency is calculated for a decorated photovoltaic cell in comparison with a PV cell with an antireflection coating of 50 nm Si3N4.

Tailoring polypyrrole-based Janus aerogel for efficient and stable solar steam generation

Seeking an efficient and robust evaporator is the central goal of solar-driven interfacial steam generation. Here, a free-standing Janus aerogel with rough hydrophobic upper layer and smooth hydrophilic bottom layer was tailored by freeze drying of polypyrrole-polyvinyl alcohol (PPy/PVA) colloidal solution, followed by acetalization treatment and selective fluorination decoration. PPy/PVA-F Janus aerogel has a structure integrated in the form of different microscopic layers and chemical compositions, endowing it with omnidirectional light absorption, stable floating on water, enhanced evaporation performance, and salt rejection capability. Due to the reduced vaporization enthalpy, evaporation rate of 1.68 kg·m−2·h−1 with solar-to-vapor efficiency of 94.7% was obtained under 1-sun irradiation using the Janus aerogel. Furthermore, the Janus aerogel demonstrated good recyclability and long-term durability at different radiation intensities. This Janus aerogel had satisfactory availability for solar desalination and sewage treatment. The strategy to integrate different microscopic layers and chemical compositions in a Janus aerogel holds great promise for the design and application of Janus evaporators in the field of solar steam generation.

Viscothermal Effects in a Two-Dimensional Acoustic Black Hole: A Boundary Element Approach

The acoustic analog of the quantum black hole for airborne sound in two dimensions was denominated as an omnidirectional acoustic absorber by Climente et al. [see Appl. Phys. Lett., 100, 144103 (2012)], who characterized its absorbing properties without providing any theoretical support. The viscothermal losses of the underlying structure, which consists of an absorbing core and a surrounding gradient-index (GRIN) lens both made of periodic distributions of cylindrical rods, are here comprehensively studied by using the boundary element method (BEM) in two dimensions. It is shown that the numerical simulations in two dimensions reproduce fairly well the increase in absorption of the core when the GRIN lens is added and reveal that the discrepancy between measured and calculated values of absorbance is an artifact of the experimental setup. The possibility of independent calculation of viscous and thermal losses contributions in the two-dimensional (2D) BEM algorithm is employed for the comparison with a homogenization theory in which the cluster of cylinders is represented by a single fluidlike viscous cylinder with effective parameters. We conclude that viscous losses represent about 90% of the total energy dissipated in the core. The homogenization approach results are only 2% below the results calculated with 2D BEM, indicating that the effective parameters obtained by the homogenization are very accurate.

A Composite Perforated Partitioned Sandwich Panel With Corrugation for Underwater Low-Frequency Sound Absorption

Underwater acoustic wave absorption and control play an important role in underwater applications. Various types of underwater acoustic metamaterials have been proposed in recent years with the vigorous development of acoustic metamaterials. Compared with airborne sound, underwater sound waves have a longer wavelength and much smaller propagation loss, making them more difficult to control. In addition, given that the acoustic impedance of water is much greater than that of air, numerous conventional materials and structures are not suited to underwater use. In this paper, we propose a composite structure based on an excellent broadband low-frequency sound absorber of air using aluminum mixed with rubber. Our composite structure possesses broadband low-frequency (<1,000 Hz) sound absorption underwater, omnidirectional high sound absorption coefficient under the oblique incidence (0–75°), and pressure resistance. It has promising applications for underwater acoustic wave control and contributes to the design of underwater acoustic metamaterials.

Nanobowls-assisted broadband absorber for unbiased Si-based infrared photodetection.

Hot electrons from the nonradiative decay of surface plasmons have drawn extensive attention due to the outstanding performance in realizing below-bandgap photodetection. However, the widely employed metallic nanostructures are normally complex and delicate with a great challenge in large-area fabrication, and there is a great limitation to achieve substantial photoresponse at relatively long wavelengths (e.g., 2000nm) with polarization- and incident-angle independence. In this study, we theoretically and experimentally demonstrate a broadband, omnidirectional, and polarization-insensitive absorber based on wafer-scale silicon honeycomb nanobowls with 20-nm-thick gold overlayer. The average absorption across the long wave near infrared band (LW-NIR, i.e., 1100-2500 nm) is higher than 82%, which is contributed from the random nature and multimode localized plasmonic resonances excited on the side walls of nanobowls. Benefitted from the well-connected thin Au film and relatively low Schottky barrier, the generated hot electrons have a high transport probability to reach Schottky interface and participate in the interfacial charge transfer process. As a result, the hot-electron photodetector under no bias realizes a broadband photodetection up to 2000nm wavelength with a responsivity of 0.145 mA/W, and its cutoff wavelength is predicted up to 3300 nm by fitting the experimental result with Fowler theory. Our proposed Au/Si nanobowls photodetector could open a pathway to further extend the detection wavelength of Si-based photodetectors with a large-area and low-cost fabrication process, which promotes practical hot-electron applications.

Spray coating of a perfect absorber based on carbon nanotube multiscale composites

We report the design and simple spray coating fabrication process of a perfect absorber based on carbon nanotubes (CNTs) and carbon black particles with omnidirectional high absorption efficiency beyond 99.9% over the broad wavelength range from 400 nm to 20 μm. The carbon black particles are introduced into the CNT solution to form multi-scale all-carbon-based nanomaterials, effectively modifying the morphology of the sprayed layer to improve the light absorption. The promising absorption characteristics of the CNT–carbon black layer are attributed to not only the intrinsic optical properties of the carbon nanomaterials but also the unique surface nanostructures that efficiently trap light. In addition to serving as absorbers in the coating layer, the CNTs, more importantly, act as connectors between carbon black particles for absorber preparation at a large scale, which is the key to modifying the surface morphology. The all-carbon-based coating layer is also shown to be effective for solar thermal harvesting and self-cleaning function. The developed mixing method for CNTs and carbon black particles in solutions opens a new path for the realization of multi-functional sprayed layers based on CNTs and advances the technology of CNT coatings for scale-up to industry applications.

Settled fast measurement of topological charge by direct extraction of plane wave from vortex beam* *Project supported by the National Natural Science Foundation of China (Grant Nos. 61975015, 61575022, and 61421001) and the Beijing Natural Science Foundation, China (Grant No. L191004).

A method of measuring the vortex beam topological charge (TC) is proposed based on a device that can directly extract the plane wave form from the vortex beam in which the different propagation angles of the plane waves are uniquely related to the different TCs. Then the TC can be obtained by simply comparing the energy values perceived by two fixed sensors in the detection location with the help of twin omnidirectional energy absorbers (OEAs). Because the settled detection relies only on the simple quantitative value at two fixed positions, neither pattern recognition nor field analysis procedure is applied, thus allowing faster measurement. Some features of the methodology are investigated, and the numerical simulations verify the feasibility and robustness of the system.

Ultrathin, broadband, omnidirectional, and polarization-independent infrared absorber using all-dielectric refractory materials

Abstract Broadband long-wavelength infrared (LWIR) optical absorbers have important applications in thermal emission and imaging, infrared camouflaging, and waste heat and biothermal energy utilization. However, the practical application of broadband LWIR optical absorbers requires low-cost and facile fabrication of large-area structures with limited thickness. This paper reports the design and fabrication of an ultrathin, broadband, omnidirectional, and polarization-independent LWIR optical absorber composed of anodized aluminum oxide and highly doped Si using the gradient refractive index strategy. The average absorption of the broadband optical absorber is higher than 95% in the 8–15 μm wavelength range, and it has wide incident angle and polarization tolerances. More than 95% of the optical energy in the wavelength range from 8 to 13 μm was absorbed within a depth of 8 μm, making this absorber the thinnest broadband LWIR dielectric absorber so far. The absorption remained above 90% after annealing at 800 °C in air. The infrared camouflage of the proposed absorber was successfully demonstrated with a human body background. With the advantages of facile fabrication, low-cost materials, restricted absorption thickness, and excellent thermal stability, the developed broadband LWIR optical absorber is very promising for the practical applications mentioned above.

Ultra-broadband, lithography-free, omnidirectional, and polarization-insensitive perfect absorber

Perfect absorbers (PAs) at near infrared allow various applications such as biosensors, nonlinear optics, color filters, thermal emitters and so on. These PAs, enabled by plasmonic resonance, are typically powerful and compact, but confront inherent challenges of narrow bandwidth, polarization dependence, and limited incident angles as well as requires using expensive lithographic process, which limit their practical applications and mass production. In this work, we demonstrate a non-resonant PA that is comprised of six continuous layers of magnesium fluoride (MgF2) and chromium (Cr) in turns. Our device absorbs more than 90% of light in a broad range of 900–1900 nm. In addition, such a planar design is lithography-free, certainly independent with polarization, and presents a further advantage of wide incidence up to 70°. The measured performance of our optimized PA agrees well with analytical calculations of transfer matrix method (TMM) and numerical simulations of finite element method, and can be readily implemented for practical applications.

Study on the optical characteristics of gradient Al component AlGaN nanowires for ultraviolet photocathode

We numerically studied the effect of the geometric structure and Al component on the optical capture performance of gradient Al component AlxGa1 − xN photocathodes. The effects of geometric parameters, such as base radius (R), wire-to-wire spacing, cone rate, and angle of incident light, on the optical response were systematically studied based on the finite element method. In the radial direction, we study the optical response of rectangular periodic structure and hexagonal periodic structure. Simulation results show that pencil nanostructure can achieve omnidirectional and broadband light absorption of AlxGa1 − xN nanowires with hexagonal periodic structure. In addition, we used the Spicer three-step emission model to establish the photoemission efficiency of the AlxGa1 − xN nanostructure. As a result, the photocathode achieves optimal quantum efficiency when the Al component is in the range of 0 to 0.75 and sublayer thickness of 240, 180, 120, and 60 nm.

К ОЦЕНКЕ ПРИМЕНИМОСТИ АНАЛИТИЧЕСКИХ СООТНОШЕНИЙ МАССОПЕРЕНОСА ДЛЯ ОПРЕДЕЛЕНИЯ КОНСТАНТ СОРБЦИИ В ПОЛИМЕРНЫХ МАТЕРИАЛАХ РДТТ

The use of analytical solutions for Frick`s diffusion equation to define the main sorption constants leading to the necessity to solve the united problem is shown. To be more exact this is the application assessment of analytical solutions for more often used experimental results and the determination of the representative sample dimensions in the experiments which are need to construct the sorption/desorption curves. By numerical experiments it is established that the most exact values of masstransfer constants are determined for small and middle values of diffuser absorption temporary process with the help of analytical solutions. Because of the difference in real sample dimensions and structural models used for analytical solutions which as a rule are presented in the form of semi-infinite plates it is necessary to minimize the influence of side surfaces in real samples on the accuracy of masstransfer parameters determination. The numerical-analytic study according to the geometry of polymeric samples from the point of view their representation to define diffusion, solubility and swelling coefficients is conducted. Herewith it is brought out that to obtain the values of masstransfer constants to an accuracy of not more 3 %, the sample geometric features must be chosen with relative dimensions L/h >10 (L – width, h – height of sample, accordingly) – the conditions of plane omnidirectional absorption of the diffuser into the right-angle-formed plates. To obtain the diffusion, solubility and swelling coefficients to an accuracy of not more 3 % the cylinder samples with relative dimensions D/h >15 (D – diameter, h – height, accordingly) should be preferably used. The algorithm of masstransfer constants determination when jointly use the sorption curves and the numerical method of diffusion analysis – finite element method is proposed. Herewith the advantages of such approach removing the sample size and shape limits are shown in comparison with the analytical methods of diffusion analysis.

Broadband refractory plasmonic absorber without refractory metals for solar energy conversion

Electromagnetic wave absorbers with full-spectrum near-unity solar absorption are extremely desirable for solar energy conversion. Here, we theoretically proposed a broadband refractory plasmonic absorber without refractory metal, which has nearly perfect absorption over the whole solar spectrum. The absorber is supported by a periodic refractory dielectric framework of alumina and exhibits superior solar-selective and omnidirectional absorption. The simulated average absorption efficiency is up to 96% from 300 to 1400 nm, and the solar energy absorption efficiency can reach up to 90.8% over the entire solar spectrum range. Moreover, the steady-state temperature profile indicates that the absorber heats uniformly. The broadband refractory plasmonic absorber paves the way for the selective thermal absorption under high-temperature environment and holds great promise for solar energy harvesting and desalination applications.

Admittance analysis of broadband omnidirectional near-perfect absorber in epsilon-near-zero mode.

In this paper, we propose a broadband omnidirectional near-perfect absorber that transforms light energy into heat. In contrast to previous research on structural metamaterials, this study focuses on light absorption in the epsilon-near-zero (ENZ) layers without any structural patterns. Chromium (Cr) thin films were applied as ENZ layers. Using the admittance method, we found the proper thicknesses of SiO2 layers to match the incident medium and achieve perfect absorption. Also, the absorber is angular insensitive up to 60°. The temperature of the absorber increases from room temperature to 42°C, which is 4°C higher than the uncoated substrate at 38°C, after exposure to sunlight for 20 min.

Strong Omnidirectional and Broadband Absorption of the Solar Light with Arrays of Submicrometer‐Scaled Compound Parabolic Light Concentrators

Efficient omnidirectional broadband absorption is important to various applications involving the harvest of the solar spectrum. The optical absorption dependency on oblique illumination in arrays of submicrometer‐scaled compound parabolic concentrators (CPCs) is examined. For a non‐optimized CPC array, the broadband absorption is ≈30% higher than that of a nanopillar (NP) array that is corrected for the absorption of the solar light for an angle‐of‐illumination range of 0°–80°. It is shown that enhanced absorptivity of CPC arrays, relative to the optically corrected NP array, is due to the repressed transmission for both transverse electric and transverse magnetic illumination. It is determined that the superior omnidirectional broadband absorption of a CPC array is due to strong “light bending”; in the CPC array, the optical intensity is predominantly distributed laterally, whereas in the NP array, the optical intensity is mainly directed in the vertical direction. It is shown that for the CPC array, the intensity lateral components are up to 80% higher than the lateral components in the NP array. Finally, the CPC array omnidirectional broadband absorption is governed by substantial light bending, which induces competent excitation of various asymmetric photonic states.

Designs of photoabsorption-enhanced variable Al component GaN nanostructure for UV photodetectors

Abstract In this paper, we study the effect of geometric structure of gradient Al component AlxGa1-xN nanowires on their optical response. Based on the finite element simulation software Comsol Multiphysics, we systematically studied the optical properties of different geometric parameters of base radius (R), axis inclination (θ) and Angle of incident light (AOI). In the radial mode, we investigate the optical response of nanopillar and nanohole. Simulation results show that the AlxGa1-xN cathode can be omnidirectional and unified broadband light absorption by pillar nanostructured arrays with an axial inclination of 24°. In addition, Three-step Spicer emission model was used to establish the photoemission efficiency of graded Al component AlxGa1-xN nanostructure. All these findings not only indicate that the gradient Al component AlxGa1-xN material has a significant potential advantage over the UV photocathode, but it also provides an effective broadband and omnidirectional light absorption UV photocathode.

Omnidirectional, Broadband Light Absorption in a Hierarchical Nanoturf Membrane for an Advanced Solar‐Vapor Generator

AbstractSolar‐thermal materials have been intensively studied in the context of production and localization of thermal energy, targeting an industry level application. Although photonic and optical strategies for enhancing light absorption have increased the efficiency of photo excitation/conversion into thermal energy, most of them have several limitations such as large area fabrication, thermal stability and broadband/omnidirectional light absorption. In this study, a gold‐coated hierarchical nanoturf membrane (Au/h‐Nanoturf membrane) incorporated with randomly distributed high aspect ratio (AR) nanostructures and micro‐through holes is proposed. The Au/h‐Nanoturf has peculiar black structures that provide advantages in forming a membrane with a large area and in absorbing broadband solar light spectrum. Furthermore, the membrane is combined with micro‐cone array which makes it exhibit exceptionally omnidirectional light absorption properties. Using computational analysis, it is confirmed that the micro‐cone array substantially contributes to the omnidirectional solar absorption irrespective of the wavelength. The optimized structural parameters for the maximum efficiency of the solar‐thermal materials are also found. The prepared solar‐vapor generator with the optimized structural features exhibits 91% average conversion efficiency under one sun condition. The efficiency is sustainable for up to four weeks. The highly efficient and omnidirectional broadband‐absorbing solar‐thermal membrane can be readily employed, targeting an industry level application.

Omnidirectional absorption properties of a terahertz one-dimensional ternary magnetized plasma photonic crystal based on a tunable structure

In this paper, by using the transfer matrix method and introducing the plasma as the dispersion medium, we propose the tunable one-dimensional magnetized ternary plasma photonic crystal structure (APB)N, and study its omnidirectional absorption properties of TM wave in terahertz band, where A and B are two different dielectric layers, P is plasma layer, and studied the regulation of the frequency and peak of the absorption channel by the number of periods, the relative refractive index of the dielectric layer, length, density, collision frequency and magnetic field intensity of the plasma layer to the absorption channel frequency and peak value. Therefore, this paper provides an idea for the design of terahertz absorbers for omnidirectional plasma photonic crystals.

Integrated Evaporator for Efficient Solar-Driven Interfacial Steam Generation.

Solar-driven interfacial steam generation is a promising technique for clean water production because it can minimize thermal loss by localizing solar-to-heat conversion at the air/liquid interface. Here we report an integrated solar evaporator by partially growing 2D polypyrrole microsheets within a melamine foam through chemical vapor polymerization. These microsheets can induce multiple light reflections within the foam, enable omnidirectional light absorption, provide abundant surfaces to promote heat transfer, and achieve spatially defined hydrophobicity to facilitate vapor escape. Meanwhile, the inherent hydrophilicity of the bottom part of the foam promotes spontaneous upward water transport and suppresses heat loss. The composite foam exhibits an excellent apparent evaporation rate of ∼2 kg/(m2·h) and solar-to-vapor efficiency of ∼91%. The combined advantages of large surface area, high efficiency, low cost, all-weather application, excellent durability, and scalable manufacturing make our integrated design promising for fabricating large-scale solar steam generation systems that are suitable for practical clean water production.

A Novel Omnidirectional Self-Locked Energy Absorption System Inspired by Windmill

Abstract Impact accidents cause great damage to lives and properties because the destructiveness, direction, and action mode of impact loadings can hardly be predicted. Ordinary thin-walled tube systems for energy absorption require outside constraints or inside fasteners to avoid tube splashing, which affects the modifiability of the systems and limits their application in emergencies. In an effort to break through this limitation, inspired by windmill, a novel omnidirectional self-locked energy absorption system has been proposed. The proposed system is made up of thin-walled tubes with windmill-liked cross section, which are specially designed to interlock with adjacent tubes and thus provide constraints among individual tubes to resist impact loadings in spatial arbitrary directions. The spatial omnidirectional self-locking capability of the windmill-inspired system is demonstrated under distributed and concentrated impact loadings. Moreover, the windmill-inspired system shows higher energy absorption efficiency than that of the widely used round tube system and previous self-locked system under loadings in various directions, and their energy absorption properties can be further improved by combining with the round tube system, adjusting the geometric parameter of each tube and designing the arrangement of tubes with different properties in the system. This work may shed light on the energy absorption system design and expand the application of self-locked energy absorption systems.