Ultrathin planar broadband absorber through effective medium design

Ultrathin planar absorbers hold promise in solar energy systems because they can reduce the material, fabrication, and system cost. Here, we present a general strategy of effective medium design to realize ultrathin planar broadband absorbers. The absorber consists of two ultrathin absorbing dielectrics to design an effective absorbing medium, a transparent layer, and metallic substrate. Compared with previous studies, this strategy provides another dimension of freedom to enhance optical absorption; therefore, destructive interference can be realized over a broad spectrum. To demonstrate the power and simplicity of this strategy, we both experimentally and theoretically characterized an absorber with 5-nm-thick Ge, 10-nm-thick Ti, and 50-nm-thick SiO2 films coated on an Ag substrate fabricated using simple deposition methods. Absorptivity higher than 80% was achieved in 15-nm-thick （1/50 of the center wavelength） Ge and Ti films from 400 nm to near 1 btm. As an application example, we experimentally demonstrated that the absorber exhibited a normal solar absorptivity of 0.8 with a normal emittance of 0.1 at 500 ~C, thus demonstrating its potential in solar thermal systems. The effective medium design strategy is general and allows material versatility, suggesting possible applications in real-time optical manipulation using dynamic materials.

Recent progress on nanostructure-based broadband absorbers and their solar energy thermal utilization

Nanostructure-based broadband absorbers are prominently attractive in various research fields such as nanomaterials,nanofabrication,nanophotonics and energy utilization.A highly efficient light absorption in wider wavelength ranges makes such absorbers useful in many solar energy harvesting applications.In this review,we present recent advances of broadband absorbers which absorb light by nanostructures.We start from the mechanism and design strategies of broadband absorbers based on different materials such as carbon-based,plasmonic or dielectric materials and then reviewed recent progress of solar energy thermal utilization dependent on the superior photo-heat conversion capacity of broadband absorbers which may significantly influence the future development of solar energy utilization,seawater purification and photoelectronic device design.

Bidirectional visible light absorber based on nanodisk arrays

A perfect bidirectional broadband visible light absorber composed of titanium nitride and tungsten nanodisk arrays is proposed.The average absorption of the absorber exceeds 89%at 400 nm–800 nm when light is normally incident on the front-side.Illumination from the opposite direction(back-side)results in absorption of more than 75%.Through the theoretical analysis of the electric and magnetic fields,the physical mechanism of the broadband perfect absorption is attributed to the synergy of localized surface plasmons,propagating surface plasmons,and plasmonic resonant cavity modes.Furthermore,the absorber also exhibits excellent polarization-independence performance and a high angular tolerance of~30°for both front-and back-side incidence.The designed bidirectional broadband visible light absorber here has wide application prospects in the fields of solar cells and ink-free printing.

Tunable broadband metamaterial absorber consisting of ferrite slabs and a copper wire

A tunable broadband metamaterial absorber is demonstrated at microwave frequencies in this paper. The meta- material absorber is composed of ferrite slabs with large resonance beamwidths and a copper wire. The theoretical analysis for the effective media parameters is presented to show the mechanism for achieving the perfect absorptivity characteristic. The numerical results of transmission, reflectance, and absorptivity indicate that the metamaterial ab- sorber exhibits a near perfect impedance-match to free space and a high absorptivity of 98.2% for one layer and 99.97% for two layers at 9.9 GHz. The bandwidth with the absorptivity above 90% is about 2.3 GHz. Moreover, the absorption band can be shifted linearly in a wide frequency range by adjusting the magnetic bias. This metamaterial absorber opens a way to prepare perfectly matched layers for engineering applications.

Design of Broadband Metamaterial Absorbers for Permittivity Sensitivity and Solar Cell Application

A broadband and ultra-thin absorber for solar cell application is designed. The absorber consists of three layers, and the difference is that the four split ring resonators made of metal gold are encrusted in the gallium arsenide （GaAs） plane in the top layer. The simulated results show that a perfect absorption in the region from 481.2 to 684.0THz can be obtained for either transverse electric or magnetic polarization wave due to the coupling effect between the material of GaAs and gold. The metamaterial is ultra-thin, having the total thickness of 56nm, which is less than one-tenth resonance wavelength, and the absorption coefficients at the three resonance wavelengths are above 90%. Moreover, the effective medium theory, electric field and surface current distributions are adopted to explain the physical mechanism of the absorption, and the permittivity sensing applications are also discussed. As a result, the proposed structure can be used in many areas, such as solar cell, sensors, and integrated photodetectors.

Polarization Insensitive Broadband Zero Indexed Nano-Meta Absorber for Optical Region Applications

Broadband response metamaterial absorber(MMA)remains a challenge among researchers.A nanostructured new zero-indexed metamaterial(ZIM)absorber is presented in this study,constructed with a hexagonal shape resonator for optical region applications.The design consists of a resonator and dielectric layers made with tungsten and quartz(Fused).The proposed absorbent exhibits average absorption of more than 0.8972(89.72%)within the visible wavelength of 450–600 nm and nearly perfect absorption of 0.99(99%)at 461.61 nm.Based on computational analysis,the proposed absorber can be characterized as ZIM.The developments of ZIM absorbers have demonstrated plasmonic resonance characteristics and a perfect impedance match.The incidence obliquity in typically the range of 0◦–90◦both in TE and TM mode with maximum absorbance is more than 0.8972(∼89.72%),and up to 45◦angular stability is suitable for solar cell applications,like exploiting solar energy.The proposed structure prototype is designed and simulated by studying microwave technology numerical computer simulation(CST)tools.The finite integration technique(FIT)based simulator CST and finite element method(FEM)based simulator HFSS also helps validate the numerical data of the proposed ZIM absorber.The proposed MMA design is appropriate for substantial absorption,wide-angle stability,absolute invisible layers,magnetic resonance imaging(MRI),color images,and thermal imaging applications.

A Polarization-Insensitive Broadband Metamaterial Absorber at the Optical Regime

We present a polarization-insensitive broadband absorber which has a feature of metal-insulator-metal structures. The top metal layer consists of four-fan-rings-shaped gold. Simulations show that the absorber exhibits an absorption of nearly unity at the wavelength of 386.1 nm and a relative absorption bandwidth of 0.548, which refers to the ratio of the full absorption bandwidth over an absorption of 0.9 to the central wavelength. Meanwhile, the absorption is nearly independent of the polarized direction of the incident wave. This absorption bandwidth with insensitive polarization is widely reported to date for such metal-insulator-metal structures. Such a structure offers a way of realization of a polarization-insensitive broadband absorber ranging in ultraviolet-to-visible wavelengths.

Lightweight broadband microwave absorbing metamaterial with CB-ABS composites fabricated by 3D printing

The self-similarity,high geometric symmetry and spatial utilization properties of fractal structures provide new methods for the development of absorbing metamaterials.In this paper,the microwave absorption properties of the gradient dendritic fractal metamaterial structure(GDFMs)based on carbon black and acrylonitrile-butadiene-styrene composites were investigated.The optimal metamaterial structure has an effective absorption in the frequency range of 4.5-40 GHz.The rotational-symmetry GDFMs leads to the polarization independence,and the GDFMs exhibits a wide-angle absorption performance for both TE and TM waves.It is expected that the proposed GDFMs has good application prospects in electromagnetic wave absorption.

A radar-infrared compatible broadband absorbing surface:Design and analysis

A radar-infrared compatible stealth surface is designed and analyzed.Without modifying the radar absorbing material(RAM),the design can theoretically achieve radar-infrared compatibility and broadband radar absorption through surface patterns and structures.A transmission-line-based model(TLM)is developed to analyze the radar absorbing performance of the surface.Optimization of the structure geometries is conducted aiming to maximize the-10 d B absorption bandwidth in 2–18 GHz.Surface with optimized structure geometries exhibits a superior absorption bandwidth,more than twice the bandwidth of the original 1.5 mm RAM slab,while maintaining a relatively low infrared emissivity.

Broadband and High-Temperature-Resistant Microwave Absorber Using Additively Manufactured Ceramic Substrate

This paper presents an approach to achieve broadband absorption and temperature resistance using ceramic sub-strates.A specially formulated slurry suitable for additive manufacturing technology was developed to fabricate ceramic substrates with lattice structures.The lattice structure not only reduces the weight of the absorber but also facilitates the broadening of the absorption bandwidth.The experimental results demonstrate that the pro-posed structure exhibits absorption rates exceeding 88%within the frequency range of 19.9-30.41 GHz,with a relative absorption bandwidth of 41.8%under normal incidence.Furthermore,the absorber’s performance was assessed under high temperatures of up to 200℃,revealing absorption spectra that closely match the initially measured spectrum.Additive-manufactured ceramic lattice structures present a promising avenue for designing multifunctional broadband microwave absorbers capable of withstanding elevated temperatures.

Super broadband absorbing hierarchical CoFe alloy/porous carbon@carbon nanotubes nanocomposites derived from metal-organic frameworks

The exploitation of electromagnetic wave absorption(EMA) materials has attracted ever-increasing attention,not only because electromagnetic wave(EMW) originated from overuse of electronic products has threatened human' health seriously,but also because EMA materials can effectively protect radar stealth from being detected.However,it is still a challenge to obtain broadband and efficient EMA materials to satisfy practical applications.In this work,we developed a series of hierarchical CoFe alloy/porous carbon@carbon nanotubes(CoFe/PC@CNTs) nanocomposites through revising the proportion of Fe^(2+) in the metal-organic frameworks(MOFs)(CoFe-ZIF) precursor and one-step simple pyrolysis process.The cross deposition between CoFe alloy,carbon nanotubes(CNTs),and porous carbon(PC) formed a double conductive network,favoring for achieving excellent EMA property.Surprisingly,when the filler mass ratio is only 10 wt%,the optimized CoFe/PC@CNTs nanocomposites display the best EMA capability,whose minimum reflection loss(RL_(min)) value is-68.94 dB at 13.69 GHz and the effective absorption band(EAB,<-10 dB) reaches up 9.14 GHz with a matching thickness of 2.63 mm.In addition,the largest EAB can achieve up to 11 GHz(3.35-14.35 GHz) during all matching thicknesses.The splendid EMA performance benefits the favorable dielectric loss provided by the double conductive network,the good magnetic loss benefited from the evenly distributed CoFe alloy,the excellent impedance matching,rich transmission paths,and multiple polarization.Therefore,such EMA materials with super broadband absorbing provide desirable candidates for lightweight and high-efficient microwave absorbers.

Nano-Cs_(x)WO_(3):Ultra-broadband nonlinear optical modulator for near-infrared and mid-infrared ultrafast fiber lasers generation

Saturable absorbers(SAs)covering wavelengths from near-infrared(NIR)to mid-infrared(MIR)band are required for mode-locking and Q-switching lasers in muti-band wavelengths.Here,broadband nonlinear optical property was discovered in Cs_(x)WO_(3) nanorods(NRs),which as a novel non-stoichiometric SA for realizing ultrafast fiber lasers is first demonstrated.The Cs_(x)WO_(3) NRs based SA exhibited good mode-locking ability in three key wavelengths from NIR to MIR region,which is a key advantage over the most reported broadband SAs.The given Cs_(x)WO_(3) NRs showed a broadband optical absorption from 800 to 3,200 nm,and excellent SA properties at 1-μm,1.5-μm,and 2-μm optical bands.Employing such SA,the ultrashort pulse lasers with a pulse duration/repetition rate of 530 fs/37.42 MHz at 1,567 nm and 5.6 ps/41.50 MHz at 1,965 nm from Er-and Tm-doped fiber lasers(TDFL)were realized separately.In addition,a stable mode locked operation at 1,030 nm with a repetition rate of 48.80 MHz was also achieved from Yb-doped fiber laser(YDFL).This work not only offers a new and reliable broadband mode locker for ultrafast laser generation,but also broadens the application of Cs_(x)WO_(3) materials in the field of nonlinear fiber optics.