Broadband Perfect Absorption Research Articles

A polarization-insensitive broadband terahertz absorber with a multilayer structure

In this paper, a polarization-insensitive broadband terahertz absorber (PBTA) is presented and demonstrated, which can realize a polarization-insensitive, and broadband perfect absorption in the terahertz regime. By simulation, the polarization-insensitive broadband absorption is over 90%, which runs from 4.904 THz to 6.632 THz (the relative bandwidth is 29.96%), and the obtained absorption remains a good absorption performance with a wide incident angle for both TE and TM waves. The surface current distributions, power loss densities, electric and magnetic field distributions of such an absorber are investigated to figure out the physical mechanism of such a PBTA. The effects structure parameters on the absorption performance are also studied, which will be a guiding to realize a PBTA. The simulated results show that the proposed multilayer structure can help to design a PBTA.

Tunable broad-band perfect absorber by exciting of local surface plasma modes resonances at terahertz frequency

A broadband metamaterial absorber is proposed and measured in this paper, which consisted of two patterned square metal patches and a same-size patterned square dielectric patch alternately stacking on a bottom dielectric-metallic ground plane. The measured metamaterial absorber gives shows a broadband absorption performance, while the single layer structure with the same structural parameters only achieves a narrow absorption peak. The full width at half maximum (FWHM) is 2.2 THz, which is higher than that of the single-layer structure. Electric field intensity distribution calculation results display that the resonance and vertical coupling of local surface plasma (LSP) modes between two patterned square metal patches results in the absorption band. Effective parameter extraction results display that the impedance matching condition is reached in the absorption band. Measured results indicate that the absorption band is shifted the lower frequency with the side length w of square metal plates increasing, while the absorption band is shifted the higher frequency with the dielectric layer thickness h1 and h2 increasing. An equivalent LC resonance current model is established to explain the effect of geometric parameters on the absorption performance. Moreover, the measured absorber exhibits the property of broadband absorption at the incident angle of 60°.

Perfect light absorption in graphene by two unpatterned dielectric layers and potential applications

In the spectral range from ultraviolet to near infrared, graphene lacks the capability to support plasmon polaritons, and has low optical absorptivity for applications due to its extremely small thickness. Many photonic structures based on sophisticated nanofabrication or metal plasmonics have been adopted to conquer this limitation, but they suffer from high expenses or metal parasitic losses. Here, a single-channel coherent perfect absorber simply based on two unpatterned dielectric layers is proposed to reach ∼100% light absorption in monolayer and few-layer graphene. The schemes for narrowband and broadband perfect absorption in graphene are systematically demonstrated, and their potential applications on fibre-integrated narrowband perfect absorbers, high-performance optical sensors, electric-optic modulators and broadband perfect absorbers are also investigated. Our research provides a simple and cost-effective method to completely trap the light from ultraviolet to near infrared in a sub-nanometre scale for a lot of high-performance photonic and optoelectronic devices based on graphene and potentially other 2D materials.

Tunable broadband, wide-angle, and polarization-dependent perfect infrared absorber based on planar structure containing phase-change material.

A multilayer absorber composed of SiO2, Fe, Ge2Sb2Te5 (GST), and Al is designed, and the absorptive properties are theoretically investigated based on the Fresnel coefficients method in the wavelength range of 300-2500nm by changing the thickness and crystallization rate of GST, the incident angle, and the polarization. The results show that the thin Fe layer plays a key role in obtaining an ultra-broadband perfect absorption. The absorption properties are polarization-dependent, and the perfect absorption can be nearly realized for a p-polarized wave at the incident angle smaller than 75° with the bandwidth larger than 316nm at 90% of max absorption value. The absorptive peak of this absorber can be tuned with the crystallization rate of GST by temperature, and the peak wavelength moves from 1433nm in the amorphous phase to 2051nm in the crystalline phase. This structure can provide a feasible route to design the tunable broadband, wide-angle, and polarization-dependent perfect absorber without lithographic patterns in the infrared band.

Monolayer-graphene-based broadband and wide-angle perfect absorption structures in the near infrared

Broadband optical absorption structures in the near infrared by coupling monolayer-graphene with periodical metal structures are proposed and demonstrated numerically. Optical absorption of graphene with over-50%-absorption bandwidth up to hundreds of nanometer caused by magnetic dipole resonances and magnetic coupling effect are investigated in detail, and the demonstrated bandwidths are one order higher than those caused by dielectric guiding mode resonances. In addition, the influences of geometrical parameters of structures are fully analyzed and these demonstrated structures show angular-insensitive absorption for oblique incidence in a large angular range. The demonstrated absorption structures in this work provide new design ideas in the realization of advanced graphene-based optoelectronic devices.

Actively tunable broadband terahertz absorption using periodically square-patterned graphene

We propose an actively tunable broadband perfect absorber using a square-patterned graphene–spacer–polysilicon–spacer–metal structure. The simulated results show that the absorber can reach nearly perfect broadband terahertz absorption with over 99.5% (90%) absorbance from 1.52 (1.27) to 2.23 (2.51) THz, corresponding to the normalized bandwidth of 37.5% (65.6%) under normal incidence with the graphene Fermi level of 0.7 eV. The absorption spectra show a clear independence of the polarization and the angle of incidence. By adjusting the graphene Fermi level from 0 to 0.7 eV, the peak absorbance can be continuously tuned from 15 to 100% without shifting the absorption frequency band.

A broadband and switchable VO2-based perfect absorber at the THz frequency

We demonstrate a broadband and switchable THz metamaterial absorber by utilizing the phase transition of VO2, which is a stacked structure composed of VO2 periodic array, dielectric layer, VO2 film, Au periodic array, dielectric layer and Au reflective layer, respectively. The absorption band from 0.76 THz to 0.86 THz at room temperature can be changed into the absorption band from 1.12 THz to 1.25 THz when temperature increases above the phase change temperature of VO2, with the absorptivity in both bands over 90%. Furthermore, the effective medium theory is introduced to explain the perfect absorption mechanism. Such absorber can work well over a wide range of incidence around 40°. Thickness of the absorber is only about one twentieth of the working wavelength. The proposed structure can be applied to absorbers working at other frequencies.

Simplified Design for Broadband and Polarization-Insensitive Terahertz Metamaterial Absorber

Broadband perfect light absorbers have application promising in many areas. However, current design concept of broadband absorbers using the configurations of super-unit coplanar or stacked has shortcomings of complexity in design and challenge in fabrication. We here only employ a patch resonator without using the complex structures of coplanar or stacked to achieve the broadband and polarization-insensitive terahertz light absorber. Larger than 80% absorption can cover a continuous range of 2.83 THz, and its relative bandwidth can reach 77.64%. The combination of fundamental mode of patch resonator and two different coupling intensities of localized resonance and surface propagation mode results in the broadband absorption, and field distributions are given to reveal its physical mechanism. Furthermore, the design can offer a deal of freedom for widening the bandwidth of the device through some parameters redesigning.

Broad-band and polarization-independent perfect absorption in graphene-gold cylinder arrays at visible and near-infrared wavelengths

A wavelength tunable perfect absorber with graphene-hexagonal gold (Au) cylinder array on a ground plate is investigated theoretically. The interactions between electromagnetic (EM) waves and monolayer graphene are analyzed through the field distributions and spectral responses in detail. The finite-difference-time-domain (FDTD) method is used to investigate the tunable properties of the absorber. It is demonstrated that in an optimized configuration, monolayer graphene can interact with light via critical coupling, and the absorptance can be greatly enhanced and reach to 100% for both transverse magnetic (TM) and transverse electronic (TE) polarizations. Furthermore, the influence of geometrical parameters of the structure on the response of the hybrid structure is studied. It is expected that the proposed graphene perfect absorbers can be applied for many applications in the visible (VIS) and the near-infrared (NIR) spectral ranges such as wavelength selective infrared photodetectors and plasmonic sensors.

Field-Effect Tunable and Broadband Epsilon-Near-Zero Perfect Absorbers with Deep Subwavelength Thickness

We report perfect light absorption due to the excitation of bound and radiative p-polarized optical modes in epsilon-near-zero (ENZ) conducting oxide nanolayers with thicknesses as thin as λENZ/100. Perfect absorption in the wavelength range of 600 nm to 2 μm may be achieved for unpatterned indium tin oxide (ITO) nanolayers with an electron density of 5 × 1020 to 2 × 1021 cm–3. Multilayer stacks of ITO nanolayers with a gradient of electron densities and optimized thicknesses enable broadband perfect absorption. The postfabrication tuning, of the perfect absorption wavelength, of 32 nm is achieved in a metal-oxide-semiconductor (MOS) geometry with applied voltage of 5 V. Such ultrathin and tunable broadband perfect absorbers have many potential applications in nonlinear flat ENZ optics, thermophotovoltaics, hot-electron generation in the ENZ regime, and other fields.

Optimization of a perfect absorber multilayer structure by genetic algorithms

BackgroundAn increasing interest has been recently grown in the development of nearly perfect absorber materials for solar energy collectors and more in general for all the thermophotovoltaic applications.MethodsWide angle and broadband perfect absorbers with compact multilayer structures made of a sequence of ITO and TiN layers are here studied to develop new devices for solar thermal energy harvesting. Genetic Algorithms are introduced for searching the optimal thicknesses of the layers so to design a perfect broadband absorber in the visible range, for a wide range of angles of incidence from 0° to 50°, and for both polarizations.ResultsGenetic Algorithms allow to design several optimized structures with 6, 8, and 10 layers reaching a very high average absorbance of 97%, 99% and 99.5% respectively together with a low hemispherical total emissivity (<20%) from 200 °C till 400 °C.ConclusionsThe proposed multilayer structures use materials with high thermal stability, and high melting temperature, can be fabricated with simple thin film deposition techniques, appearing to have very promising applications in solar thermal energy harvesting.

Ultra-broadband absorber from visible to near-infrared using plasmonic metamaterial.

We propose a design of an ultra-broadband absorber based on a thin metamaterial nanostructure composed of a periodic array of titanium-silica (Ti-SiO2) cubes and an aluminum (Al) bottom film. The proposed structure can achieve nearly perfect absorption with an average absorbance of 97% spanning a broad range from visible to near-infrared (i.e., from 354 nm to 1066 nm), showing a 90% absorption bandwidth over 712 nm, and the peak absorption is up to 99.8%. The excitation of superior surface plasmon resonance combined with the resonance induced by the metal-insulator-metal Fabry-Perot (FP) cavity leads to this broadband perfect absorption. The polarization and angle insensitivity is demonstrated by analyzing the absorption performance with oblique incidences for both TE- and TM-polarized waves. In addition, we discuss the impact of various metal materials and geometry structure on absorption performance in detail. The proposed broadband metamaterial absorber shows a promising prospect in applications such as solar cell, infrared detection, and imaging. Moreover, the use of a thin titanium cap and an aluminum film instead of noble metals has the potential to reduce production cost in applications.

Ultrabroadband Plasmonic Absorber Based on Biomimetic Compound Eye Structures

A perfect absorber based on plasmonic resonance for a variety of remarkable properties has motivated vivid research during the last few decades. However, most of the perfect absorbers had been focused on polarization-insensitive and narrow-band absorption, due to the existence of a theoretical limit. It is urgent to extend the optical properties of absorbers with omnidirectional and broadband performances to satisfy the requirements for practical application. Hence, we adopt a new convenient, energy-saving, and environmentally friendly silk mediated layer nanoimprint technology to obtain biomimetic moth eye structures with omnidirectional and broadband perfect absorption. Experimental and simulation results show that the absorbers can realize the absorption efficiency to a maximum of 99% over the entire visible spectrum for angles up to 70° from normal. In addition, it also exhibits excellent bidirectional absorptive characteristic of this design. We anticipate that this absorber may have greatly potential application in the photovoltaic and thermal imaging area.

Optimization of Perfect Absorbers with Multilayer Structures

We study wide-angle and broadband perfect absorbers with compact multilayer structures made of a sequence of ITO and TiN layers deposited onto a silver thick layer. An optimization procedure is introduced for searching the optimal thicknesses of the layers so as to design a perfect broadband absorber from 400 nm to 750 nm, for a wide range of angles of incidence from $$0{^{\circ }}$$ to $$50{^{\circ }}$$ , for both polarizations and with a low emissivity in the mid-infrared. We eventually compare the performances of several optimal structures that can be very promising for solar thermal energy harvesting and collectors.

Realization of near-perfect absorption in the whole reststrahlen band of SiC.

Materials used for outdoor radiative cooling technologies need not only be transparent in the solar spectral region, but also need to have a broadband perfect absorption in the infrared atmospheric transparency window (infrared-ATW). Silicon carbide (SiC) has been thought to be a potential candidate for such materials. However, due to the near-perfect reflection of electromagnetic waves in the whole reststrahlen band (RB) of SiC, which is within the infrared-ATW, perfect absorption in the whole RB remains a challenge. Here by constructing a cone-pillar double-structure surface on SiC, a near-perfect absorption (>97%) of normally incident electromagnetic waves in the whole RB has been realized experimentally. Simulation results reveal that the dominant reason for the near-perfect absorption is the efficient coupling of incident electromagnetic waves into the bulk evanescent waves in the free-space wavelength range (10.33 μm, 10.55 μm) and the efficient coupling of incident electromagnetic waves into the surface phonon polaritons in the free-space wavelength range (10.55 μm, 12.6 μm). Our findings open up an avenue to enhance the absorption performance of SiC in infrared-ATW, and may lead to many new applications.

Near-infrared tunable multiple broadband perfect absorber base on VO2 semi-shell arrays photonic microstructure and gold reflector

We proposed a metamaterial absorber which is aimed to achieve a multiple broadband absorption and tunable absorption peak in the near-infrared region. The absorber is based on VO2 semi-shell coated on the top of silica nano-particle array supported on the gold-reflective layer. Measured results show that the absorber has the multiple broadband with the absorption magnitudes more than 95% in the near infrared region. The absorption peaks can be tuned through the VO2 phase transition from metallic phase to insulator phase in the short wavelength (before λ = 1500 nm), when VO2 is at the metallic state, an absorption band appears in the long wavelength (after λ = 1500 nm). The simulation results closely match those of measured. The absorption intensity becomes stronger and absorption peaks have red shift with the increase of thickness of VO2 semi-shell. Thus, this designed tunable absorption intensity and position absorber based on VO2 can be a good choice for enhancing the performance of multiple band, this would be beneficial to the field of photo detectors, sensor and solar cell.

Broadband perfect infrared absorption by tuning epsilon-near-zero and epsilon-near-pole resonances of multilayer ITO nanowires.

We numerically investigate the broadband perfect infrared absorption by tuning epsilon-near-zero (ENZ) and epsilon-near-pole (ENP) resonances of multilayer indium tin oxide nanowires (ITO NWs). The monolayer ITO NWs array shows intensive absorption at ENZ and ENP wavelengths for p polarization, while only at the ENP wavelength for s polarization. Moreover, the ENP resonances are almost omnidirectional and the ENZ resonances are angularly dependent. Therefore, the absorption bandwidth is broader for p polarization than that for s polarization when polarized waves are incident obliquely. The ENZ resonances can be tuned by altering the doping concentration and volume filling factor of ITO NWs. However, the ENP resonances only can be tuned by changing the doping concentration of ITO NWs, and volume filling factor impacts little on the ENP resonances. Based on the strong absorption properties of each layer at their own ENP and ENZ resonances, the tuned absorption of the bilayer ITO NWs with the different doping concentrations can be broader and stronger. Furthermore, multilayer ITO NWs can achieve broadband perfect absorption by controlling the doping concentration, volume filling factor, and length of the NWs in each layer. This study has the potential to apply to applications requiring efficient absorption and energy conversion.

Lithography-free thin-titanium-nanocone metamaterial perfect absorbers using ZnO nanostructures

In this work, thin Ti nanocones are deposited on top of the arrays of ZnO nanopagodas, and the whole structure works as an efficient nanostructured metamaterial perfect absorber (MPA) without using lithography and dry etching. In this design, ~1µm long ZnO nanopagoda arrays are grown on a 100 nm ZnO buffer layer over the silicon/glass substrate by a treatment with an aqueous solution of L-ascorbic acid. Growth direction and the degree of lamination in the ZnO nanostructures can be easily controlled by adjusting the concentration of L-ascorbic acid. Afterward, these ZnO nanopagodas are coated with a 30nm thin top and a 500nm thick bottom layer of Ti to achieve the proposed nanocone resonant cavity structure with electromagnetic wave field penetration. The overall structure encapsulates three physical concepts, namely, field penetration, adiabatic coupling and cavity resonance, which contribute the broadband perfect absorption. The entire process is carried out at a low temperature (<90°). We believe the proposed tapered Ti nanocones MPA structure facilitates ultra-broadband perfect spectral absorption with promising nature of low-cost, large-area, and lithography-free.

Ultrathin microwave metamaterial absorber utilizing embedded resistors

We numerically and experimentally studied an ultrathin and broadband perfect absorber by enhancing the bandwidth with embedded resistors into the metamaterial structure, which is easy to fabricate in order to lower the Q-factor and by using multiple resonances with the patches of different sizes. We analyze the absorption mechanism in terms of the impedance matching with the free space and through the distribution of surface current at each resonance frequency. The magnetic field, induced by the antiparallel surface currents, is formed strongly in the direction opposite to the incident electromagnetic wave, to cancel the incident wave, leading to the perfect absorption. The corresponding experimental absorption was found to be higher than 97% in 0.88–3.15 GHz. The agreement between measurement and simulation was good. The aspects of our proposed structure can be applied to future electronic devices, for example, advanced noise-suppression sheets in the microwave regime.

Broadband polarization-independent wide-angle and reconfigurable phase transition hybrid metamaterial absorber

We report the simulation, fabrication, and experimental characterization of a single-layer broadband, polarization-insensitive and wide-angle near perfect metamaterial absorber (MA) in the microwave regime. The topology of the resonators is chosen in such a way that is capable of supporting simultaneously multiple plasmon resonances at adjacent frequencies, which lead to a broadband operation of the MA. Absorption larger than 80% at normal incidence covering a broad frequency range (between 7.4 GHz and 10.4 GHz) is demonstrated experimentally and through numerical simulations. Furthermore, the performance of the metamaterial absorber is kept constant up to an incident angle of 30°, for both TE and TM-polarizations. In addition, a hybrid model of the MA is proposed and implemented numerically in order to dynamically tune the absorption window. The hybrid MA is controlled by incorporating vanadium dioxide (VO2) temperature-driven metal-insulator phase transition material, which enables the transition from broadband (80% absorption and 3 GHz bandwidth) to narrowband (80% absorption and 0.7 GHz bandwidth) absorption window. Our proposed single-layer MA offers substantial advantages due to its low-cost and simplicity of fabrication. The results are very promising, suggesting a potential use of the MA in wide variety of applications including solar energy harvesting, biosensing, imaging, and stealth technology.