Broadband Perfect Absorption Research Articles

Enhanced broadband perfect absorption in visible and near-infrared regimes

ABSTRACT It is still a challenge to achieve perfect ((near-unity)) absorption across a wide wavelength range with a three-layer structure. This paper presents a metamaterial absorber (MA) based on MXene that can achieve broadband and perfect absorption in the near-infrared and visible regions. The MA with a simple three-layer structure consists of a top Ti3C2T x layer, an intermediate dielectric layer and a bottom gold layer. It can achieve perfect absorption (the absorptivity over 99%) in the wavelength range of 730–900 nm. The electric field distribution reveals that the localized surface plasmon resonance occurring at the interface of Ti3C2T x and SiO2 leads to strong absorption over the wide wavelength range. Furthermore, the MA exhibits excellent angular stability. This work provides a method to construct MAs with broadband perfect absorption in the near-infrared and visible band.

High-performance ultra-simple solar absorber with extremely high tolerance for fabrication errors

Broadband metamaterial perfect absorbers have drawn considerable attention from researchers for a broad range of applications in electromagnetic stealth, solar energy harvesting and other fields. However, it is extremely challenging to integrate the excellent characteristics of ultra-broadband, perfect absorption, polarization independent, and insensitivity to wide incidence angles into a structurally simple absorber. Herein, we present an ultra-wideband solar perfect absorber with an ultra-simple structure, which consists of a Si3N4-Ti-Si3N4-Ti four-layer structure. The optimized structure offers more than 92 % absorption in the bandwidth range of 450–2750 nm, and the average absorption is up to 97.3 %. Moreover, we elaborate the physical mechanism to achieve the broadband perfect absorption. For solar thermal systems, the absorber is capable of absorbing 95.7 % of the sunlight, its blackbody radiation absorption is as high as 98.0 % at 1900 K and it has a photothermal conversion efficiency of 90.2 % at 1000 K. Furthermore, the absorber displays the characteristics of being independent of polarization and insensitive to wide incidence angles. Additionally, the absorber possesses a very high tolerance for manufacturing errors. It is shown that the designed absorber is ultra-simple in structure and excellent in performance, and has excellent application prospects in fields including solar thermal conversion.

Numerical study of mid-infrared broadband perfect absorber based on dielectric/aluminium doped zinc oxide multilayer films

We propose a lithography-free wide-angle polarization-insensitive ultra-broadband absorber by using multilayer films consist of five pairs of aluminium doped zinc oxide (AZO) and a dielectric film which is supposed to be fabricated on glass substrate. The absorption spectrum of the proposed model is calculated by using transfer matrix method. The analytic results show that the absorptivity is larger than 95% with normal incidence light in the wavelength range from 7.5μm to 20μm and having high transmittance at the visible wavelength range from 0.4μm to 0.8μm range. Further, the effect of incident angle and polarization on the absorption spectrum are also investigated. The absorption spectrum of the proposed design is highly tunable on changing the damping constant of the AZO film. The electric field and magnetic distributions at different wavelengths show that the absorption is mainly originated due to the constructive interference of electromagnetic waves in the multilayer films. Such physical mechanism of broadening bandwidth based on increasing the carrier concentration of the highly visible transparent conducting films have pave a new direction for the design of tunable broadband absorber in different frequency band. Meanwhile, this broadband absorber is a good candidate for the potential applications in daytime radiation cooling system in the mid-infrared frequency band and also used as transparent conducting electrode in solar cell applications.

Meta-earplugs: Innovative concepts for alleviating the occlusion effect

Passive earplugs are commonly used to reduce workers' exposure to excessively high noise levels. Yet, they are associated with various discomforts of diverse origins (e.g., acoustic, physical, functional, or psychological). The occlusion effect, characterized by an increased perception of physiological sounds transmitted through bone conduction to the cochlea, presents a challenge, leading to acoustic discomfort, especially at low frequencies (0.1 to 1 kHz) and for shallow or moderate insertion depths of the earplug. Building upon acoustic meta-material principles, this study presents “meta-earplug” concepts designed to alleviate the occlusion effect. The approach focuses on reducing the input impedance of the earplug medial surface either to the characteristic impedance of air, using broadband perfect absorption, or to the acoustic impedance of the open ear canal, resulting in a zero objective occlusion effect. For these purposes, the proposed meta-earplug concepts are made of Helmholtz resonators arranged in parallel or series. Transfer matrix models are used in an optimization process to refine the geometry of the meta-earplugs. Although meta-earplug concepts involving multiple Helmholtz resonators have been preliminarily assessed using an artificial ear, a meta-earplug featuring a single resonator has undergone testing with human participants. The evaluation encompasses both objective measurements and subjective assessments.

Switchable quadruple narrowband to broadband terahertz perfect absorber based on graphene and VO2 metamaterials

The article describes a switchable and adjustable terahertz perfect absorber to achieve quadruple narrowband absorption and broadband perfect absorption, consisting of graphene and VO2. Two operating modes can be toggled on account of the electrical tunability of graphene and the reversible insulating property-metallic property transformation of VO2. If VO2 displays as the insulating property, our proposed terahertz absorber manifests as quadruple narrowband perfect absorption mode. Four absorption formants can be altered effectively via adjusting the graphene Fermi energy. And then VO2 is adjusted to the metallic property, the proposed terahertz absorber possesses over 99 % absorption in 6.71–9.75 THz as broadband perfect absorption mode. A variation in the conductance of VO2 can regulate broadband absorption efficiency from 0 % to nearly 100 %. And under this condition, if the conductance of VO2 maintains in 200 S/m, it is possible to utilize broadband perfect absorber as a total reflector. Moreover, the absorption performances of both quadruple narrowband perfect absorption mode and broadband perfect absorption mode are rather polarisation-insensitive and wide-incident-angle tolerant. Generally speaking, the proposed meta-device possesses a better absorption performance, accomplishing the switch of different absorption modes and dynamic regulation of absorption performances under different modes, which can satisfy the function needs under different situations. It is believed that our design can facilitate the development of potential applications of terahertz technology in the field of filters, optical switches and sensors.

A study on the optimized area ratio of unit cell assembly for the broadband perfect sound absorption of acoustic metasurface

A study on the optimized area ratio of unit cell assembly for the broadband perfect sound absorption of acoustic metasurface

Thermal controlled multi-functional metasurface for freely switching of absorption, reflection, and transmission.

Metasurfaces have garnered significant attention in recent years due to their substantial electromagnetic (EM) wave manipulation capabilities. However, most previously documented metasurfaces have been limited to controlling just a single EM wave mode, encompassing transmission, reflection, or absorption. Such limitations have impeded the broader applications of metasurfaces. To address this issue, this study introduces a multi-functional metasurface (MFM) in the utilization of Ge2Sb2Te5 (GST), vanadium dioxide (VO2), and graphene. This novel design enables real-time control over the transmission, absorption, and reflection of EM waves as necessitated through thermal control, allowing for seamless transitions from complete transmission to complete reflection. Furthermore, this configuration achieves extensive broadband perfect absorption, spanning up to 1.83 THz. The optical response mechanism of this MFM across distinct operational modes is meticulously analyzed through electric field distribution. Remarkably, this proposed MFM exhibits polarization insensitivity and maintains good optical performance even under conditions of wide-angle incidence. With the ability to switch to different operating modes according to the needs of different environments, the proposed MFM has the potential to be used in a wide range of scenarios, including radar stealth, wireless communications, and military search.

Bimetallic metasurface broadband perfect absorber for high-efficiency thermal photovoltaics

We theoretically propose and demonstrate that broadband polarization-insensitive perfect visible light absorption can be realized in a periodic bimetallic nanotriangle structure with a thick gold plate. The average absorption is 95.79% from 400 nm to 800 nm. The multiple localized surface plasmon resonances of the bimetallic nanostructure are the major contributions to this broadband perfect absorption in the visible region. Meanwhile, the physical mechanism of the broadband perfect absorption of the bimetallic nanostructure can be well interpreted by impedance matching theory. Moreover, the average photo-thermal conversion efficiency is up to 93% over a wide temperature range.

Broad-band conical-shaped perfect absorber metamaterial for solar cells

In this paper, we present a design, simulation, and measurement of a single broad-band metamaterial absorber composed of a consecutive layer of conical-shaped SiO2/Al/SiO2/Al. The circular symmetry of the conical-shaped resonator makes the metamaterial absorber polarization insensitive so that the full intensity of the sunlight can be harvested. The physical behavior of the broad-band mechanism is investigated both numerically by finite difference time domain (FDTD) and experimentally. The numerical calculations indicate the origin of broadband absorption is due to the couplings between the two metal layers sandwiched in between two dielectric SiO2 layers. The effect of geometrical parameters and the layer thicknesses are studied numerically and experimentally. The experimental results show the average absorption strength of the metamaterial absorber is around 80% within the spectral range of 400–1000 nm. The experimental and simulation spectra are well-consistent. The proposed platform has potential use in many potential applications, such as solar cells, photodetectors, cameras, etc.

A broadband absorber with multiple tunable functions for terahertz band based on graphene and vanadium dioxide

A broadband perfect absorber tunable in terahertz band based on vanadium dioxide (VO2) and graphene metamaterial is proposed. We have carried out numerical simulations using Finite-Difference Time-Domain (FDTD) method. This structure can tune the absorption rate of VO2 by changing its conductivity. Moreover, due to the addition of graphene, the absorption is improved and the bandwidth reaches 3.24 THz. The structure allows spectral shift by varying the Fermi level of graphene, which can achieve >90 % ultrawideband absorption in 3.74 to 6.96 THz range when the Fermi level is 0.8 eV. In the range of 4.338 to 6.332 THz, nearly 100 % absorption can be achieved. The proposed absorber also exhibits good polarization insensitivity and wide angle of incident stability. Our design has potential applications in terahertz spectroscopy and imaging, also can be applied to modulators, sensors, stealth and photoelectric switches.

Dispersion-enabled control of photonic density of states in photonic hypercrystals.

In this work, we investigate possibility of engineering photonic density of states (PDOS) in photonic hypercrystals (PHCs). In the course of our analysis, we have demonstrated that it is possible to obtain photonic bandgap for selected polarization of light as well as to achieve significant broadband PDOS enhancement. We have also presented for the first time that anomalous dispersion, that arises from effective resonance of hyperbolic medium constituting the PHC structure, may lead to negative PDOS, which is photonic equivalent of mobility gap, observed in electronic crystals. Furthermore, we have demonstrated that application of PHC structure, instead of standalone hyperbolic medium, allows to obtain more versatile electromagnetic response, such as broadband perfect absorption of adjustable spectral range of operation.

Terahertz broadband near-perfect absorber with a single-layer coating on doped semiconductor

We demonstrate that a single-layer coating on a doped GaAs or Si substrate enables broadband antireflection and, hence, broadband perfect absorption in the terahertz frequency range. This broadband behavior can be generally expected when the substrate material has a Drude-type dispersion. Our mathematical analyses show that the reflection from Drude-type material may have an anomalously dispersive phase shift. The anomalous dispersion of the reflection phase is used to compensate for the normal dispersion of the accumulation phase in the single-layer coating film. Consequently, the antireflection conditions are satisfied in a wide frequency range, and broadband antireflection is achieved. Thus, broadband perfect absorption is realized with only a single-layer coating film on the substrate. Our method provides a simple and efficient approach to achieving broadband perfect absorption, which is critical in many applications such as radar stealth techniques and solar cells.

Multi-mode resonance plasmonic solar absorber based on pyramid multiary-grating

In the manuscript, a broad-band perfect absorber based on multilayer-grating and MDM film structure is proposed and numerically studied. Finite difference time domain (FDTD) simulations indicate the absorption performances are originated from Fabry–Perot (FP) resonance effect, localized, and propagating surface plasmons (LSPs and PSPs) effect. The designed absorber possesses over 95% absorption from 697 nm to 2906 nm, and an average absorption of 98.7% is achieved with TM-polarized. For TM-polarized, the designed absorber possesses over 95% between 534 nm and 2475 nm when the oblique-angle is up to 45°, while for a TE-polarized light, the corresponding average absorption remains 80.48% when the oblique-angle is up to 30°. The average absorption is larger than 94% in different environment refractive indexes (1 < RI < 1.5), which illustrates the designed absorber possesses excellent environment RI stability. Another absorber with triple layer grating of different sizes is also designed, and the absorber has more than 95% absorption between 400 nm and 2376 nm, based on the high absorption and waveband, the absorber can find potential applications in solar energy capture. It is believed the proposed work can be applied in plasmonic solar absorber design, thermal emitter, and plasmonic imaging.

Ultra-broadband polarisation-insensitive far infrared absorber based on an umbrella tungsten array structure

The technical study of metamaterial broadband perfect absorbers is gradually gaining widespread attention in the ongoing research. Due to its different aspects such as high absorption, frequency selectivity and ease of design, its application in various fields is showing more and more possibilities and prospects. In this research project, we propose an ultrabroadband, polarisation-insensitive​ metamaterial terahertz absorber based on an umbrella absorption cell that can be used as a far-infrared absorber, which uses tungsten (W) refractory metal as the absorber layer. From the results of our simulations, we can see that the absorber can achieve an absorption bandwidth of 3.3–15 THz, and the average absorption of the absorber can reach more than 99.0%. The absorber has a large absorbance in the far-infrared band and can be considered as a far-infrared absorber.

Switchable dual-broadband to single-broadband terahertz absorber based on hybrid graphene and vanadium dioxide metamaterials

Switchable and tunable broadband perfect absorbers have drawn great interest in a wide range of applications, including modulation, energy harvesting, and spectroscopy. Here, we propose a switchable and tunable terahertz (THz) metamaterial absorber with dual-broadband, single-broadband perfect absorption using graphene and vanadium dioxide (VO2). Simulation results show that, when VO2 is in the insulated state, this design behaves as a dual-broadband absorber with over 90% absorption in 0.73 THz–1.53 THz and 3.03 THz–3.64 THz under normal incidence. When VO2 is in the metallic state and the graphene Fermi energy is set as 0.01 eV, this design acts as a single-broadband absorber with over 90% absorption in 1.00 THz–3.55 THz and the fractional bandwidth reaches 112%, which is much larger than the previously reported values. The absorption rate can be dynamically tuned by individually changing the conductivity of VO2 and the Fermi energy of graphene. Moreover, this design exhibits polarization-insensitive and wide-incident-angle behaviors for both TE and TM waves. Such a design may have potential applications in many fields, such as thermal detectors, modulators, and stealth technology.

Absorption properties and mechanisms of metallic moth-eye structures

The antireflection and absorption properties of non-metallic moth-eye structures have been intensively studied, however the research on those of metallic moth-eye structures, especially their absorption mechanisms, is far from enough. In this paper, the absorption properties and mechanisms of a metallic Au moth-eye structure are studied in detail. The results show that broadband perfect absorption covering the entire visible to the near infrared range with polarization independence and in a wide incident angle range can be easily realized using a single-layer single-material Au moth-eye structure. As for the absorption mechanism, in the short wavelength range of 300 nm–550 nm, due to the high loss of the Au material, the antireflection effect plays a main role. While in the long wavelength range of 550 nm–1000 nm, the widely accepted absorption mechanisms (the antireflection effect as well as the cascaded multiple plasmonic resonances, and even the combination of them) cannot completely account for the broadband perfect absorption. We found that the adiabatic nanofocusing effect of the plasmonic resonances enabled by the ultra-sharp air grooves plays a key role in the absorption. It is the nanofocusing effect that effectively increases the absorption to nearly 100% in a broad wavelength band. These findings provide a deeper insight into the absorption mechanisms of metallic moth-eye structures and offer an effective guide to explore demanded excellent absorption performance.

Broadband low-frequency sound absorption of honeycomb sandwich panels with rough embedded necks

A hexagonal honeycomb sandwich panel with rough embedded necks is proposed as a new structure for broadband low-frequency sound absorption. A theoretical model is established to study the sound absorption performance of the structure, and a finite element model is also developed to explore its sound absorption mechanism. The results show that the structure has excellent broadband low-frequency sound absorption performance. The energy dissipation mainly occurs in the neck, and the energy dissipation of the neck can be enhanced by adjusting the relative roughness of the neck. As a result, the sound absorption peak of the structure increases and the peak frequency shifts towards low frequencies. Also, thinner structures will shift the peak frequency to high frequencies. By reasonably adjusting the height of the honeycomb cavity and the relative roughness of the neck, the effects of these two changes can be balanced to achieve the perfect sound absorption of the thinner structure in the low frequency band. Based on the electro-acoustic analogy method, the broadband perfect sound absorption of thin structure in low frequency can be realized by reasonably paralleling different units with rough embedded necks. Experimental measurements are carried out, and favorably verify the theoretical model and the finite element model, as well as the broadband low-frequency design scheme. This work opens up a new way to design thin acoustic metamaterials for broadband low-frequency sound absorption.

Optimization of TiN-based ultra-flexible materials for photothermal and solar harvesting applications

We propose a theoretical model to investigate photothermal heating of ultra-flexible metamaterials, which are obtained by randomly mixing TiN nanoparticles in polydimethylsiloxane (PDMS). Due to the plasmonic properties of TiN nanoparticles, incident light can be perfectly absorbed in a broadband range (300-3000 nm) to generate heat within these metamaterials. Under irradiation of an 808 nm near-infrared laser with different intensities, our predicted temperature rises as a function of time agree well with recent experimental data. For a given laser intensity, the temperature rise varies non-monotonically with the concentration of TiN nanoparticles. Increasing the TiN concentration leads to a decrease in the heating process since the thermal conductivity grows. A small TiN concentration significantly reduces the absorbed energy and, thus, the system is less heated. When we apply this model to solar heating, we find that the temperature rise is no longer non-monotonic, and the heating efficiency is much lower than in the laser case. Our studies would provide good guidance for future experimental studies on the photothermal heating of broadband perfect absorbers.

Broadband actively tunable metamaterial absorber based on vanadium dioxide and Fabry-Perot cavity

The dynamic tunable broadband perfect absorber based on vanadium dioxide (VO2) is proposed in this paper. And the VO2 conductivity can be changed by temperature, thus changing the absorption characteristics of the absorber. Under normal incident conditions, absorption is more than 90% of the absorber in the range of 5.8–17.2 THz (THz), with an average absorption rate of more than 96%. And impedance matching theory and magnetic resonance theory are used to illustrate the mechanism of operation. The effect of the incidence angle on the absorption performance was studied, and the results show that the absorbers still maintain excellent absorption when the incidence angle is less than 45∘. In addition, the absorption characteristics of the absorber working in water are also studied. We demonstrate that the absorber's absorption is more than 90% in the range of 5–18 THz with an absorption bandwidth of 13 THz at a water depth of 5.2 μm. And by adjusting the VO2 conductivity, the average absorption rate can be adjusted from 2.5% to 96.1% with a water depth of 5.2 μm. The proposed structure has potential significant applications in the design of heat emitters, optical switches, cloaking devices, and energy harvesting devices.

Photo-to-heat conversion of broadband metamaterial absorbers based on TiN nanoparticles under laser and solar illumination

We theoretically investigate photothermal heating of ultra-flexible metamaterials, which are obtained by randomly mixing TiN nanoparticles in polydimethylsiloxane (PDMS). Due to the plasmonic properties of TiN nanoparticles, incident light is perfectly absorbed in a broadband range (300–3000 nm) to generate heat within these metamaterials. Under irradiation of an 808 nm near-infrared laser with different intensities, our predicted temperature rises as a function of time agree well with recent experimental data. For a given laser intensity, the temperature rise varies non-monotonically with concentration of TiN nanoparticles because the enhancement of thermal conductivity and absorbed energy as adding plasmonic nanostructures leads to opposite effects on the heating process. When the model is extended to solar heating, photothermal behaviors are qualitatively similar but the temperature increase is less than 13 K. Our studies would provide good guidance for future experimental studies on the photo-to-heat conversion of broadband perfect absorbers.