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Freely switching light transmission and absorption via an achromatic reflectionless screen is highly desired for many photonic applications (e.g., energy-harvesting, cloaking, etc.), but available meta-devices often exhibit reflections out of their narrow working bands. Here, we rigorously demonstrate that an optical metasurface formed by two resonator arrays coupled vertically can be perfectly reflectionless at all frequencies below the first diffraction mode, when the near-field (NF) and far-field (FF) couplings between two constitutional resonators satisfy certain conditions. Tuning intrinsic loss of the system can further modulate the ratio between light transmission and absorption, yet keeping reflection diminished strictly. Designing/fabricating a series of metasurfaces with different inter-resonator configurations, we experimentally illustrate how varying inter-resonator NF and FF couplings can drive the system to transit between different phase regions in a generic phase diagram. In particular, we experimentally demonstrate that a realistic metasurface satisfying the discovered criteria exhibits the desired achromatic reflectionless property within 160–220 THz (0–225 THz in simulation), yet behaving as a perfect absorber at ~ 203 THz. Our findings pave the road to realize meta-devices exhibiting designable transmission/absorption spectra immune from reflections, which may find many applications in practice. [ABSTRACT FROM AUTHOR]

One of the fastest-expanding study areas in optics over the past decade has been metasurfaces (MSs). These subwavelength meta-atom-based ultrathin arrays have been developed for a broad range of functions, including lenses, polarization control, holography, coloring, spectroscopy, sensors, and many more. They allow exact control of the many properties of electromagnetic waves. The performance of MSs has dramatically improved because of recent developments in nanofabrication methods, and this concept has developed to the point that it may be used in commercial applications. In this review, a vital topic of sensing has been considered and an up-to-date study has been carried out. Three different kinds of MS absorber sensor formations, all-dielectric, all-metallic, and hybrid configurations, are presented for biochemical sensing applications. We believe that this review paper will provide current knowledge on state-of-the-art sensing devices based on MSs. [ABSTRACT FROM AUTHOR]

Solar energy is a clean and renewable energy source and solves today's energy and climate emergency. Near-perfect broadband solar absorbers can offer necessary technical assistance to follow this route and develop an effective solar energy-harvesting system. In this work, the metamaterial perfect absorber operating in the ultraviolet to the near-infrared spectral range was designed, consisting of a periodically aligned titanium (Ti) nanoarray coupled to an optical cavity. Through numerical simulations, the average absorption efficiency of the optimal parameter absorber can reach up to 99.84% in the 200–3000 nm broadband range. We show that the Ti pyramid's localized surface plasmon resonances, the intrinsic loss of the Ti material, and the coupling of resonance modes between two neighboring pyramids are highly responsible for this broadband perfect absorption effect. Additionally, we demonstrate that the absorber exhibits some excellent features desirable for the practical absorption and harvesting of solar energy, such as precision tolerance, polarization independence, and large angular acceptance. [ABSTRACT FROM AUTHOR]

In this study, a perfect metamaterial absorber (PMMA) based on an indium antimonide temperature-sensitive material is designed and investigated in the terahertz region. We demonstrate that it is an ideal perfect narrow-band absorber with polarization-insensitive and wide-angle absorption properties. Numerical simulation results show that the proposed PMMA can be operated as a temperature sensor with a sensitivity of 21.9 GHz/K. A graphene layer was added to the PMMA structure to improve the sensitivity, and the temperature sensitivity was increased to 24.4 GHz/K. Owing to its excellent performance, the proposed PMMA can be applied in thermal sensing, detection, and switching. [ABSTRACT FROM AUTHOR]

Infrared detection technology has important applications in laser ranging, imaging, night vision, and other fields. Furthermore, recent studies have proven that hot carriers which are generated by surface plasmon decay can be exploited for photodetection to get beyond semiconductors' bandgap restriction. In this study, silicon nanopillars (NPs) and gold film at the top and bottom of silicon nanopillars were designed to generate surface plasmon resonance and Fabry–Perot resonance to achieve perfect absorption. The absorption was calculated using the Finite Difference Time Domain (FDTD) method, and factors' effects on resonance wavelength and absorption were examined. Here we demonstrate how this perfect absorber can be used to achieve near-unity optical absorption using ultrathin plasmonic nanostructures with thicknesses of 15 nm, smaller than the hot electron diffusion length. Further study revealed that the resonance wavelength can be redshifted to the mid-infrared band (e.g., 3.75 μm) by increasing the value of the structure parameters. These results demonstrate a success in the study of polarization insensitivity, detection band adjustable, and efficient perfect absorption infrared photodetectors. [ABSTRACT FROM AUTHOR]

A temperature‐tunable terahertz (THz) perfect absorber (PA) composed of a periodic array of deep subwavelength micro‐cross‐shaped (MCS) structures of the strontium titanate (STO) resonator is proposed and investigated theoretically, which can be applicable for the temperature sensing. Simulation results indicate that the absorbance of 99.8% at 0.221 THz can be achieved when the designed PA is under the room temperature of T = 300 K, which is in good agreement with the calculation done by the coupling mode theory (CMT). The simulated distributions of electric and magnetic fields in the unit‐cell structure of the designed PA reveal that the observed perfect absorption is mainly attributed to the Mie resonance of the all‐dielectric MCS structure STO. In addition, this designed PA is polarization‐insensitive and wide‐angle absorption for both transverse magnetic (TM) and transverse electric (TE) waves. The resonance absorption properties of the designed PA can be controlled by changing the geometrical parameters of STO resonator structure. The designed PA can be served as a temperature sensor, which has a sensitivity of about 0.37 GHz K−1 since the electrical property of the STO is dependent on the variation of surrounding temperature. Furthermore, the perfect absorption can also be achieved by the PAs using the square, circular, and ring STO structures. The proposed design concepts of the STO‐based tunable PAs can find potential THz applications in sensing, detecting, imaging, and so on. [ABSTRACT FROM AUTHOR]

The present paper addresses the theoretical design and circuit model details of a square- and circle-shaped broadband perfect ring absorber in a wide wavelength range. This absorber has a three-layered structure, which is respectively made up of tungsten, silica, and tungsten. The optimal design is related to tungsten since its impedance matches the free area of the visible spectrum. It records a more desirable response at significant temperatures that can be attributed to the significantly high melting point of tungsten (3422°C). The effects of structure-related parameters and different metals on absorption-related performance are also detailed. The results suggest that the circular ring absorber registers an ultra-broadband absorption response in the range of 550–676 nm, as absorptivity rises above 99%. Comparatively, the square ring absorber records the maximum possible absorption rate of 96.61% at 482 nm and the minimum absorption rate of 94.40% at 642 nm in the visible light regime (400–800 nm). Finally, an exact circuit model is used to obtain the analytical expressions for the proposed absorber. We contrast the findings for this circuit model with the results of full-wave simulation, which shows a good match. The proposed circuit model is basic, as it suggests the physical results of the design along with the operation of a wide range of subwavelength structures over an extensive frequency range. [ABSTRACT FROM AUTHOR]

Abstract Triple-band terahertz metamaterial absorber with design of miniaturization and compactness is presented in this work. The unit cell of the terahertz absorber is formed by an analogy I-typed resonator (a rectangular patch with two small notches) deposited on top of dielectric sheet and metallic mirror. The miniaturized structure design exhibits three discrete frequency points with near-perfect absorption at terahertz regime. The three absorption peaks could be ascribed to localized resonances of analogy I-typed resonator, while the response positions of these absorption peaks at the analogy I-typed resonator are different by analyzing the near-field patterns of these resonance peaks. Changes in structure parameters of the analogy I-typed resonator are also investigated. Simulation results revealed that the notch sizes of the rectangular patch are the key factor to form the triple-band near-perfect absorption. Further structure optimization is given to demonstrate triple-band polarization insensitive performance. Moreover, actively tunable absorption properties are realized by inserting or introducing vanadium dioxide with adjustable conductivity into the metamaterial structure. It is revealed that the insulator–metal phase transition of vanadium dioxide is the main reason for the modulation of absorption performance. Compared with previous multiple-band absorbers, the device given here has excellent features of high degrees of simplification, miniaturization, and active modulation, these are important in practical applications.

• The absorber exhibits high absorption, the remarkable anisotropy and flexible tunability characteristics. • The absorption reach nearly 100 % thought bound states in the continuum even at electronic doping of n s = 0.1 × 1013 cm−2. • The quality factor can achieve a value of 611.2. Black phosphorus (BP) is a promising two-dimensional (2D) material with direct band gap and distinctive lattice structure. Control over its bandgap can be achieved through physical or chemical methods, enabling selective light absorption within specific wavelength ranges. This study designed a narrow-bandwidth perfect absorber with highly quality (Q) factor, anisotropy and flexible tunability based on BP. The proposed structure comprises a monolayer BP sheet on the square array of Si hollow nanodisks placed on a SiO 2 spacer with a gold substrate backing. We numerically demonstrate the narrowband perfect absorption, which is achieved through bound states in the continuum resonance supported by metasurfaces critically coupled to BP monolayer. Notably, simulation results show that the absorption magnitude can reach nearly 100 % even at low electronic doping of n s = 0.1 × 1013 cm−2, and the Q factor can achieve a value of 611.2. The absorption peak and the resonance wavelength can be flexibly tuned by adjusting the electron doping of BP, the geometrical parameters of the structure and the angles of incident. Additionally, due to the inherent in-plane anisotropic feature of BP, the proposed structure exhibits high polarization-dependence. These results make our design a promising candidate for polarization-selective and tunable high performance mid-infrared devices, including polarizers, modulators, and photodetectors. [ABSTRACT FROM AUTHOR]

• A MEMS reconfigurable THz metamaterial absorber (TMA) is presented. • MEMS-based TMA shows tunable free spectra range (FSR) characteristic. • MEMS-based TMA also shows polarization-dependent characteristic. • Remarkable linear sensitivity to variations in surrounding environment is demonstrated. This study presents and demonstrates a design of a micro-electro-mechanical system (MEMS) reconfigurable terahertz (THz) metamaterial absorber (TMA) that shows tunable free spectra range with a maximum value as 166 GHz in transverse magnetic mode and switchable resonance from 1.24 THz to 1.53 THz. The unit structure of TMA consists of two movable rectangular resonators aside and an x-shaped resonator at the center comprising two c-shaped resonators connected upside down. The material configuration of TMA is typical of the metal–insulator-metal sandwich configuration. That is a bottom Au layer preventing the penetration of the THz wave, a dielectric SiO 2 layer to accommodate and absorb the incident wave, and a patterned Au structure array to select the resonant frequency. The electromagnetic response of TMA could be flexibly modulated when the structure is reconfigured by changing the gaps between x-shaped resonators and rectangular resonators based on the MEMS platform. Meanwhile, owing to the asymmetry of resonant structure, the electromagnetic response of TMA could be tuned from single-, dual-, triple- quad-resonances by varying the polarization state of incidence. Furthermore, the sensitivity of TMA is explored and demonstrated with remarkable linearity under a variation of the surrounding environment refractive index from 1.0 to 2.0. The feasibility of TMA is validated as the experimental absorption spectra are well meet the simulation results. The above results indicate the proposed TMA possesses great potential applications in multiple resonance switches, polarization switches, and highly efficient environmental sensors. [ABSTRACT FROM AUTHOR]

Plasmonic nanostructures as narrowband perfect absorbers have garnered significant attention due to their potential applications in biosensing and environment detection. This study emphasizes the investigation of arrayed split ring nanostructures within the configuration of metal-insulator-metal (MIM) multilayers, resulting in a maximum light absorption of 99.94% in the near-infrared (NIR) spectral range. The exceptional absorption efficiency of the device is attributed to the strong resonance of electric and magnetic fields arising from the Fabry–Pérot cavity resonance. The resonant peak can be flexibly tuned by engineering the dielectric layer thickness, the period, and the geometric parameter of split rings. Remarkably, the device exhibits promising capabilities in sensing, demonstrating a sensitivity of 326 nm/RIU in visible wavelengths and 504 nm/RIU in NIR wavelengths when exposed to bio-analytes with varying refractive indices. This designed nanostructure can serve as a promising candidate for biosensors or environmental detection.

Metamaterials and metasurfaces have been widely developed recently for extraordinary acoustic and elastic wave control at a deep subwavelength scale. Perfect wave absorption as an extreme case to totally absorb the impinged waves has gained great attention, whereas most existing designs based on local resonance lack tunabilities, making perfect absorption be observed at a single frequency. To overcome this drawback, in this work, we design and fabricate a tunable inductance-resistance (LR) shunted mechanical resonator via a bonded piezoelectric patch for perfect flexural wave absorption at low frequency. The LR shunted absorber could be reconfigured to a broad frequency range for perfect flexural wave absorption. The tunable perfect absorption performances are validated through experiments and unit absorption is achieved in experiments. In the end, to further highlight the advantages of shunted damping we numerically demonstrate that the absorption spectrum could be enhanced to broadband absorption with a negative capacitance and an inductance-resistance circuit (NC-LR) connected in parallel. The approach proposed provides an alternative solution to achieve perfect wave absorption in the low-frequency range and enables practical application in complex engineering structures.

In this letter, a terahertz metamaterials broadband perfect absorber based on molybdenum disulfide (MoS2) is proposed. The absorber is composed of a single layer of MoS2 and a metal plate separated by thin polyimide. The single-layer MoS2 structure consists of four rectangular strips transformed by fillets. Numerical simulations using the full-vector finite element method show that the new hybrid mode can be excited and the electric fields of the modes can be redistributed due to the introduction of fillet MoS2 metamaterial structure, which effectively improves the absorption performance of devices. In the range of 1.8 to 2.2 THz, the absorptivity is close to 100%, and the absorption curve is much flatter, which provide a useful reference for terahertz modulation equipment. [ABSTRACT FROM AUTHOR]

I constructed two models for achieving a perfect absorption acoustic metamaterial using a hexagonal honeycomb structure in the air with a change in folding number. The purpose of these models was to construct a hexagonal honeycomb metamaterial derived from Polydimethylsiloxane (PDMS) polymer. I carried out finite element simulations using COMSOL Multiphysics software to take theoretical measurements for our honeycomb structure and to show the influence of structural parameters in our models. Our simulations revealed that, depending upon the theoretical analysis, an acoustic metamaterial that supports resonance at 210 Hz for folding number n = 6 can be developed to construct a low-frequency sound-absorbing technology. I demonstrate that the dissipative loss effect can be controlled by folding number and high space utilization through adjusting the hexagonal dimensions to achieve perfect absorption. I also demonstrate the important effects of folding number, rotation angle, and structural parameters on improving acoustic absorption performance for honeycomb structural design. The results are of extraordinary correspondence at low frequency for achieving an ideal sound absorbing material. [ABSTRACT FROM AUTHOR]

A new design of a triple band perfect metamaterial absorber based on Pythagorean fractal geometry is proposed and analyzed for terahertz sensing applications. The proposed design showed an enhanced sensing performance and achieved three intensive peaks at 33.93, 36.27, and 38.39 THz, corresponding to the absorptivity of 98.5%, 99.3%, and 99.6%, respectively. Due to the symmetrical nature of the recommended design, the structure exhibited the characteristics of independency on the incident wave angles. Furthermore, a parametric study was performed to show the effects of the change in substrate type, resonator material, and substrate thickness on the absorption spectrum. At a fixed analyte thickness (0.5 μm), the resonance frequency of the design was found to be sensitive to the refractive index of the surrounding medium. The proposed design presented three ultra-sensitive responses of 1730, 1590, and 2050 GHz/RIU with the figure of merit (FoM) of 3.20, 1.54, and 4.28, respectively, when the refractive index was changed from 1.0 to 1.4. Additionally, the metamaterial sensor showed a sensitivity of 1230, 2270, and 1580 GHz/μm at the three resonance frequencies, respectively, when it was utilized for the detection of thickness variation at a fixed analyte refractive index (RI) of 1.4. As long as the RI of the biomedical samples is between 1.3 and 1.4, the proposed sensor can be used for biomedical applications. [ABSTRACT FROM AUTHOR]

We propose and numerically investigate an angle-tuned polarization-independent multi-narrowband perfect absorber, which comprises a simple gold nanocube array on SiO2/oxidized aluminum mirror layers. The oxidized aluminum mirror is able to support the surface plasmon polariton mode on the SiO2/Al2O3 interface. The multi-narrowband absorption contributes to the simultaneously excited quasi-metalâ€"insulatorâ€"metal guided modes and standing wave cavity mode in the thick insulator layer by the incident light normal to the structure in the wavelength range 1400â€"2400 nm. Moreover, the absorption can be actively modulated by adjusting the incidence angle, and it is polarization independent. The perfect absorber is also suitable for sensing, with the figure of merit reaching 102 RIUâˆ'1 within the refractive index range from 1.3 to 1.36. [ABSTRACT FROM AUTHOR]

As solar energy is a low-cost and clean energy source, there has been a great deal of interest in how to harvest it. To absorb solar energy efficiently, we designed a broadband metamaterial absorber based on the principle of Fabry–Pérot (FP) cavities and surface plasmon resonance (SPR). We propose a broadband perfect absorber consisting of a four-layer structure of silica–tungsten–silica–titanium (SiO2–W–SiO2–Ti) for the incident light wavelength range of 300–2500 nm. The structure achieves perfect absorption of incident light in the wavelength range of 351.8–2465.0 nm (absorption > 90%), with an average absorption of 96.3%. The advantage of our proposed structure is that it combines the characteristics of both high and broadband absorption, and has high overall absorption efficiency for solar radiation. It is also independent of polarization and insensitive to incident angle. We investigated how absorption was affected by different structures, materials, geometric parameters, and refractive indices for different dielectric materials, and we explored the reasons for high absorption. This structure is refractory and ultrathin, and it offers a good tradeoff between bandwidth and absorption. It therefore has premium application prospects and value. [ABSTRACT FROM AUTHOR]

The Q factor in a quasi-BIC-based optical device can approach infinity and has therefore been attracting the attention of many researchers in recent years. However, this mode is barely applied to absorbers since it mainly tunes the radiative loss. The resonant wavelength of quasi-BICs normally couples with the Q factor, and it is difficult to independently tune one of them while maintaining the other, which weakens the flexibility of tuning. In this work, a quasi-BIC-based high-Q perfect absorber with some unique features is proposed. It shows a decoupled relationship between the resonant wavelength and the Q factor such that these two properties can be independently tuned by changing different structure parameters. In addition, both radiative and resistive losses are tunable. An easy method is proposed to design a perfect absorber with different resonant wavelengths and different Q factors, and a near-infrared perfect absorber with a Q factor as high as 5.13 × 105 is designed. This work proposes a method to tune the quasi-BIC mode, thereby introducing a new paradigm for the design of a high-Q perfect absorber. [ABSTRACT FROM AUTHOR]

This paper presents a metamaterial perfect absorber as a biosensor in the near-infrared region for biomedical molecular detection, such as urine concentration, malaria infection, bacillus bacteria, and cancer cell detection. In the proposed biosensor, an Au layer with a bracket-shaped cavity is utilized to boost electric field enhancement intensity in the vicinity of the nanohole. According to the capacitance of the bracket-shaped nanohole, changing the length of the hole controls the resonance wavelength. Due to the significant confinement of the electromagnetic field in the sharp edges and gaps, which excites Localized Surface Plasmon Resonances (LSPRs) in the metal-dielectric interface, near-unity absorption is obtained. Through evaluating the optical responses of the structure, via the finite element method (FEM), for different biomedical samples, high sensitivity of 2120.86 nm/RIU is acquired. In order to increase the sensitivity of the biosensor, a graphene layer is added between the dielectric layer and the gold nanohole layer. Adding graphene increases the LSPRs excitation and losses which yields a maximum sensitivity of 2500 nm/RIU. The presented biosensor with high sensitivity and compact size is a good candidate for biomedical purposes. [ABSTRACT FROM AUTHOR]

In this paper, a novel tunable dual-band graphene-based perfect absorber which features a three-layer structure consisting of a monolayer graphene sheet and a metallic mirror separated by a thin Si layer with circular microcavity array is designed and systematically investigated. The simulation results show that the two perfect absorption peaks (99.7% and 99.5%) are achieved at 4.13 and 4.38 THz. The two resonance modes are caused by different electric and magnetic field distributions, resulting in different tuning properties. The absorption peaks could be tuned by adjusting the chemical potential of the monolayer graphene. In addition, the influences of the structural parameters such as period, thickness of the Si layer, depth and radius of the circular microcavity on the absorption characteristics of the absorber have been studied and presented.

A tunable multiple band THz metamaterial perfect absorber (MPA) is proposed and investigated numerically, which is believed to be applicable for both temperature and refractive index sensing in THz region. Distinct from previous designs, the proposed MPA is only consisted of a structured indium antimonide (InSb) metamaterial layer, a dielectric layer and a metallic layer. Numerical results demonstrate that three resonant peaks with 100% absorptance at 1.17 THz, 1.60 THz and 2.17 THz can be achieved simultaneously. The impedance matching theory, the electric field, surface current and magnetic field distribution are used to explain the physical mechanism of the MPA. It is worth to mention that the number of perfect absorption peaks can be flexibly tuned from two to five band though breaking the symmetry of patterned InSb array. Furthermore, the absorption property of the designed MPA is found to be highly sensitive to the variations of the surrounding mediums and fluctuations of external environment temperature. Thus, the proposed MPA can not only operate as a temperature sensor with a sensitivity of 22.0 GHz/K, but also a refractive index sensor with a sensitivity of 287 GHz/RIU (refractive index unit). Due to its high sensing performance, our proposed tunable multiple band THz MPA can be widely applied into the material detection, thermal biosensing and other related optoelectronic fields.

A switchable ultra-wideband THz absorber based on vanadium dioxide was proposed, which consists of a lowermost gold layer, a PMI dielectric layer, and an insulating and surface vanadium dioxide layer. Based on the phase transition properties of vanadium dioxide, switching performance between ultra-broadband and narrowband can achieve a near-perfect absorption. The constructed model was simulated and analyzed using finite element analysis. Simulations show that the absorption frequency of vanadium dioxide above 90% is between 3.8 THz and 15.6 THz when the vanadium dioxide is in the metallic state. The broadband absorber has an absorption bandwidth of 11.8 THz, is insensitive to TE and TM polarization, and has universal incidence angle insensitivity. When vanadium dioxide is in the insulating state, the narrowband absorber has a Q value as high as 1111 at a frequency of 13.89 THz when the absorption is more excellent than 99%. The absorber proposed in this paper has favorable symmetry properties, excellent TE and TM wave insensitivity, overall incidence angle stability, and the advantages of its small size, ultra-widebands and narrowbands, and elevated Q values. The designed absorber has promising applications in multifunctional devices, electromagnetic cloaking, and optoelectronic switches.

We present a metamaterial-based perfect absorber (PA) that strongly supports four resonances covering a wide spectral range from 1.8 µm to 10 µm of the electromagnetic spectrum. The designed perfect absorber has metal–dielectric–metal layers where a MgF2 spacer is sandwiched between an optically thick gold film and patterned gold nanoantennas. The spectral tuning of PA is achieved by calibrating the geometrical parameters numerically and experimentally. The manufactured quad-band plasmonic PA absorbs light close to the unity. Moreover, the biosensing capacity of the PA is tested using a 14 kDa S100A9 antibody, which is a clinically relevant biomarker for brain metastatic cancer cells. We utilize a UV-based photochemical immobilization technique for patterning of the antibody monolayer on a gold surface. Our results reveal that the presented PA is eligible for ultrasensitive detection of such small biomarkers in a point-of-care device to potentially personalize radiotherapy for patients with brain metastases. [ABSTRACT FROM AUTHOR]

Six band metamaterial perfect absorber at the terahertz region is made and designed of two rectangular bars and a metallic board separated by a dielectric spacer. The simulated results show that the absorber has six distinctive absorption modes at frequencies 1.80 THz, 3.35 THz, 4.5 THz, 5.25 THz, 5.95 THz, and 6.1 THz with absorption rates of 97%, 96.2%, 97.07%, 91.1%, 100%, and 83%, respectively. The absorption mechanism of the six-band absorber was investigated by near-field distribution. The frequencies of these absorption bands can be adjusted by controlling the structural parameters of the proposed design. Furthermore, the influence of the geometric parameters on the performance of six-band light absorption was also analyzed to further verify the underlying mechanism of these six bands. In addition, two conventional parameters, the sensitivity (S) and figure of merit (FOM) were used to investigate the sensing performance of the proposed design. FOM of the fifth absorption peak can reach up to 26.9 which is much higher than the previously reported values for the terahertz region. This type of six-band absorber finds many potential applications in sensing and detection, filters, solar energy, and stealth technology. [ABSTRACT FROM AUTHOR]

In this paper, we have presented a new design of a metamaterial perfect absorber (MPA) consisting of three layers of metal-dielectric-metal in which the top layer is considered of special kind square patches at different places in a unit cell. This MPA exhibits wideband, wide-angle, and polarization-independent absorption performance in the visible region. The proposed MPA structure is composed of a resonator of metal and spacer of a dielectric layer. The interactions between the resonator and the coupling of metal-dielectric create the plasmonics effects which are responsible for the perfect absorption. Under a specific condition, this simulated absorber structure exhibits an extremely high broadband absorption between 591.54 and 704.40 nm wavelength range with near-unity absorption, and a single peak observed at 385.33 nm with the absorption of 94.16%. We extracted the impedance of the absorber and matched it with free space, and also demonstrated the effective permittivity and permeability. Moreover, the parametric study of the resonators, dielectric layer, and multi-band topology has also been investigated. The polarization-insensitive-based metamaterial may be utilized to improve the efficiency of different devices in the visible range. Furthermore, we have calculated the absorption of the proposed MPA under solar radiation (AM1.5) for different structural parameters. The proposed absorber greatly enhances the conversion efficiency which is highly useful for solar cells. We also determined the short circuit current density of this absorber for different thicknesses of the GaAs layer. The meta-surface of Al metal provides a good performance in comparison to other costly metals and the proposed structure may be used for different devices. [ABSTRACT FROM AUTHOR]

Terahertz metamaterial absorbers (TMAs) can efficiently absorb electromagnetic wave in the range of 0.1–10 THz based on the tunable electromagnetic properties of artificially designed unit cells. TMAs can achieve perfect absorption, as well as broad band absorption. Specifically, TMAs can break the thickness limit of a device at a quarter wavelength, realizing ultra‐thin devices so as to facilitate integration with other systems. Accordingly, TMAs have high application value in communication, imaging, detection, security inspection, and other fields because of their characteristics of small size, perfect absorption, less restriction of natural materials, and tunable electromagnetic parameters. This review covers the fundamental principles and recent advances of TMAs, from the essential structural features of TMAs and working principle of various types of TMAs to the design and applications of corresponding structures, especially the TMAs that can be tuned by various approaches. The development trend and challenges of TMAs are briefly discussed too. It is envisioned that this review can help R&D of the future TMAs for real applications. [ABSTRACT FROM AUTHOR]

Broadband absorbers are essential components of many light detection, energy harvesting, and camouflage schemes. Current designs are either bulky or use planar films that cause problems in cracking and delamination during flexing or heating. In addition, transferring planar materials to flexible, thin, or low-cost substrates poses a significant challenge. On the other hand, particle-based materials are highly flexible and can be transferred and assembled onto a more desirable substrate but have not shown high performance as an absorber in a standalone system. Here, we introduce a class of particle absorbers called transferable hyperbolic metamaterial particles (THMMP) that display selective, omnidirectional, tunable, broadband absorption when closely packed. This is demonstrated with vertically aligned hyperbolic nanotube (HNT) arrays composed of alternating layers of aluminum-doped zinc oxide and zinc oxide. The broadband absorption measures >87% from 1,200 nm to over 2,200 nm with a maximum absorption of 98.1% at 1,550 nm and remains large for high angles. Furthermore, we show the advantages of particle-based absorbers by transferring the HNTs to a polymer substrate that shows excellent mechanical flexibility and visible transparency while maintaining near-perfect absorption in the telecommunications region. In addition, other material systems and geometries are proposed for a wider range of applications.

This paper is investigated a silicon-dielectric-metal nanostructure, it supports the optional anapole mode due to the destructive interference between electric and toroidal dipoles. Such the proposed nanostructure exhibits a near-perfect ultra-narrow absorption spectrum in the near infrared region with the absorption bandwidth (FWHM) of 0.48 nm and a high-quality (Q) factor up to 1763. Besides inherently low dissipative losses and strong anapole response, this optical nanostructure can demonstrate subtle refractive index sensing properties. Our work shed new light on active ultra-narrow band perfect absorption based on the optical anapole mode, and provides potential applications such as optical sensors, narrow-band filter, non-radiative data transmission and biomolecules manipulation.

Abstract This study proposes using double-layer wire-grid structures to create narrow-band, perfect plasmonic absorbers, which depend on polarization, for the short-wavelength visible and near-ultraviolet regions of the electromagnetic spectrum. A rigorous coupled-wave analysis reveals that the maximum absorption attained using Ag and Al is ~ 90% at 450 and 375 nm. Experiments using Ag yielded results similar to those predicted by simulations. These results demonstrate that narrow-band perfect plasmonic absorbers, which depend on the polarization, can be realized at 450 and 375 nm using Ag or Al.

• 40% smaller in size than similar designs, enabling PIC miniaturization. • Exceptional efficiency: >90% transmission (filter) & >99% absorption. • Unparalleled versatility: 5 functions in 1 device: filter, PIT, absorber. • Simulation-driven design of novel plasmonic device with GA-optimized stubs & perturbations for precise spectral tuning. This paper presents the design and numerical investigation of a novel, tunable, and highly efficient metal–insulator-metal (MIM) plasmonic device specifically designed for near-infrared (NIR) applications. The device leverages strategically placed stub resonators within a MIM waveguide. We introduce two small perturbations, a triangle and a rectangle, to achieve remarkable functional versatility. Comprehensive numerical analysis, employing the finite element method (FEM) and validated by the transmission line method (TLM), demonstrates the working principle and excellent agreement between the approaches. Our simulation-driven approach, utilizing a Genetic Algorithm (GA) for accelerated optimization, was crucial in achieving performance levels difficult or costly to reach through purely experimental methods. The GA enabled efficient exploration of a vast parameter space, iterative refinement of device configurations, and fine-tuning of geometric characteristics. This meticulous optimization allows us to control complex interactions within the simulated structure. The proposed device offers diverse functionalities based on adjusted geometrical parameters, including: A. Flat-top band-pass filtering: Achieving a maximum transmission efficiency of 95.8 % within a compact footprint of 420 nm × 540 nm. B. Dual-band band-pass filtering: Maintaining a high transmission efficiency of 88.4 % within a slightly larger size of 450 nm × 540 nm. C. Triple-band notch filtering: Exhibiting minimum transmission (below 1 %) at specific resonance wavelengths for targeted signal suppression. D. Plasmon-induced transparency (PIT) effect: Offering potential applications in various optical functionalities. And, E. Perfect absorption: Achieving a maximum absorption efficiency of 99.62 %, paving the way for efficient light harvesting and manipulation. This multifunctional plasmonic device excels in its combination of compactness, tunability, and diverse NIR functionalities. It holds promise for miniaturized optical components, integrated photonic circuits, and advanced light-matter interactions. Our findings contribute significantly to the advancement of compact, efficient, and readily manufacturable photonic technologies. [ABSTRACT FROM AUTHOR]

A frequency magnetically tunable perfect absorber based on graphene in the terahertz (THz) region is proposed. The performance is analysed using the 4 × 4 transfer matrix method, demonstrating that the perfect absorption frequency of the proposed absorber for a left-handed circularly polarized (LCP) wave can be dynamically tuned by varying the external static bias magnetic field in three frequency ranges (0.95–2.2 THz, 4.15–5.4 THz, and 7.3–8.55 THz). Due to the destructive interference of the reflected waves and the graphene-induced photonic band gap, the maximum absorption of the LCP wave can reach 99.91%. In addition, the proposed absorber can tolerate a wide range of incident angles for the LCP wave. This study may have great potential for various applications, such as detectors, sensors, and other optoelectronic devices in the THz region.

In this paper a perfect absorber with a photonic crystal cavity (PhC-cavity) is numerically investigated for carbon dioxide (CO2) gas sensing application. Metallic structures in the form of silver are introduced for harnessing plasmonic effects to achieve perfect absorption. The sensor comprises a PhC-cavity, silver (Ag) stripes, and a host functional material—Polyhexamethylene biguanide polymer—deposited on the surface of the sensor. The PhC-cavity is implemented within the middle of the cell, helping to penetrate the EM waves into the sublayers of the structure. Therefore, corresponding to the concentration of the CO2 gas, as it increases, the refractive index of the host material decreases, causing a blue shift in the resonant wavelength and vice versa of the device. The sensor is used for the detection of 0–524 parts per million (ppm) concentration of the CO2 gas, with a maximum sensitivity of 17.32 pm (pico meter)/ppm achieved for a concentration of 366 ppm with a figure of merit (FOM) of 2.9 RIU−1. The four-layer device presents a straightforward and compact design that can be adopted in various sensing applications by using suitable host functional materials.

In this work, the Ag-SiO2-Ag metamaterial with elliptical nano-slits was proposed to investigate the multi-band polarization-dependent perfect absorber. It was found that multi-band perfect absorptions can be induced under TE and TM-polarized illuminations. Simulation results showed that the absorption peaks for TE-polarized wave appeared at 337.6 THz and 360.0 THz with 98.5% and 97.6% absorbance, respectively. Conversely, the absorption peaks for TM-polarized wave appeared at 325.7 THz and 366.1 THz with 96.3% and 97.9% absorbance, respectively. As a result, the elliptical metamaterial presented polarization-selectivity property for perfect absorption, and so, the metamaterial can filter out different frequencies of TE- and TM-reflected waves, i.e., the elliptical metamaterial can be used as a reflecting filter. In addition, this work studied the sensing performance of the elliptical metamaterial and showed that the dual-band sensing performances were different at low and high frequencies. The sensitivities (S) to the refractive index reached up to 151.1 THz/RIU and 120.8 THz/RIU for the TE and TM-polarized waves around 337.6 THz and 325.7 THz, which provide promising potential in near-infrared photoelectric sensor and detector. However, both the absorption frequency and intensity of TM-polarized wave were insensitive to the refractive index of the medium around 366.1 THz, and so, the study provides a theoretical basis for infrared stealth of different media.

Achieving absorption–transmission switchable devices is still a great challenge due to the complex competitions among absorption, reflection, and transmission. Here, we propose a nanolayered terahertz (THz) metasurface exhibiting switchable functionality from quad-band near-perfect absorption to strongly enhanced transmission. The switchable performances are attributed to the electrical tunability of graphene and insulator–metal transition in vanadium dioxide (VO2). When VO2 functions as a metal, the proposed metasurface performs as a quad-band perfect absorber. The four absorption peaks can be effectively adjusted by altering the related graphene Fermi energy levels. When VO2 is in an insulating state, the proposed device functions as a THz antireflection coating, which diminishes reflection and substantially enhances transmission. In addition, a theoretical interference model is employed to clarify the physical mechanisms behind the near-perfect absorber and antireflection coating. The obtained theoretical results significantly agree with the numerical simulations. Furthermore, owing to the rotationally symmetric patterns, the performances of both the quad-band perfect absorber and antireflection coating are rather insensitive to light polarization and angle of incidence. Hence, our results can facilitate the progress of nanoscale switchable metadevices in the THz regime. [ABSTRACT FROM AUTHOR]

Optically anisotropic materials show important advantages in constructing polarization-dependent optical devices. Very recently, a new type of two-dimensional van der Waals (vdW) material, known as α-phase molybdenum trioxide (α-MoO3), has sparked considerable interest owing to its highly anisotropic characteristics. In this work, we theoretically present an anisotropic metamaterial absorber composed of α-MoO3 rings and dielectric layer stacking on a metallic mirror. The designed absorber can exhibit ultra-narrowband perfect absorption for polarizations along [100] and [001] crystalline directions in the visible light region. Plus, the influences of some geometric parameters on the optical absorption spectra are discussed. Meanwhile, the proposed ultra-narrowband anisotropic perfect absorber has an excellent angular tolerance for the case of oblique incidence. Interestingly, the single-band perfect absorption in our proposed metamaterials can be arbitrarily extended to multi-band perfect absorption by adjusting the thickness of dielectric layer. The physical mechanism can be explained by the interference theory in Fabry–Pérot cavity, which is consistent with the numerical simulation. Our research results have some potential applications in designs of anisotropic optical devices with tunable spectrum and selective polarization in the visible light region. [ABSTRACT FROM AUTHOR]

In this paper, we propose a highly selective and efficient gas detection system based on a narrow-band IR metasurface emitter integrated with a resistive heater. In order to develop the sensor for the detection of specific gases, both the microheater and metasurface structures have been optimized in terms of geometry and materials. Devices with different metamaterial structures and geometries for the heater have been tested. Our prototype showed that the modification of the spectral response of metasurface-based structures is easily achieved by adapting the geometrical parameters of the plasmonic micro-/nanostructures in the metasurface. The advantage of this system is the on-chip integration of a thermal source with broad IR radiation with the metasurface structure, obtaining a compact selective radiation source. From the experimental data, narrow emission peaks (FWHM as low as 0.15 μm), corresponding to the CO2, CH4, and CO absorption bands, with a radiant power of a few mW were obtained. It has been shown that, by changing the bias voltage, a shift of a few tens of nm around the central emission wavelength can be obtained, allowing fine optimization for gas detection applications. [ABSTRACT FROM AUTHOR]

Graphene as a mono-atomic sheet has recently grabbed attention as a material with enormous properties. It has also been examined for enhancing absorbance in the current plasmonic structure. This has led to an increment in the sensitivity of the plasmonic sensors. In this paper, we present theoretical investigation of the novel graphene-based plasmonic metamaterial perfect absorber for biosensing applications. The simulation study performs the analysis of the novel plasmonic metamaterial absorber structure by adding coatings of graphene sheets. Each sheet of graphene enhances absorbance of the structure. In this study, we demonstrate three layers of graphene sheets lead to perfect absorbance (100%) for multiple bands in the visible and near-infrared regions. Furthermore, we also computed the sensitivity of the graphene-based proposed structure by varying the refractive index (RI) of the sensing region from 1.33–1.36 with RI change of 0.01. Proposed fabrication steps for realization of the device are also discussed. [ABSTRACT FROM AUTHOR]

Triple-band terahertz metamaterial absorber with design of miniaturization and compactness is presented in this work. The unit cell of the terahertz absorber is formed by an analogy I-typed resonator (a rectangular patch with two small notches) deposited on top of dielectric sheet and metallic mirror. The miniaturized structure design exhibits three discrete frequency points with near-perfect absorption at terahertz regime. The three absorption peaks could be ascribed to localized resonances of analogy I-typed resonator, while the response positions of these absorption peaks at the analogy I-typed resonator are different by analyzing the near-field patterns of these resonance peaks. Changes in structure parameters of the analogy I-typed resonator are also investigated. Simulation results revealed that the notch sizes of the rectangular patch are the key factor to form the triple-band near-perfect absorption. Further structure optimization is given to demonstrate triple-band polarization insensitive performance. Moreover, actively tunable absorption properties are realized by inserting or introducing vanadium dioxide with adjustable conductivity into the metamaterial structure. It is revealed that the insulator–metal phase transition of vanadium dioxide is the main reason for the modulation of absorption performance. Compared with previous multiple-band absorbers, the device given here has excellent features of high degrees of simplification, miniaturization, and active modulation, these are important in practical applications. [ABSTRACT FROM AUTHOR]

In this paper, we proposed a tunable graphene-based metamaterial absorber (MA) at dual terahertz band to achieve perfect absorption. The electric field intensity distributions of demonstrated structure is illustrated to explain the origin of two absorption peaks. Simulation result shows the proposed MA owning polarization-insensitive property and wide incident angle tolerance. An inversion algorithm is used to extract the equivalent impedance and explore the physical mechanism of multiple-peak absorption. The equivalent impedance is explored through algorithm when the Fermi energy level alters, consequently the variation of resonant frequency of corresponding absorption peak is determined. Simultaneously, the resonant frequencies of absorption peaks can be adjusted by changing the geometric parameters of the graphene. Furthermore, we adapt the proposed structure into reversely designed pattern, achieving similar characteristic of dual-band absorption peaks and tunability. Thus, our work provides new insight and useful design guidance to various tunable multiple band terahertz metamaterial perfect-absorbers.

We report a theoretical study of a perfect absorber based on the metal-insulator-metal (MIM) structure, which achieves perfect absorption of single and double peaks in the visible range. The top Ag and middle SiO2 are arranged in a periodic nanopillar array. Adjusting the structural parameters of the nanopillars to indirectly control the absorption spectrum, theoretically the maximum single-peak absorption can reach more than 99.0% and the maximum double-peak absorption can reach more than 90.0%. We investigate its absorption mechanism through simulations and calculations, and explain the results well with the SPP scattering mode and F-P cavity theory. We find that through the double-peak absorption process, the mode split phenomenon appears as the thickness of SiO2 increases. Therefore, the selective appearance of absorption peaks can be achieved, which provides the possibility of application in absorbers.

Graphene is a powerful 2-D matter with the capability of extraordinary transparency, and tunable conductivity is employed in emerging optoelectronics devices. In this article, the design of an electrically tunable graphene-based perfect terahertz absorber is proposed and evaluated numerically. The introduced structure is composed of two graphene layers with a sharp absorption peak in the terahertz band. These graphene layers are combline and stripline separated by the insulator substrate. The position of the absorption peak is tunable on the absorption band by means of manipulation in geometric parameters of the combline graphene layer. Furthermore, the intensity and frequency of the absorption peak can be flexibly modulated by varying Fermi potential of the combline graphene layer, which can be controlled through external DC voltages without the need of changing the geometry of the structure. It is shown that the absorption band can be tuned in the bandwidth from 5 to 15 in terahertz. The findings of this paper can promote a new perspective in designing perfect ribbon absorbers based on graphene properties that can be utilized for future photodetectors, solar cells, and thermal sensors with an absorption intensity above 2 × 105(nm2) with narrow absorption bandwidth of 0.112 THz. [ABSTRACT FROM AUTHOR]

Adaptive metasurfaces (MSs) provide immense control over the phase, amplitude and propagation direction of electromagnetic waves. Adopting phase-change materials (PCMs) as an adaptive medium allows us to tune functionality of MSs at the meta-atom length scale providing full control over MS (re-)programmability. Recent experimental progress in the local switching of PCM-based MSs promises to revolutionize adaptive photonics. Novel possibilities open new challenges, one of which is a necessity to understand and be able to predict the phase transition behavior at the sub-micrometer scale. A meta-atom can be switched by a local deposition of heat using optical or electrical pulses. The deposited energy is strongly inhomogeneous and the resulting phase transition is spatially non-uniform. The drastic change of the material properties during the phase transition leads to time-dependent changes in the absorption rate and heat conduction near the meta-atom. These necessitate a self-consistent treatment of electromagnetic, thermal and phase transition processes. Here, a self-consistent multiphysics description of an optically induced phase transition in MSs is reported. The developed model is used to analyze local tuning of a perfect absorber. A detailed understanding of the phase transition at the meta-atom length scale will enable a purposeful design of programmable adaptive MSs.

In this work, we theoretically propose an anisotropic metasurface absorber, unit cell of which consists of a continuous monolayer black phosphorus (BP) sheet, sandwiched between a circular silver ring and a dielectric layer stacking on a silver substrate. Numerical results reveal that perfect enhancement absorption can be achieved in both armchair and zigzag directions, in which the resonant absorption peaks occur at different wavelengths due to the anisotropic property of BP. The calculated results by finite-difference time domain (FDTD) simulations have a good agreement with the coupled-mode theory. The physical mechanism of the perfect absorption is attributable to the critical coupling effect. Furthermore, the anisotropic perfect absorber not only exhibits tunable characteristics by varying the electron doping concentration, but also shows a wide-angle tolerance. In addition, our proposed absorber can achieve an arbitrary number of absorption bands by setting a suitable thickness of the dielectric layer. These results may have great potential applications in spectral imaging and photodetectors in the mid-infrared region.

Resonant dielectric metasurfaces have been demonstrated to hold a great promise for manipulation of light-wave dispersion at the nanoscale due to their resonant photonic environment and high refractive index. However, the efficiency of devices based on dielectric nanostructures is usually limited by the quality ( Q ) factor of their resonant modes. The physics of the bound sates in the continuum (BICs) provide an elegant solution for control over the Q factor of resonant modes. Here, by engineering the substrate of Si-based metasurfaces, we demonstrate two eigenmodes that exhibit an intrinsic magnetic dipole (MD)character and have an infinite radiation lifetime. We reveal that they are characterized by in-plane and out-of-plane MD modes and respectively correspond to two groups of BICs, that is, Fabry–Pérot BICs and symmetry-protected BICs. Using temporal coupled-mode theory and numerical simulations, we show that these BIC modes can transform into high- Q quasi-BIC resonances with near-unity absorption under normal incidence through tuning structural parameters. Our work provides a promising route to use BIC-inspired metasurfaces for designing ultra-narrowband absorbers which can be used as absorption filters, photodetectors, and sensors.

A kind of graphene-based perfect absorber which can generate low-threshold and high-extinction-ratio optical bistability in the near-IR band is proposed and simulated with numerical methods. The interaction between input light and monolayer graphene in the absorber can be greatly enhanced due to the perfect absorption. The large nonlinear coefficient of graphene and the strong light-graphene interaction contribute to the nonlinear response of the structure, leading to relatively low switching thresholds of less than 2.5 MW/cm2 for an absorber with a Q factor lower than 1000. Meanwhile, the extinction ratio of bistable states in the absorber reaches an ultrahigh value of 47.3 dB at 1545.3 nm. Moreover, the influence of changing the structural parameters on the bistable behaviors is discussed in detail, showing that the structure can tolerate structural parametric deviation to some extent. The proposed bistable structure with ultra-compact size, low thresholds, high extinction ratio, and ultrafast response time could be of great applications for fabricating high-performance all-optical-communication devices.

Infrared detection technology has important applications in laser ranging, imaging, night vision, and other fields. Furthermore, recent studies have proven that hot carriers which are generated by surface plasmon decay can be exploited for photodetection to get beyond semiconductors’ bandgap restriction. In this study, silicon nanopillars (NPs) and gold film at the top and bottom of silicon nanopillars were designed to generate surface plasmon resonance and Fabry–Perot resonance to achieve perfect absorption. The absorption was calculated using the Finite Difference Time Domain (FDTD) method, and factors’ effects on resonance wavelength and absorption were examined. Here we demonstrate how this perfect absorber can be used to achieve near-unity optical absorption using ultrathin plasmonic nanostructures with thicknesses of 15 nm, smaller than the hot electron diffusion length. Further study revealed that the resonance wavelength can be redshifted to the mid-infrared band (e.g., 3.75 μm) by increasing the value of the structure parameters. These results demonstrate a success in the study of polarization insensitivity, detection band adjustable, and efficient perfect absorption infrared photodetectors.

In this study, a perfect metamaterial absorber (PMMA) based on an indium antimonide temperature-sensitive material is designed and investigated in the terahertz region. We demonstrate that it is an ideal perfect narrow-band absorber with polarization-insensitive and wide-angle absorption properties. Numerical simulation results show that the proposed PMMA can be operated as a temperature sensor with a sensitivity of 21.9 GHz/K. A graphene layer was added to the PMMA structure to improve the sensitivity, and the temperature sensitivity was increased to 24.4 GHz/K. Owing to its excellent performance, the proposed PMMA can be applied in thermal sensing, detection, and switching.

Solar energy is a clean and renewable energy source and solves today’s energy and climate emergency. Near-perfect broadband solar absorbers can offer necessary technical assistance to follow this route and develop an effective solar energy-harvesting system. In this work, the metamaterial perfect absorber operating in the ultraviolet to the near-infrared spectral range was designed, consisting of a periodically aligned titanium (Ti) nanoarray coupled to an optical cavity. Through numerical simulations, the average absorption efficiency of the optimal parameter absorber can reach up to 99.84% in the 200–3000 nm broadband range. We show that the Ti pyramid’s localized surface plasmon resonances, the intrinsic loss of the Ti material, and the coupling of resonance modes between two neighboring pyramids are highly responsible for this broadband perfect absorption effect. Additionally, we demonstrate that the absorber exhibits some excellent features desirable for the practical absorption and harvesting of solar energy, such as precision tolerance, polarization independence, and large angular acceptance.