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This study introduces an ultrathin metamaterial-based sensor designed for the accurate detection of microorganisms and pesticides displaying enhanced sensing capabilities. The innovative design incorporates of polyimide thin film act as dielectric layer while a single square gold patch featuring four uniformly sized notches act as sensing surface. The outcome of simulation reveals a single resonance peak with absorption rate equal to 100%. Furthermore, the design sensor sensing effectiveness has also been examined and analyzed by manipulating the refractive index and thickness of analyte layer. The sensor exhibits remarkable sensitivity with sensor sensitivity increases up to 0.175 THz/ RIU within 1 to 1.8 RI range. This increased sensitivity makes the sensor notably capable to detect microorganisms such as bacteria and pesticides revealing a significant frequency shift of 100 GHz (high sensitivity) for pesticides that has not been previously apparently in the literature. The novelty of work lies in remarkable sensing capabilities in the detection of microorganisms such as bacteria and various pesticides in terms of notable frequency shift making it significant for applications in agriculture and food products industry. The proposed research work can also be significant in other sensing applications such as chemical and bio- sensing industry applications. [ABSTRACT FROM AUTHOR]

A double-band perfect absorber is proposed based on M o S 2 metamaterial. The proposed absorber is composed of a monolayer molybdenum disulfide, a crystalline silicon layer, and a silver substrate. Utilizing the critical coupling principle of guided resonance, efficient optical absorption of molybdenum disulfide was achieved. Specifically, at the resonance wavelengths ( λ 1 = 610.431 nm , λ 2 = 654.484 nm ), two perfect absorption peaks of 99.42% and 99.31% were attained based on finite-difference time-domain (FDTD) simulations. Through detailed analysis of field distributions and spectral responses, the interactions between electromagnetic waves and this perfect absorber structure are examined. Based on the simulation results, the structure's geometric parameters wield control over the resonance wavelength range of M o S 2 , holding pivotal practical implications for bolstering its absorption potency and selectivity. Moreover, the innovative approach of employing critical coupling to amplify light-silica interactions holds promise for application across various atomic-scale thin materials. Additionally, the absorber's sensing capabilities are revealing many metrics. The highest quality factor, sensitivity, and figure of merit (FOM) of the sensors are recorded as 111.3, 178.4 nm/RIU, and 30.3 R I U - 1 , respectively. Hence, our findings potentially pave the way for enhancing light-matter interactions within monolayer transition-metal dichalcogenides, offering promise for applications in multiband absorbers and sensors. [ABSTRACT FROM AUTHOR]

In this paper, a polarization-insensitive magnetic tunable perfect metamaterial absorber is analyzed which shows its functionality as a refractive index (RI) sensor. The unit cell structure of reported absorber consists of a cross-shaped Indium Arsenide (InAs) array over aluminum ground plane and has dimensions less than λ c / 4 with thickness of 7.5 μ m and periodicity of 9 μ m making it an ultra-thin absorber. The proposed absorber provides an average absorption of 99.99% on the implementation of magnetic field (B) at 0.4 T. The resonance frequency and corresponding absorption rates can be controlled by varying the magnetic field from B = 0. 1 –0.4 T which provides high absorption as well as shift in the frequency spectrum by 0.3 THz/T. Additionally, parametric analysis has also been done to confirm the selection of the optimum value of the designed parameters of the structure. Furthermore, simulation results reveal that the proposed structure is very sensitive to the change of RI values in the surroundings thus, the proposed structure can be employed as a RI sensor high sensitivity of 1.083 THz/RIU over the range of 1–1.6 RI. In addition, the result shows that the sensor can detect different chemical compounds such as ethanol, gasoline, paraffin, sodium chloride, carbon disulfide with an average sensitivity of 0.925 THz/RIU. Thus, the proposed absorber with obtained sensitivity can be utilized as a biochemical sensor, making it one of the best contenders for chemical industry uses in diagnosis of different hazardous chemicals as well as finding its applicability in biomedical uses. [ABSTRACT FROM AUTHOR]

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 SiO2 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.

A tunable multiband terahertz metamaterial absorber, based on vanadium dioxide (VO2), is demonstrated. The absorber comprises a three-layer metal–insulator–metal (MIM) configuration with a split ring and slots of VO2 on the uppermost layer, a middle dielectric substrate based on silicon dioxide (SiO2), and a gold reflector on the back. The simulation results indicate that, when VO2 is in the metallic state, the proposed metamaterial exhibits nearly perfect absorption at six distinct frequencies. The design achieves an average absorption of 98.2%. The absorptivity of the metamaterial can be dynamically tuned from 4% to 100% by varying the temperature-controlled conductivity of VO2. The proposed metamaterial absorber exhibits the advantages of polarization insensitivity and maintains its absorption over 80% under different incident angle conditions. The underlying physical mechanism of absorption is explained through impedance matching theory, interference theory, and the distribution of electric fields. The ability to achieve multiband absorption with tunable characteristics makes the proposed absorber a promising candidate for applications in terahertz sensing, imaging, communication, and detection. The polarization insensitivity further enhances its practicality in various scenarios, allowing for versatile and reliable performance in terahertz systems. [ABSTRACT FROM AUTHOR]

The increasing demand for renewable energy has increased in the last decade due to climate change resulting from conventional energy and solar cells become popular. The perfect absorption in solar cells can be achieved with artificially engineered materials at low cost for solar absorbers with high efficiency. A perfect ultra-broadband solar absorber can be achieved by using metamaterial (MM) based on surface plasmon resonance phenomena. We design a near-ideal ultra-broadband plasmonic MM absorber for solar energy harvesting over an ultra-broadband wavelength range (0.4–4 μ m ). The structure is made up of periodic taper arrays consisting of a thin, multilayered titanium nitride–titanium dioxide MM. The proposed structure has an absorption greater than 96% between the visible to infrared (IR) regime. It is independent of both polarization and incident angle. The large operational bandwidth, high absorption percentage, and compact thin structure combined with the strong thermal stability of metal TiN make an advantageous choice for solar thermophotovoltaics. [ABSTRACT FROM AUTHOR]

In this paper, we theoretically and numerically demonstrate a thermally controlled broadband absorber based on the phase change material Ge2Sb2Te5 (GST). When GST operates in the amorphous state, the proposed metamaterial acts as a broadband nearly perfect absorber. The absorption can reach more than 90% in the wavelength range from 0.9 to 1.41 μm. As an application of the GST-based metamaterial absorber, the near-field imaging effect is achieved by using the intensity difference of optical absorption. Moreover, the thermally controlled switchable imaging can be performed by changing the phase transition characteristics of GST, and the imaging quality and contrast can be adjusted by changing the geometrical parameters. This designed metamaterial may have potential applications in near-infrared temperature control imaging, optical encryption, and information hiding. [ABSTRACT FROM AUTHOR]

Wideband perfect absorbers are widely demanded for various applications, including efficient photodetection, radiation cooling. However, achieving perfect absorption across an extensive range of wavelengths on engraved structured substrate remains a challenge due to the complex light responses. It presents a hierarchical conical metasurface that demonstrates distinct perfect absorption from visible to far‐infrared range (0.4–16 µm), which is composed of surface‐engraved high aspect ratio nanogratings on microcones. The perfect absorption in the typical reststrahlen band of 4H‐SiC primarily relies on a light‐trapping‐enhanced surface phonon polaritons (SPhPs) mechanism, where the microcones effectively confine and enhance the SPhPs excited by the nanogratings through light trapping. Outside the reststrahlen band, the dominant mechanism for light antireflection is the light trapping of microcones assisted by nanogratings, which act as an equivalent antireflection layer. The hierarchical cones exhibit exceptional broadband performance, achieving an average absorptance of 98% over the spectrum of 0.4–16 µm with wide‐angle feasibility, and are fabricated using one‐step ultrafast‐laser ablation with ring‐shaped vector beams. Notably, the hierarchical cones can be applied to various materials, showing significant potential for use in photodetectors. [ABSTRACT FROM AUTHOR]

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.

This paper introduces a metamaterial energy harvester that is compact, highly efficient, and capable of operating at wide angles. The proposed design has an outer ring resonator housing inverted T-shaped resonators, and it can operate at two distinct frequencies, 3.2 GHz and 5.4 GHz. The structure's impedance is carefully designed to align with that of free space, ensuring efficient capture of incident electromagnetic power with minimal reflection. This enables the resistor load to receive power in the most efficient manner. Based on the simulation findings, the proposed harvester exhibits a notably higher conversion efficiency of around 97 %. To ensure the accuracy and reliability of the simulation outcomes, we fabricated a 3x3 cell array of the proposed design and conducted experimental tests within an anechoic chamber. The simulation and experimental results exhibit a strong correlation. Existing metamaterial-based energy harvesting designs frequently confront size, absorption band, and polarization sensitivity limitations. Our compact design is distinguished by its ability to accomplish near-unity absorption and greater power conversion efficiencies at the desired frequency bands. This makes it an ideal option for energy harvesting systems for wireless sensor networks prioritizing efficiency and size.

In this paper, a broadband, polarization-independent metamaterial absorber for millimeter waves is proposed. Operating across the frequency range of 22.7 GHz to 37.2 GHz, our broadband, polarization-independent absorber demonstrates exceptional efficacy by capturing over 90% of incident wave power. Distinguished by an array of conductive square patches on a dielectric substrate, the proposed structure boasts a remarkably low thickness, measuring less than a quarter of the free space wavelength of the central point within its operational spectrum (excluding the ground layer thickness). Leveraging a combination of circuit modeling and transmission line theory, this paper presents the design of this absorber and also elucidates its underlying operating principles. The combination of broadband operation, low thickness, and systematic design is rarely seen in previous works. To validate the proposed approach, a physical prototype is realized using 3D printing technology, employing conductive polylactic acid (C-PLA) patches on a light polyvinyl chloride (L-PVC) substrate. The proposed absorber has various potential applications in communication, electromagnetic shielding, and satellite technologies. Finally, a comparison with recent works is performed to demonstrate the feasibility of the proposed structure for millimeter-wave applications. [ABSTRACT FROM AUTHOR]

In this article, we propose and numerically investigate a graphene-based electromechanically tunable perfect absorber design. The fundamental motivation is to demonstrate the electromechanically tunable feature in graphene metasurface at the mid-infrared regime, which otherwise exploited only the chemical potential feature in the literature. The structure consists of a gold grating on a gold substrate separated by an oxide spacer with an overlay of monolayer graphene with free-standing segments. By applying external DC voltages, the free-standing region of the graphene layer deflects, owing to the electrostatic force it experiences. This shifts the resonance absorption wavelength. In the mid-infrared region of 5–10 µm, a very low actuation voltage of 1.6 V displaces the graphene membrane by 1 nm, resulting in a wide shift of 60 nm in resonance wavelength. A continuous tunability with near-perfect absorption over a wavelength range of 200 nm for an applied voltage of only 7 V is demonstrated. At a voltage greater than 7 V, a mode hopping phenomenon is observed, hampering the perfect absorber operations with a shift in wavelengths. It is shown that the perfect absorption is again retained at some higher voltage. Such implementations hold promising applications in nanophotonics sensors, detectors, real-time beam steering, etc. [ABSTRACT FROM AUTHOR]

We consider the propagation of acoustic waves in a waveguide containing a penetrable dissipative inclusion. We prove that as soon as the dissipation, characterized by some coefficient η, is non zero, the scattering solutions are uniquely defined. Additionally, we give an asymptotic expansion of the corresponding scattering matrix when η → 0+ (small dissipation) and when η → +∞ (large dissipation). Surprisingly, at the limit η → +∞, we show that no energy is absorbed by the inclusion. This is due to the so-called skin-effect phenomenon and can be explained by the fact that the field no longer penetrates into the highly dissipative inclusion. These results guarantee that in monomode regime, the amplitude of the reflection coefficient has a global minimum with respect to η. The situation where this minimum is zero, that is when the device acts as a perfect absorber, is particularly interesting for certain applications. However it does not happen in general. In this work, we show how to perturb the geometry of the waveguide to create 2D perfect absorbers in monomode regime. Asymptotic expansions are justified by error estimates and theoretical results are supported by numerical illustrations. [ABSTRACT FROM AUTHOR]

This paper introduces a metamaterial energy harvester that is compact, highly efficient, and capable of operating at wide angles. The proposed design has an outer ring resonator housing inverted T-shaped resonators, and it can operate at two distinct frequencies, 3.2 GHz and 5.4 GHz. The structure's impedance is carefully designed to align with that of free space, ensuring efficient capture of incident electromagnetic power with minimal reflection. This enables the resistor load to receive power in the most efficient manner. Based on the simulation findings, the proposed harvester exhibits a notably higher conversion efficiency of around 97 %. To ensure the accuracy and reliability of the simulation outcomes, we fabricated a 3x3 cell array of the proposed design and conducted experimental tests within an anechoic chamber. The simulation and experimental results exhibit a strong correlation. Existing metamaterial-based energy harvesting designs frequently confront size, absorption band, and polarization sensitivity limitations. Our compact design is distinguished by its ability to accomplish near-unity absorption and greater power conversion efficiencies at the desired frequency bands. This makes it an ideal option for energy harvesting systems for wireless sensor networks prioritizing efficiency and size. [ABSTRACT FROM AUTHOR]

This paper introduces a four-band tunable perfect absorber built on Al-Cu-Fe quasicrystal. The operating frequency range is 4.75 THz to 9.25 THz, which is in the terahertz band. The absorber exhibits multiple narrow bands, polarization incoherence, high sensitivity, and tunability. The structure comprises a gold layer, a silica dielectric layer, and a top layer of Al-Cu-Fe quasicrystal from bottom to top. The Al-Cu-Fe quasicrystal structure at the top consists of a circular ring left after being cut by a 45° rotation of the cruciform and a cruciform design containing a central circular cavity. Simulation results reveal four absorption peaks attributed to cavity resonance and surface plasmon resonance, with absorption efficiencies of 0.998, 0.994, 0.989, and 0.998, respectively. Furthermore, changing the radius of the inner ring in the top layer structure allows for varying the number of absorption peaks. We also propose a new fabrication process for Al-Cu-Fe quasicrystals absorbers. The calculated results demonstrate that the designed absorber, characterized by a simple structure and high sensitivity of 2620 GHz/RIU, holds significant potential for applications in optical switches, optical modulators, biomedicine, and sensing.

The metal-dielectric-metal (MDM) perfect absorber (PA) is an important kind of resonant metasurface with promising applications in selective thermal emitting, solar energy harvesting, biosensing and so on. Establishing a direct link between resonant features and structural parameters is essential for guiding design processes and exploring novel applications. In this paper, we conduct a comprehensive investigation of the MDM PA, utilizing scattering singularity (pole/zero) engineering. We propose a straightforward design methodology to achieve a MDM PA operating at a specific wavelength, and demonstrate a design example with a maximum absorption of 99.93 % at 1200 nm and a full width of half maximum of about 155 nm, which is subsequently experimentally validated. The results indicate high absorption across a wide range of angles. This study sheds new light on fast design and analysis of MDM PAs.

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. [ABSTRACT FROM AUTHOR]

A plasmonic perfect absorber is proposed using a silver substrate in the visible near-infrared wavelength range. The unit cell of the periodic structure is designed and evaluated in this paper. The Ag layer is etched so that four trapezoidal trenches filled with Si3N4 couple plasmonic modes on the metal–insulator interfaces. Various optical modes are formed, from which, a surface and a bulk mode are thoroughly studied. The surface mode can be coupled to the graphene sheet surface through a thin dielectric layer. Moreover, the effect of the variation of the structure's geometrical parameters on these modes was discussed. The bulk mode changes by the volume of the trenches and the effective refractive index, whereas the surface mode shifts with the period of the unit cell, the trenches depth, and other parameters that change the coupling criteria. The variation of the surface mode of the proposed structure can be used in sensors and photodetectors. As an application instance, an optimized structure is proposed at the end of the paper for sensing applications, showing high sensitivity (560 to 600 nm/RIU) and a figure of merit (70 ~ 120 RIU−1) in a wide refractive index range. [ABSTRACT FROM AUTHOR]

A dual-band terahertz metamaterial absorber having narrow discrete spacing is presented in this work. The basic cell of the dual-band absorption comprises two parallel metallic strips and metallic backplane separated by an insulating dielectric layer. Two near-perfect absorption peaks with narrow discrete spacing of 0.24 THz are obtained, and their relative discrete spacing is 10.62%. Importantly, its discrete spacing or relative discrete spacing can be narrowed by choosing appropriate structure parameters. Results demonstrated that the discrete spacing of only 0.1 THz (or relative discrete spacing of 4.57%) is obtained by adjusting the length of one of the metallic strips. Moreover, narrow discrete spacing of a triple-band terahertz metamaterial absorber formed by three parallel metallic strips and an insulating dielectric layer placed on the metallic plate is further designed. The triple-band absorption device possesses two narrow discrete spacing, which are respectively 0.24 THz and 0.22 THz with the relative discrete spacing of 10.39% and 10.05%, respectively. Results further revealed that the triple-band absorber also has the possibility of narrowing the discrete spacing by employing different lengths of metallic strips. These proposed multiple-band absorption devices with narrow discrete spacing could have considerable application potential in many technology-related fields. [ABSTRACT FROM AUTHOR]

In this work, we designed and theoretically investigated an adjustable perfect absorber metamaterial according to vanadium dioxide (VO 2 ). The VO 2 is a phase change material. By controlling its electrical conductivity from 2 × 10 2 to 2 × 10 5 S/m, the absorption continuously changed from 3.92 % to 100 % . The wider bandwidth and flexibility in selecting perfect absorption in the frequency range of 6.5 until 7.90 Thz are enhanced compared with other published VO 2 -based terahertz absorbers. We applied four common physical approaches to show the validation of results. These results are confirmed using the interference cancellation, modulation depth, electric field, surface current analysis, and impedance matching theory. Also, we investigate the influence output absorption spectrum by changing structure parameters. Therefore, the present THz perfect absorber has a vast capacity for profitable usages, such as photochemical energy absorption, modulating, sensing, cloaking, and stealth devices. [ABSTRACT FROM AUTHOR]

Metamaterial absorbers over a broadband spectrum with high absorption, good angular tolerance, and easy configurations have essential importance for optical and optoelectronic devices. In this study, a hybrid metamaterial absorber comprising multilayered cermet thin films (multi-cermet) with tapered structure is designed and experimentally demonstrated. Combining optical interference of multi-cermet films and optical field localization of nano-cone structures, the average absorbance of both simulation and measurement are more than 98% in an ultrabroad bandwidth (300–3000 nm), and the proposed absorber shows a good angular tolerance as well. The composite process of two easy-operated and efficient methods, colloidal lithography, and magnetron sputtering, is employed for large-area fabrication. In addition, owing to flexible polyimide substrate, the proposed absorber also shows good bending and heating resistance, which reflects its potential in engineering application.

A tunable multiband terahertz metamaterial absorber, based on vanadium dioxide (VO2), is demonstrated. The absorber comprises a three-layer metal–insulator–metal (MIM) configuration with a split ring and slots of VO2 on the uppermost layer, a middle dielectric substrate based on silicon dioxide (SiO2), and a gold reflector on the back. The simulation results indicate that, when VO2 is in the metallic state, the proposed metamaterial exhibits nearly perfect absorption at six distinct frequencies. The design achieves an average absorption of 98.2%. The absorptivity of the metamaterial can be dynamically tuned from 4% to 100% by varying the temperature-controlled conductivity of VO2. The proposed metamaterial absorber exhibits the advantages of polarization insensitivity and maintains its absorption over 80% under different incident angle conditions. The underlying physical mechanism of absorption is explained through impedance matching theory, interference theory, and the distribution of electric fields. The ability to achieve multiband absorption with tunable characteristics makes the proposed absorber a promising candidate for applications in terahertz sensing, imaging, communication, and detection. The polarization insensitivity further enhances its practicality in various scenarios, allowing for versatile and reliable performance in terahertz systems.

In this paper, we theoretically and numerically demonstrate a thermally controlled broadband absorber based on the phase change material Ge2Sb2Te5 (GST). When GST operates in the amorphous state, the proposed metamaterial acts as a broadband nearly perfect absorber. The absorption can reach more than 90% in the wavelength range from 0.9 to 1.41 μm. As an application of the GST-based metamaterial absorber, the near-field imaging effect is achieved by using the intensity difference of optical absorption. Moreover, the thermally controlled switchable imaging can be performed by changing the phase transition characteristics of GST, and the imaging quality and contrast can be adjusted by changing the geometrical parameters. This designed metamaterial may have potential applications in near-infrared temperature control imaging, optical encryption, and information hiding.

We realize a tunable narrowband and broadband coexisting absorber based on a simple step-shaped all-metal grating structure. The absorber presents an ultra-narrow absorption band of 1.5 nm and a relatively broad absorption band of 29.8 nm, both with nearly 100% absorption in the infrared region. The mechanism underlying the dual-band perfect absorption is the interaction between two diffraction coupled surface plasmon polariton (SPP) modes with one of them modulated by a cavity resonance. Influences of structure parameters on the absorption performance are numerically investigated. It is found that the positions of the two perfect absorption peaks can be easily tuned both independently and together by changing the structural parameters. In addition, the designed grating structure presents excellent sensing performance with sensitivity and figure of merit as high as 2,514 nm/RIU and 1,600 RIU−1, respectively. The excellent sensing performance, flexible tenability and simple structure design endow this grating absorber with great potential in high-precision biochemical sensing applications.

Ultrathin and broadband metamaterial absorber with loaded four lumped resistors is proposed and analyzed. The reported design is based on increasing the absorptivity by reducing the reflection and transmission coefficients simultaneously. Therefore, continuous metallic ground is used to achieve zero transmission while the reflection is reduced by matching the impedance of the proposed metamaterial absorber with the impedance of free space (Z = Z0 or μr = εr). Additionally, electric and magnetic resonances are achieved simultaneously with perfect absorptivity. The finite element method is used to simulate and analyze the reported absorber. The suggested absorber shows higher absorption than 90% over large frequency range (14.35–29.18 GHz) for both transverse electric and transverse magnetic polarizations. Further, the proposed design has high absorption through incident angle variation from 0° to 50°. Therefore, the reported perfect metamaterial absorber has good potential applications in communications, stealth and imaging fields. [ABSTRACT FROM AUTHOR]

In this paper, a dual-band ultra-narrow metamaterial absorber for sensing applications is presented. The unit cell of the structure consists of an Ag disk resonator located on a thin stack of graphene/SiO2/Au layers. The absorber shows high reflectivity in the whole of the incident light, ranging from 1.05 to 1.45 μm. Simulation results obtained by the finite-difference time-domain (FDTD) method indicate that the resonance behavior of the absorber depends on different factors including geometric parameters, the materials of the structure's components, the chemical potential of the graphene sheet as well as the shape of the resonators. For the optimal structure, the sensitivity, figure-of-merit (FOM) and quality-factor characteristics are as high as 760 nm/RIU, 109 RIU−1, and 193, respectively. Also, the absorption can reach 99.96% in the on-resonance wavelengths. As an optofluidic sensor, this structure can detect industrial analytes such as Methanol, Ethanol, Pentanol, Butanol, and Propanol with high accuracy. The proposed sensor has the advantages of high absorption, high sensitivity, and a fabrication-friendly structure, promising a great platform for the detection of industrial analytes. [ABSTRACT FROM AUTHOR]

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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. [ABSTRACT FROM AUTHOR]

Metamaterial absorbers show great promise for applications in optical manipulation, photodetection, solar energy harvesting, and photocatalysis. In this work, we present a twisted stacked metamaterial design that serves as a plasmonic perfect absorber with polarization selectivity. Leveraging effective energy localization, the metamaterial realizes a near-unity absorbance of up to 99.6% for right circularly polarized incidence and 97.2% for left circularly polarized incidence. At a longer wavelength in the visible range, the chiral metamaterial becomes more sensitive to the polarization state of the incident wave, retaining an ultrahigh absorption of light (∼94%) for only a given polarization state, that is, light in this polarization state is effectively shielded. A giant circular dichroism signal of up to 7° can be simultaneously observed. Electromagnetic field and charge distribution simulations further reveal that the ultrahigh performance of the design is attributed to the interplay between cavity coupling, magnetic resonances, and plasmonic coupling. Besides switchable and tunable chirality, the plasmonic metamaterial presents a near-perfect absorption band with tunable operational wavelengths. We envision that the high-performance chiral gold metamaterial proposed here can serve as a good candidate for light trapping, chirality sensing, polarized light detection, and polarization-enhanced photocatalysis. [ABSTRACT FROM AUTHOR]

This work reported a novel terahertz sensor based on metamaterial absorber. The proposed structure is composed of rectangular metallic resonator made of gold placed above on dielectric medium of a polyimide and a metallic board at the bottom. Due to high-intensity field energy confinement in the sensing regime, two resonance peaks occur at 1.355 THz and 2.785 THz with absorption nearly 100% is achieved. The proposed structure represents sharp resonances with a highest Q factor of 69.6 in the dual-band absorption spectra. The proposed structure is highly sensitive to the change of refractive index (RI) of the surrounding medium at fixed analyte. Furthermore, the proposed sensor exhibits high sensitivity of 0.735THz/RIU, and high FOM of 18.4 RIU−1 in the refractive index ranging from 1 to 2 with fixed analyte thickness of 35 μm. Moreover, the sensitivity and absorption strength influence of the sensor on the sample thickness covered by the sensor surface is also analyzed. Due to high Q factor and sensitivity, the design RI sensor is successfully employed in sensing and detection. [ABSTRACT FROM AUTHOR]

We have designed a multilayer metamaterial structure which exploits the epsilon-near-zero (ENZ) mode for perfect absorption. The design procedure is based on the effective medium theory (EMT) model. The proposed multilayer structure consists of layers of Ag, InSb, SiO2. The thickness of the layers is adjusted so that the ENZ mode is at the communication wavelength of 1550 nm. Two alternations of the layered structure are placed on a metal (Ag) substrate to prevent light transmission. Placing the nano-ring cavity arrays at the top of the multilayer structure leads to excitation of the surface plasmon polaritons (SPPs) and the cavity mode. Consequently, a resonance peak with nearly perfect absorption of 99.92% for normal incidence is obtained. Since the dimensions of the nano-ring affect the resonance wavelength, it is tuned to be exactly at the wavelength of the ENZ mode, resulting in perfect absorption. Simulation results based on the finite difference frequency domain method indicate that the absorption is insensitive to polarization and the absorption efficiency remains above 90% up to a 60° incident angle. The proposed absorber can be used for various optical communication applications such as filters, detectors and sensors. Finally, the structure's sensitivity to the environmental refractive index variations has been used here for refractive index sensing. A sensitivity of 200 nm/Refractive Index Unit (RIU) is obtained in this case. [ABSTRACT FROM AUTHOR]

A triple-band metamaterial perfect absorber (MPA) is presented in monolayer graphene in the THz region, which is adjustable and polarization-independent. The first layer of the structure from the top is patterned graphene which contains a graphene ring at the center of the frame with four graphene wheel-shaped around it and four graphene triangles in the corner. This proposed structure has caused the absorber to achieve 98.64%, 99.97%, and 99.98% perfect absorption peaks at 8.17 THz, 9.74 THz, and 11.95 THz, respectively. We can vary the absorption peak frequencies to our desirable frequencies by changing graphene's Fermi level. This absorber has good tolerance for up to 60-degree change in the angle of incident waves. Moreover, a significant matter about the suggested absorber structure design is that it is polarization independent, so by tuning the polarization angle, the frequency peaks and absorption value remain unchanged. These aspects make the suggested absorber proper for imaging, detecting, filtering, and sensing applications. We have investigated the application of the MPA in refractive index sensing. The refractive index of unknown material can be measured by measuring the shift of the frequency peaks. The perfect metamaterial absorber is a good candidate for biosensor application based on the obtained results. [ABSTRACT FROM AUTHOR]

In this work, a 3-5 μm mid-infrared broadband perfect absorber is proposed. The proposed absorber is only composed of an aluminium oxide (Al2O3) dielectric layer on the top, a silicon (Si) dielectric layer with an embedded titanium (Ti) ring in the middle and a high loss metal substrate. It exhibits an extremely perfect absorption performance in the entire range of 3-5 μm mid-infrared band, with the highest and average absorption rates of 99.91% and 98.57%, respectively. The excellent absorption at 3-5 μm is attributed to the intrinsic absorption of the two dielectrics and the excitation of the surface plasmon resonance (SPR). Moreover, the average absorption rate can be still maintained above 91.33% when the incident angle of TE/TM or unpolarized wave reaches 60°, and there is almost unchanged in the absorption rate when the polarization angle of the incident wave gradually changes from 0° to 90°, which shows the insensitivity of the absorber to the incident and polarization angle. The absorber with simple structure and high stability has potential application prospects in infrared stealth and detection.

Perfect absorption over the entire visible spectrum can create a dark background for acquiring images with high contrast and improved resolution, which is crucial for various applications such as medical imaging, biological detection, and industrial non-destructive testing. The broadband absorption is desired to be achieved in an ultrathin structure for low noise as well as high integration. Here, we experimentally demonstrate a metasurface broadband perfect absorber with an ultrathin thickness of 148 nm and a large area of ∼10 cm2. Such a metasurface, with more than 97% absorption in the wavelength range from 400 to 800 nm, is composed of chromium nanodisk hexagonal array deposited on a chromium substrate with a silica spacer. A self-assembly based colloidal lithography nanofabrication method is developed for the scalable fabrication of the proposed nanostructure. We attribute the broadband absorption to the spectrally overlapped Fabry–Perot resonance, surface plasmon polariton, and localized surface plasmon resonances. Our results offer a novel approach to wafer-scale and low-cost manufacturing of absorption-based devices for applications such as high-contrast imaging and optical modulation.

Abstract 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.

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. [ABSTRACT FROM AUTHOR]

Although many multi-band terahertz (THz)–metasurface perfect absorbers (MPAs) have been proposed, many efforts are still suffering from complex structure design and high fabrication cost, thus limited in practical application. Here, a simple design of the dual-band MPA was proposed in the THz region, which is composed of the complementary-star-shaped (CSS) structure all-dielectric GaAs array. Simulation results show that the designed MPA can achieve an absorbance of over 99.9% at 0.715 THz and 0.861 THz, respectively, which can be confirmed by the coupling mode theory (CMT). In addition, the high Q-factors of about 79.58 and 78.27 of the designed MPA can be achieved, respectively. The observed perfect absorption of the MPA is mainly contributed to the excitations of the surface plasmonic polaritons (SPPs) modes. The absorption properties of the MPA can be adjusted by varying the geometric parameters of the unit-cell structure. Due to its excellent properties, the designed dual-band MPA may find many potential THz applications in sensing, detecting, thermal emitting, and imaging. [ABSTRACT FROM AUTHOR]

A plasmonic perfect absorber (PA) utilizes high absorption of incident radiation and near-field enhancement which is so advantageous for many applications such as solar cell, infrared (IR) spectroscopy, and refractive index (RI) sensitivity. In this study, the numerically presented PA system has resonances from mid-infrared (MIR) to near-infrared (NIR) regions for different periodicities at 2.0, 2.4, and 2.8 µm. A fine-tuning mechanism is obtained to control the spectral response of PA through the geometrical parameters. Dual-band plasmonic resonances have nearly 98% absorption. Near-field intensity distributions are shown with the enhancement factor of 103 at the resonance frequencies. Refractive index sensitivity is also analyzed by embedding PA in different cladding mediums to show the sensing performance. Relationship between resonance wavelength and refractive index of the medium is observed for all resonant modes. The highest sensitivity value is 2382.48 nm/RIU and the figure of merit reaches as high as 28.29 RIU−1. [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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This paper presents a graphene-based polarization-insensitive metamaterial perfect absorber at terahertz frequency range. The absorber consists of a graphene pattern layer at the top of the structure, a dielectric spacer layer, and an Au layer at the bottom of the structure. In the proposed structure, the ability of perfect absorption is investigated by the complete suppression of radiation and reflected light and the complete dissipation of incident energy. At the first, the proposed structure and its design process are verified and it is shown that in the three bands, the results of perfect absorption at frequencies of 2, 2.95 and 3.75 terahertz are achieved with absorption rates of 93.5%, 99.8% and 98.1%, respectively. Also, the physical mechanism of structure performance is investigated by the surface distribution of the electric field, as well as the geometric changes of the structure. In addition, the design of the proposed structure with graphene provides this advantage so that the resonant frequency can be adjusted by changing the Fermi energy level and graphene relaxation time without changing the proposed structure again. With the study, it is found that the proposed metamaterials perfect absorber is not sensitive to polarization and is more tolerant to the angle of incident radiation. Accordingly, the proposed broadband absorber in this paper has potential in imaging, detection, filtering, solar tracking and other applications.

In recent years, metamaterials have been widely used as a new type of optoelectronic device materials, such as optical stealth, holographic imaging, and metamaterial absorbers. In particular, the research of metamaterial absorbers with multi-narrowband, high absorption and tunability has attracted much attention. However, it is a great challenge to integrate multi-narrowband, high absorption, polarization independence, high sensitivity and tunability into a simple absorption structure. In this paper, we use FDTD to design a multi-narrow band, polarimetric independent, high sensitivity and tunability of metamaterial perfect absorber, which is a metal-dielectric-metal three-layer structure. From the bottom to the top of the structure, they are Au layer, SiO2 layer and Au pattern layer. The simulation results show that three resonance absorption peaks are obtained under the coaction of local surface plasmon resonance, magnetic resonance and cavity resonance. The absorption peaks are located at 878 nm, 1020 nm and 1276 nm, and the absorption rates are 98.7%, 99.3% and 99.5%, respectively. In addition, it is found that the number of absorption peaks can be changed by adjusting the top pattern layer structure. Further, the position of the absorption peak and the absorption rate can be altered by adjusting the geometric parameters of the structure. Besides its simple structure, the analysis also show that the structure has high sensitivity under different refractive index environment. Finally, it can also be used for optical conversion devices, such as optical absorption switches, splitters, optical modulators.