Near-unity Absorption Research Articles

A Dual-Band Guided Laser Absorber Based on Plasmonic Resonance and Fabry-Pérot Resonance.

We numerically investigated a dual-band metamaterial absorber based on the combination of plasmonic resonance and Fabry–Pérot (FP) resonance, which can achieve near-unity absorption for guided lasers. The absorber is constructed by a three-layer metal-insulator-metal (MIM) periodic configuration. In each unit cell, there is a gold-silicon cross on a thin silicon layer and a bottom nickel film. Numerical results show that, at normal incidence, the structure strongly absorbs light at wavelengths of 1.064 μm and 10.6 μm, with absorption rates higher than 94%. It is revealed that the two absorption peaks result from FP resonance in the thin silicon layer and plasmonic resonance in the cross, respectively. In addition, the absorber is polarization insensitive and is tolerant to the incident angle. The proposed combination of different resonances has the advantage of easily producing double absorption peaks with very large wavelength differences, and provides a new approach to the design of metamaterial absorbers.

Tunable Optimal Dual Band Metamaterial Absorber for High Sensitivity THz Refractive Index Sensing.

We present a simple dual band absorber design and investigate it in the terahertz (THz) region. The proposed absorber works in dual operating bands at 5.1 THz and 11.7 THz. By adjusting the graphene chemical potential, the proposed absorber has the controllability of the resonance frequency to have perfect absorption at various frequencies. The graphene surface plasmon resonance results in sharp and narrow resonance absorption peaks. For incident angles up to 8°, the structure possesses near-unity absorption. The proposed sensor absorber’s functionality is evaluated using sensing medium with various refractive indices. The proposed sensor is simulated for glucose detection and a maximum sensitivity of 4.72 THz/RIU is observed. It has a maximum figure of merit (FOM) and Quality factor (Q) value of 14 and 32.49, respectively. The proposed optimal absorber can be used to identify malaria virus and cancer cells in blood. Hence, the proposed plasmonic sensor is a serious contender for biomedical uses in the diagnosis of bacterial infections, cancer, malaria, and other diseases.

Refractive Index Biosensor Using Metamaterial Perfect Absorber Based on Graphene in Near-Infrared for Disease Diagnosis

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.

Model and Measurements of an Optical Stack for Broadband Visible to Near-Infrared Absorption in TiN MKIDs

Typical materials for optical Microwave Kinetic Inductance Detetectors (MKIDs) are metals with a natural absorption of sim 30–50% in the visible and near-infrared. To reach high absorption efficiencies (90–100%) the KID must be embedded in an optical stack. We show an optical stack design for a 60 nm TiN film. The optical stack is modeled as sections of transmission lines, where the parameters for each section are related to the optical properties of each layer. We derive the complex permittivity of the TiN film from a spectral ellipsometry measurement. The designed optical stack is optimised for broadband absorption and consists of, from top (illumination side) to bottom: 85 nm SiO2, 60 nm TiN, 23 nm of SiO2, and a 100 nm thick Al mirror. We show the modeled absorption and reflection of this stack, which has >80% absorption from 400 to 1550 nm and near-unity absorption for 500–800 nm. We measure transmission and reflection of this stack with a commercial spectrophotometer. The results are in good agreement with the model.

Graphite based metal-free and polarization-insensitive multiband THz absorber with wide incident angle

In this paper, a novel non-metallic graphite-based tri-layered absorber is proposed to achieve multi-band absorption with perfect resonant peaks using a graphite disk resonator. The basic unit cell for the proposed metal-free absorber comprises a simple tri-layered structure with a graphite disk at the top, which acts as a resonator, followed by a dielectric substrate in the middle, and a graphite sheet at the bottom. Here, the dielectric substrate and the graphite sheet collectively behave as a cavity oscillator, responsible for generating the multi-band absorption. Further, a graphite disk at the top, owing to its high absorption coefficient and refractive indices in the lower THz range, is responsible here for achieving the near-perfect absorption for all the resonant peaks. Hence, the complete structure behaves as a resonant cavity oscillator, producing a standing wave pattern at multiple resonant frequencies. This results in four discrete absorption peaks with narrow bandwidth profiles and high absorption rates in the frequency range of 1–6 THz, with near-unity absorption peaks at 1.665 THz, 2.89 THz, 4 THz, and 5.23 THz. The optimized parameters, like the thickness of the dielectric substrate, confirm the applicability of this multiband metal-free absorber for a wide frequency range. The response of this proposed absorber is validated numerically using the standing wave mechanism by considering its equivalence to a Metal–Insulator–Metal (MIM) waveguide model. The results are further verified using the resonant mode analysis method. It is worth mentioning that the proposed absorber is operative for a wide incident angle of 75°and is non-susceptible to polarization as well.

Quad-Band Polarization-Insensitive Square Split-Ring Resonator (SSRR) with an Inner Jerusalem Cross Metamaterial Absorber for Ku- and K-Band Sensing Applications

The development of metamaterial absorbers has become attractive for various fields of application, such as sensing, detectors, wireless communication, antenna design, emitters, spatial light modulators, etc. Multiband absorbers with polarization insensitivity have drawn significant attention in microwave absorption and sensing research. In this paper, we propose a quad-band polarization-insensitive metamaterial absorber (MMA) for Ku- and K-band applications. The proposed patch comprises two square split-ring resonators (SSRR), four microstrip lines, and an inner Jerusalem cross to generate four corresponding resonances at 12.62 GHz,14.12 GHz, 17.53 GHz, and 19.91 GHz with 97%, 99.51%, 99%, and 99.5% absorption, respectively. The complex values of permittivity, permeability, refractive index, and impedance of MMA were extracted and discussed. The absorption mechanism of the designed MMA was explored by impedance matching, equivalent circuit model, as well as magnetic field and electric field analysis. The overall patch has a rotational-symmetrical structure, which plays a crucial role in acquiring the polarization-insensitive property. The design also shows stable absorption for both transverse electric (TE) and transverse magnetic (TM) modes. Its near-unity absorption and excellent sensing performance make it a potential candidate for sensing applications.

Semiconductor nanowire metamaterial for broadband near-unity absorption

The realization of a semiconductor near-unity absorber in the infrared will provide new capabilities to transform applications in sensing, health, imaging, and quantum information science, especially where portability is required. Typically, commercially available portable single-photon detectors in the infrared are made from bulk semiconductors and have efficiencies well below unity. Here, we design a novel semiconductor nanowire metamaterial, and show that by carefully arranging an InGaAs nanowire array and by controlling their shape, we demonstrate near-unity absorption efficiency at room temperature. We experimentally show an average measured efficiency of 93% (simulated average efficiency of 97%) over an unprecedented wavelength range from 900 to 1500 nm. We further show that the near-unity absorption results from the collective response of the nanowire metamaterial, originating from both coupling into leaky resonant waveguide and transverse modes. These coupling mechanisms cause light to be absorbed directly from the top and indirectly as light scatters from one nanowire to neighbouring ones. This work leads to the possible development of a new generation of quantum detectors with unprecedented broadband near-unity absorption in the infrared, while operating near room temperature for a wider range of applications.

Selective Properties of Mid-Infrared Tamm Phonon-Polaritons Emitter with Silicon Carbide-Based Structures.

Electromagnetic (EM) absorbers and emitters have attracted much interest because of their versatile applications. A photonic heterostructure composed of silicon carbide (SiC) layer/germanium (Ge) cavity/distributed Bragg reflector (DBR) has been proposed. Selective emission properties have been investigated through rigorous coupled wave analysis (RCWA) method. The results illustrate that Tamm phonon-polaritons can be excited, and the magnetic field is partially centralized at the junction of Ge cavity and SiC film, aimed to improve the interactions of photon–phonon. The absorptivity/emissivity of the structure can be better optimized by controlling the coupling of surface modes with the incident wave. Near-unity absorption can be achieved through optimizing the SiC grating/Ge cavity/distributed Bragg reflector (DBR) multilayer structure with geometrical parameters of ds = 0.75 μm, dg = 0.7 μm, d1 = 1.25 μm and d2 = 0.75 μm, respectively. Physical mechanism of selective emission characteristics is deliberated. In addition, the simulation results demonstrate that the emitter desensitizes to the incidence angle and polarization state in the mid-infrared (MIR) range. This research ameliorates the function of the selective emitters, which provides more efficient design for SiC-based systems.

Perfect selective metamaterial absorber with thin-film of GaAs layer in the visible region for solar cell applications

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.

Selective Light Absorption and Spectral Manipulation via an Electro-Optical Nano-Cavity

A frequency-region selective light absorber is theoretically investigated by an electro-optical waveguide cavity coupled plasmonic micro-structure. A spectrally separated broadband near-unity absorption in the visible and the narrow band absorption peak in the near-infrared regime are simultaneously achieved due to the different resonant behaviors located in the different positions of the stereostructure. This spatial and frequency selective light absorption shows impressive distinct behaviors during the electro-optical (EO) operation. The absorption band in the visible maintains well while the absorption peak in the longer wavelength range presents a sensitive response to the external voltage. Based on the numerical simulation, a spectral shift related sensitivity reaches 7.1 nm/V for the infrared absorption peak when the applied voltage is launched. Meanwhile, the absorption efficiency is still keeping in the high level with the intensity exceeding 97.5%. These findings open a facile way for differential light absorption and spectral manipulation via the well-design of spatial geometry-related resonant metamaterial.

Monolayer crumpled graphene-based mechanically and electrically tunable infrared absorbers for high-sensitivity sensing

Enhancing the absorption of infrared light in continuous monolayer graphene layer has received much attention in various optical devices. Here, based on the uniaxial and biaxial crumpled graphene structures, two mechanically and electrically tunable infrared absorbers have been proposed and investigated. We have demonstrated that the near-unity absorption spectra of proposed absorbers are ascribed to the electric resonance modes excited along the sidewalls of crumpled graphene structures, and a broad spectra tunability can be achieved through combing mechanical stretching/compression with the conventional electrostatic doping method. Furthermore, the refractive index sensing capacity of proposed biaxial absorber has been further investigated. Benefiting from the unique stereoscopic morphologies, the sensing sensitivity of each mode of the absorber can reach up to 5.546 and 8.251 µm per refractive index unit, respectively. The broad spectra tunability, high sensing sensitivity as well as flexible structures render it holding promising potentials in high-sensitivity flexible sensing applications.

A Review on Metamaterial Absorbers: Microwave to Optical

Metamaterials (MM) are artificially designed materials that possess unique properties due to their geometrical design. They also display some peculiar properties, such as negative refractive index, Snell’s law reversal, Doppler effect reverse, and left-handed behavior. MMs are used in a myriad of applications, including invisibility cloaking, perfect lensing, perfect absorption, and sensing. In this review article, the property of electromagnetic absorption by structures known as metamaterial absorbers (MMAs) is discussed. An MMA is a composite made up of many layers of metallic patterns separated by dielectric. This novel device helps in achieving near-unity absorption by various mechanisms, which are investigated in this article. The MMAs are classified based on their absorption characteristics, such as polarization tunability, broadband operation, and multiband absorption, in different frequency regimes.

Polarization-independent near-infrared superabsorption in transition metal dichalcogenide Huygens metasurfaces by degenerate critical coupling

Strong near-infrared absorption of light in semiconducting transition metal dichalcogenides (TMDs) is essential for improving the photocarrier extraction efficiency in optoelectronic devices. Here, we numerically demonstrate that an original TMD Huygens metasurface is specifically designed to overcome the 50% absorptance limit of a subwavelength thin film. The unique metasurface comprising a TMD nanodisk array shows evidence of characteristic Mie resonances including electric and magnetic dipoles. By carefully optimizing the aspect ratio of nanodisks, the extraordinary spectral overlapping of orthogonal electric and magnetic dipole resonances is successfully realized and, thus, enables the super-high absorptance up to 87% in a subwavelength-thin ($\ensuremath{\approx}0.19{\ensuremath{\lambda}}_{0}$) semiconductor structure by degenerate critical coupling at the wavelength of ${\ensuremath{\lambda}}_{0}=903\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$. All numerical results are confirmed by the temporal coupled-mode theory and again via the multipole decomposition method. Importantly, the near-unity absorptance in this TMD Huygens metasurface by degenerate critical coupling not only is tailored throughout the entire near-infrared region but also is both polarization independent and angle insensitive. The absorptance maintaining >80% is observed with the incident angle up to \ifmmode\pm\else\textpm\fi{}10 \ifmmode^\circ\else\textdegree\fi{} at least. Our findings are not only of fundamental interest but could also offer a platform for TMD-based high-speed photodetecting, energy harvesting, and thermal emission.

Optically Transparent Broadband Microwave Absorber by Graphene and Metallic Rings

The demand for optically transparent microwave absorbers has attracted increasing interest among researchers in recent years. However, integrating broadband microwave absorption and high optical transparency remains a challenge. This report demonstrates a scheme for broadband microwave absorbers, featuring a 90% absorption bandwidth of 10 GHz covering a frequency range of 25.2-35.2 GHz and high compatibility with good optical transparency in a wide band from the visible to infrared. The absorber is based on a Jaumann structure composed of two graphene sheets sandwiched by dielectric and backed by an arrayed-metallic-rings sheet. Guided by derived formulas, this absorber exhibits complete absorption if the sheet resistance of graphene is close to 500 Ω sq-1. The bandwidth and center frequency of the absorption spectra can be readily tuned simply via changes in the thickness of the dielectric between the graphene films and arrayed-metallic-rings sheet. Moreover, the absorber is insensitive to the incident angle of radiation and can achieve broadband and near-unity absorption even at oblique incidence. The graphene-based absorber proposed herein provides a viable solution for effectively integrating broadband and near-unity microwave absorption with high optical transparency, thereby enabling widespread applications in optics, communications, and solar cells.

Polarization and angular insensitive bendable metamaterial absorber for UV to NIR range

Broadband absorbers are required for solar energy harvesting because they efficiently absorb the incident photon in the wide-ranging solar spectrum. To ensure high absorption of photons, metamaterial absorbers (MMAs) have been a growing area of interest in recent years. In this article, an MMA is proposed using a metal–insulator–metal (MIM) structure (Ni–SiO2–Ni) that shows a near-unity broadband absorption of wavelengths from 300 to 1600 nm, with a 95.77% average absorption and a peak absorption of 99.999% at 772.82 nm. The MMA is polarization insensitive as well as wide incident angle stable. Analysis of the effects of mechanical bending on the absorption of the proposed structure shows that absorption holds satisfactory values at different degrees of mechanical loading. The suggested MMA unit cell structure was computationally simulated using the Finite Integration Technique (FIT) and verified using the Finite Element Method (FEM). To analyze the feasibility of the proposed MMA as a solar cell, it is investigated with the universal AM 1.5 solar spectrum characteristics. Besides solar energy harvesting, the proposed MMA unit cell may be employed in a variety of diverse optical applications, including sensors, detectors, and imaging.

Nanostructured multilayer hyperbolic metamaterials for high efficiency and selective solar absorption.

Highly efficient solar-to-thermal conversion is desired for the renewable energy technologies, such as solar thermo-photovoltaics and solar thermo-electric systems. In order to maximize the energy conversion efficiency, solar-selective absorbers are essential with its absorption characteristics specially tailored for solar applications. Here, we propose a wideband spectral-selective absorber based on three-dimensional (3D) nanostructured hyperbolic metamaterial (HMM), which can realize near-unity absorption across the UV and NIR spectral ranges. Moreover, the optical topological transition (OTT) of iso-frequency surface (IFS) is manipulated to selectively enhance light absorption in the entire solar spectrum, crucial for improved energy utilization. Impressive solar-to-thermal conversion efficiency of 95.5% has been achieved. Particularly, such superior properties can be retained well even over a wide range of incident angles. These findings open new avenues for designing high-performance solar thermal devices, especially in the fields related to solar energy harvesting.

Multimode absorption in single-layer graphene: Disordered photonics and magneto-optic effect

The current study theoretically investigates the effect of disordered thickness and the position of layers on optical absorption properties of the graphene-based photonic crystal in the THz frequency regime. The results show that single, as well as multiple narrow-band peaks, are formed by changing the disorder strength and twisting unicells (defect). It is also found that multiple narrow-band absorption peaks with absorption up to 0.997 can be formed in highly disordered photonic structures with a single graphene layer. On the other hand, single as well as multiple near-unity absorption peak strength is increased by twisting the two most right dielectric layers as well as by increasing the disorderedness of the structures. The presence of magnetic fields has a strong impact on the optical absorption of LCP and RCP waves. The bandwidth of absorption peaks increases for LCP and decreases for RCP. This effect can be interchanged by reversing the magnetic field. Therefore, the proposed structure can be effectively used for the design of tunable THz optical absorbers, sensors as well as different optoelectronic devices.

Polarization-controllable and angle-insensitive multiband Yagi-Uda-shaped metamaterial absorber in the microwave regime

This paper reports a multiband Yagi-Uda shaped metamaterial absorber (YUMA) operating in X- and Ku-band of the microwave regime with the added functionalities of miniaturization, polarization controllability and wide incidence angle stability. The proposed YUMA shows three distinctive near-unity absorption peaks corresponding to 10.64 GHz, 12.08 GHz, and 14.09 GHz frequencies. The YUMA was analyzed under different oblique incidence angles for transverse electric (TE)-mode and different rotation angles of the top metasurface. The results showed that the proposed YUMA possesses multifunctional characteristics such as polarization controllability, and wide incidence angle stability. The comparison of simulated and measured results further demonstrates that the proposed absorber can be a potential candidate in polarization detection systems and transmissive polarizers. The proposed YUMA operating in the X- and Ku-band can have potential uses in several other applications, such as air traffic control, weather monitoring, military radar, and satellite communication.

Collective Plasmon Coupling in Gold Nanoparticle Clusters for Highly Efficient Photothermal Therapy.

Plasmonic nanomaterials with strong absorption at near-infrared frequencies are promising photothermal therapy agents (PTAs). The pursuit of high photothermal conversion efficiency has been the central focus of this research field. Here, we report the development of plasmonic nanoparticle clusters (PNCs) as highly efficient PTAs and provide a semiquantitative approach for calculating their resonant frequency and absorption efficiency by combining the effective medium approximation (EMA) theory and full-wave electrodynamic simulations. Guided by the theoretical prediction, we further develop a universal strategy of space-confined seeded growth to prepare various PNCs. Under optimized growth conditions, we achieve a record photothermal conversion efficiency of up to ∼84% for gold-based PNCs, which is attributed to the collective plasmon-coupling-induced near-unity absorption efficiency. We further demonstrate the extraordinary photothermal therapy performance of the optimized PNCs in in vivo application. Our work demonstrates the high feasibility and efficacy of PNCs as nanoscale PTAs.

Multi-functionalization of manipulative optoelectronic devices by enhanced plasmonic coupling of graphene-based hybrid asymmetrical system

Due to multimode coupling of the metal-grating assisted graphene plasmonic asymmetric system, multiple high-quality near-unity absorption enabling the quantitative manipulation, multiplexing function, high modulation efficiency and multi-functionalization of novel optical devices can be obtained. Since the spectral positions of the absorption peaks are with strong parabolic or linear relationship to the lower-layer graphene’s chemical potential or the dielectric spacer’s thickness, such system can be regarded as controllable modulators to realize electrically switchable effect, whose operation frequency can be tuned precisely over five discrete operation bandwidths. Excellent plasmonic switching is achieved, when a millimeter-level change of 0.0027 eV for the upper-layer graphene’s chemical potential can result in an intensity change ratio of 73.7%. Otherwise, the maximum optical sensitivity and FOM factor for sensing the analyte refractive index can reach up to 38.675 THz/RIU and 584, respectively. Besides, the ultrahigh sensitivity (1321.5 nm/RIU) for sensing the environment also enables the temperature detection, and the solution limit can be as down as the absolute zero (−273.15 °C). Moreover, the polarization-adjustable and incident angle-insensitive properties (up to 75°) further enhance the practicability and feasibility of the proposed system as sophisticated optoelectronic devices.