Metal-Insulator-Metal-Based Plasmonic Metamaterial Absorbers at Visible and Infrared Wavelengths: A Review

Nanoparticle manipulation based on chiral plasmon effects

Chiral plasmonic structures have garnered increasing attention owing to their distinctive chiroptical response. Localized surface plasmon resonance can significantly enhance the circular dichroism and local electromagnetic field of chiral plasmonic structures, resulting in enhanced electromagnetic forces acting on surrounding nanoparticles. Moreover, the circular dichroism response of chiral structures provides an effective means for macroscopic adjustment of microscopic electromagnetic fields. However, chiral plasmon effects are naturally related to angular momentum, and particle control studies of chirality usually focus on angular momentum. This paper proposes a particle manipulation method utilizing chiral light-matter interactions. Through optimization of the optical response of the chiral structure, the direction of electromagnetic forces exerted on surrounding polystyrene particles reverses upon a change in the incident light's handedness. According to this characteristic, the movement direction control of polystyrene particles with a diameter of 100 nm was achieved. By altering the handedness of a single circularly polarized light, more than 94% high-precision particle manipulation was achieved, reducing the complexity of particle manipulation. This microfluidic method has significant implications for advancing microfluidic research and chiral applications.

Uncooled High Detectivity Mid-Infrared Photoconductor Using HgTe Quantum Dots and Nanoantennas

Using a metal/insulator/metal (MIM) structure with a gold nanoantenna array made by electron beam lithography, the responsivity of a HgTe colloidal quantum dot film is enhanced in the mid-infrared. Simulations indicate that the spatially averaged peak spectral absorption of an 80 nm film is 60%, enhanced 23-fold compared to that of the same film on a bare sapphire substrate. The field intensity enhancement is focused near the antenna tips, being 20-fold 100 nm away, which represents only 1% of the total area and up to 1000-fold at the tips. The simulated polarized absorption spectra are in good agreement with the experiments, with a strong resonance around 4 μm. A responsivity of 0.6 A/W is obtained at a 1 V bias. Noise measurements separate the 1/f noise from the generation-recombination white noise and give a spatially averaged photoconductive gain of 0.3 at 1 V bias. The spatially averaged peak detectivity is improved 15-fold compared to the same film on a sapphire substrate without an MIM structure. The experimental peak detectivity reaches 9 × 109 Jones at 2650 cm-1 and 80 kHz, decreasing at lower frequencies. The MIM structure also enhances the spatially averaged peak photoluminescence of the CQD film by 16-fold, which is a potential Purcell enhancement. The good agreement between simulations and measurements confirms the viability of lithographically designed nanoantenna structures for vastly improving the performance of mid-IR colloidal quantum dot photoconductors. Further improvements will be possible by matching the optically enhanced and current collection areas.

Experimental Investigation of the Thermal Emission Cross Section of Nanoresonators Using Hierarchical Poisson-Disk Distributions

Effective cross sections of nano-objects are fundamental properties that determine their ability to interact with light. However, measuring them for individual resonators directly and quantitatively remains challenging, particularly because of the very low signals involved. Here, we experimentally measure the thermal emission cross section of metal-insulator-metal nanoresonators using a stealthy hyperuniform distribution based on a hierarchical Poisson-disk algorithm. In such distributions, there are no long-range interactions between antennas, and we show that the light emitted by such metasurfaces behaves as the sum of cross sections of independent nanoantennas, enabling direct retrieval of the single resonator contribution. The emission cross section at resonance is found to be on the order of λ_{0}^{2}/3, a value that is nearly 3 times larger than the theoretical maximal absorption cross section of a single particle, but remains smaller than the maximal extinction cross section. This measurement technique can be generalized to any single resonator cross section, and we also apply it to a lossy dielectric layer.

Hydrogels for active photonics

Conventional photonic devices exhibit static optical properties that are design-dependent, including the material's refractive index and geometrical parameters. However, they still possess attractive optical responses for applications and are already exploited in devices across various fields. Hydrogel photonics has emerged as a promising solution in the field of active photonics by providing primarily deformable geometric parameters in response to external stimuli. Over the past few years, various studies have been undertaken to attain stimuli-responsive photonic devices with tunable optical properties. Herein, we focus on the recent advancements in hydrogel-based photonics and micro/nanofabrication techniques for hydrogels. In particular, fabrication techniques for hydrogel photonic devices are categorized into film growth, photolithography (PL), electron-beam lithography (EBL), and nanoimprint lithography (NIL). Furthermore, we provide insights into future directions and prospects for deformable hydrogel photonics, along with their potential practical applications.

Scalable, Color-Matched, Flexible Plasmonic Film for Visible-Infrared Compatible Camouflage

The multispectral compatible infrared camouflage technology is implemented these days to counter the developing infrared detectors and detectors of other bands. However, the conflict between delicate optical structures and scalable procedures has significantly impeded the development and application of multispectral-compatible camouflage technology. Therefore, a semi-open Fabry-Perot structure is introduced, and the color and infrared emissivity by structural parameters for color-matched visible-infrared compatible camouflage are modulated. The prepared compatible camouflage film exhibits visible camouflage by the minimum color difference of 1.6 L*a*b* (under desert background) and infrared camouflage by low emission (ε3-5 µm ≈ 0.17 and ε8-14 µm ≈ 0.143). Due to its flexibility and scalability, the compatible camouflage film can be applied in practical applications and exhibits desirable visible and infrared camouflage performance in different battlefield backgrounds.

Black Phosphorus Molybdenum Disulfide Midwave Infrared Photodiodes with Broadband Absorption-Increasing Metasurfaces

Black phosphorus (BP) has been established as a promising material for room temperature midwave infrared (MWIR) photodetectors. However, many of its attractive optoelectronic properties are often observable only at smaller film thicknesses, which inhibits photodetector absorption and performance. In this work, we show that metasurface gratings increase the absorption of BP-MoS2 heterojunction photodiodes over a broad range of wavelengths in the MWIR. We designed, fabricated, and characterized metasurface gratings that increase absorption at selected wavelengths or broad spectral ranges. We evaluated the broadband metasurfaces by measuring the room temperature responsivity and specific detectivity of BP-MoS2 photodiodes at multiple MWIR wavelengths. Our results show that broadband metasurface gratings are a scalable approach for boosting the performance of BP photodiodes over large spectral ranges.

Near-perfect wide-band absorbers based on one-dimensional photonic crystal structures in 1-20 THz frequencies

This paper investigates the absorption behavior of one-dimensional (1D) photonic crystal (PhC) structures in the 1-20 THz region. The structures are analyzed by the transfer matrix method to achieve accurate results quickly with ordinary simulation facilities. The simulation results indicate a strong dependence of the absorber performance on the thickness and material of the PhC layers, as well as the frequency and angle of incident light. The combination of silica and titanium (Ti) materials as dielectric and metal layers presents a great choice for broadband high-absorption applications so that this structure can absorb, on average, more than 80% of the normal incident radiation in the studied frequency range. Additionally, this absorber has the lowest dependence on incident light with the angle varying from 0° to 80° compared to identical absorbers with silver, aluminum, gold, chromium, nickel, and tungsten metals. The excellent absorption feature of the Ti-based absorber compared to the other absorbers is attributed to the lower permittivity of Ti (in both real and imaginary parts) in comparison with the other metals. In addition to owning simple and fabrication-friendly structures, 1D PhCs can pave the way to achieve various absorption spectra proportional to the needs of photonics, communications, and aerospace applications.

Hexagonal Boron Nitride for Photonic Device Applications: A Review

Hexagonal boron nitride (hBN) has emerged as a key two-dimensional material. Its importance is linked to that of graphene because it provides an ideal substrate for graphene with minimal lattice mismatch and maintains its high carrier mobility. Moreover, hBN has unique properties in the deep ultraviolet (DUV) and infrared (IR) wavelength bands owing to its indirect bandgap structure and hyperbolic phonon polaritons (HPPs). This review examines the physical properties and applications of hBN-based photonic devices that operate in these bands. A brief background on BN is provided, and the theoretical background of the intrinsic nature of the indirect bandgap structure and HPPs is discussed. Subsequently, the development of DUV-based light-emitting diodes and photodetectors based on hBN's bandgap in the DUV wavelength band is reviewed. Thereafter, IR absorbers/emitters, hyperlenses, and surface-enhanced IR absorption microscopy applications using HPPs in the IR wavelength band are examined. Finally, future challenges related to hBN fabrication using chemical vapor deposition and techniques for transferring hBN to a substrate are discussed. Emerging techniques to control HPPs are also examined. This review is intended to assist researchers in both industry and academia in the design and development of unique hBN-based photonic devices operating in the DUV and IR wavelength regions.

Visible-Range Multiple-Channel Metal-Shell Rod-Shaped Narrowband Plasmonic Metamaterial Absorber for Refractive Index and Temperature Sensing

Multiple resonance modes in an optical absorber are necessary for nanophotonic devices and encounter a challenge in the visible range. This article designs a multiple-channel plasmonic metamaterial absorber (PMA) that comprises a hexagonal arrangement of metal-shell nanorods in a unit cell over a continuous thin metal layer, operating in the visible range of the sensitive refractive index (RI) and temperature applications. Finite element method simulations are utilized to investigate the physical natures, such as the absorptance spectrum, magnetic flux and surface charge densities, electric field intensity, and electromagnetic power loss density. The advantage of the proposed PMA is that it can tune either three or five absorptance channels with a narrowband in the visible range. The recorded sensitivity and figure of merit (S, FOM) for modes 1-5 can be obtained (600.00 nm/RIU, 120.00), (600.00 nm/RIU, 120.00 RIU^-1), (600.00 nm/RIU, 120.00 RIU^-1), (400.00 nm/RIU, 50.00 RIU^-1), and (350.00 nm/RIU, 25.00 RIU^-1), respectively. Additionally, the temperature sensitivity can simultaneously reach 0.22 nm/°C for modes 1-3. The designed PMA can be suitable for RI and temperature sensing in the visible range.

Dual-band wide-angle perfect absorber based on the relative displacement of graphene nanoribbons in the mid-infrared range

In this paper, a novel graphene-based dual-band perfect electromagnetic absorber operating in the mid-infrared regime has been proposed. The absorber has a periodic structure which its unit cell consists of a sliver substrate and two graphene nanoribbons (GNRs) of equal width separated with a dielectric spacer. Two distinct absorption peaks at 10 and 11.33 µm with absorption of 99.68% and 99.31%, respectively have been achieved due to a lateral displacement of the GNRs. Since graphene surface conductivity is tunable, the absorption performance can be tuned independently for each resonance by adjusting the chemical potential of GNRs. Also, it has been proved that performance of the proposed absorber is independent of the incident angle and its operation is satisfactory when the incident angle varies from normal to ±75°. To simulate and analyze the spectral behavior of the designed absorber, the semi-analytical method of lines (MoL) has been extended. Also, the finite element method (FEM) has been applied in order to validate and confirm the results.

Design and Fabrication of Millimeter-Wave Frequency-Tunable Metamaterial Absorber Using MEMS Cantilever Actuators

In this paper, a MEMS (Micro Electro Mechanical Systems)-based frequency-tunable metamaterial absorber for millimeter-wave application was demonstrated. To achieve the resonant-frequency tunability of the absorber, the unit cell of the proposed metamaterial was designed to be a symmetric split-ring resonator with a stress-induced MEMS cantilever array having initial out-of-plane deflections, and the cantilevers were electrostatically actuated to generate a capacitance change. The dimensional parameters of the absorber were determined via impedance matching using a full electromagnetic simulation. The designed absorber was fabricated on a glass wafer with surface micromachining processes using a photoresist sacrificial layer and the oxygen-plasma-ashing process to release the cantilevers. The performance of the fabricated absorber was experimentally validated using a waveguide measurement setup. The absorption frequency shifted down according to the applied DC (direct current) bias voltage from 28 GHz in the initial off state to 25.5 GHz in the pull-down state with the applied voltage of 15 V. The measured reflection coefficients at those frequencies were -5.68 dB and -33.60 dB, corresponding to the peak absorptivity rates of 72.9 and 99.9%, respectively.

Humidity-Controlled Tunable Emission in a Dye-Incorporated Metal-Hydrogel-Metal Cavity

Actively controllable photoluminescence is potent for a wide variety of applications from biosensing and imaging to optoelectronic components. Traditionally, methods to achieve active emission control are limited due to complex fabrication processes or irreversible tuning. Here, we demonstrate active emission tuning, achieved by changing the ambient humidity in a fluorescent dye-containing hydrogel integrated into a metal-insulator-metal (MIM) system. Altering the overlapping region of the MIM cavity resonance and the absorption and emission spectra of the dye used is the underlying principle to achieving tunability of the emission. We first verify this by passive tuning of cavity resonance and further experimentally demonstrate active tuning in both air and aqueous environments. The proposed approach is reversible, easy to integrate, and spectrally scalable, thus providing opportunities for developing tunable photonic devices.

Polarization-dependent wideband metamaterial absorber for ultraviolet to near-infrared spectral range applications

The need for wideband metamaterial absorbers (WBMA) for applications other than sensing and filtering has demanded modifications to the conventional three-layer metal-insulator-metal (MIM) absorber configuration. This modification often results in complex geometries and an increased number of layers requiring complex lithographic processes for fabrication. Here, we show that a metamaterial absorber with rectangular geometry in the simple MIM configuration can provide wideband absorption covering the ultraviolet and near-infrared spectral range. Due to its asymmetric nature, the WBMA is sensitive to the polarization of the incident light and independent of the angle of incidence up to about 45° depending on the polarization of the incident light. The characteristics of the WBMA presented here may be useful for applications such as detectors for wide spectral band applications.

Experimental Study of a Quad-Band Metamaterial-Based Plasmonic Perfect Absorber as a Biosensor

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.

Scalable-Manufactured Metamaterials for Simultaneous Visible Transmission, Infrared Reflection, and Microwave Absorption

Scalable manufacturing of metamaterials with multispectral manipulation capabilities remains highly challenging, which was generally circumvented by integrating several single-spectral metamaterials, potentially leading to complex processes, large thicknesses, and limited fabrication size. We experimentally demonstrate a standalone and scalable-manufactured multispectral metamaterial featuring simultaneous visible transmission, infrared reflection, and microwave absorption. The prepared multispectral metamaterial with an area of 255 cm2 exhibits a visible transmittance of 74.5% at wavelengths of 400-700 nm (the highest 80.2% at 510 nm), a thermal emissivity of 0.08 at the infrared (IR) wavelengths of 2.5-20 μm (the lowest 0.03 at 19.5 μm), and a microwave absorptance of 63.4% at frequencies of 8.2-12.4 GHz (the near-perfect 97.4% at 11.5 GHz) on average with a deep-subwavelength thickness of λ/47. The deep-subwavelength multispectral metamaterial consists of a submillimeter-thick polyethylene terephthalate dielectric spacer sandwiched by a patterned ultrathin metal and a metal mesh back-reflector with ultralow sheet resistances. Unlike the conventional optically transparent microwave absorbers made from indium tin oxides, the surface plasmonic modes can be excited within the submillimeter-thick multispectral metamaterial, bringing about the gap plasmon polaritons-induced microwave attenuation, together with the excellent visible transparency and high IR reflection/low IR emissivity. This work may inspire the designs and practical production of standalone multispectral metamaterials and benefit the protection against ubiquitous IR and microwave reconnaissance without impeding visual observation.

Thermoplasmonics in Solar Energy Conversion: Materials, Nanostructured Designs, and Applications

The indispensable requirement for sustainable development of human society has forced almost all countries to seek highly efficient and cost-effective ways to harvest and convert solar energy. Though continuous progress has advanced, it remains a daunting challenge to achieve full-spectrum solar absorption and maximize the conversion efficiency of sunlight. Recently, thermoplasmonics has emerged as a promising solution, which involves several beneficial effects including enhanced light absorption and scattering, generation and relaxation of hot carriers, as well as localized/collective heating, offering tremendous opportunities for optimized energy conversion. Besides, all these functionalities can be tailored via elaborated designs of materials and nanostructures. Here, first the fundamental physics governing thermoplasmonics is presented and then the strategies for both material selection and nanostructured designs toward more efficient energy conversion are summarized. Based on this, recent progress in thermoplasmonic applications including solar evaporation, photothermal chemistry, and thermophotovoltaic is reviewed. Finally, the corresponding challenges and prospects are discussed.

Artificial Intelligence in Meta-optics

Recent years have witnessed promising artificial intelligence (AI) applications in many disciplines, including optics, engineering, medicine, economics, and education. In particular, the synergy of AI and meta-optics has greatly benefited both fields. Meta-optics are advanced flat optics with novel functions and light-manipulation abilities. The optical properties can be engineered with a unique design to meet various optical demands. This review offers comprehensive coverage of meta-optics and artificial intelligence in synergy. After providing an overview of AI and meta-optics, we categorize and discuss the recent developments integrated by these two topics, namely AI for meta-optics and meta-optics for AI. The former describes how to apply AI to the research of meta-optics for design, simulation, optical information analysis, and application. The latter reports the development of the optical Al system and computation via meta-optics. This review will also provide an in-depth discussion of the challenges of this interdisciplinary field and indicate future directions. We expect that this review will inspire researchers in these fields and benefit the next generation of intelligent optical device design.

Towards Perfect Absorption of Single Layer CVD Graphene in an Optical Resonant Cavity: Challenges and Experimental Achievements

Graphene is emerging as a promising material for the integration in the most common Si platform, capable to convey some of its unique properties to fabricate novel photonic and optoelectronic devices. For many real functions and devices however, graphene absorption is too low and must be enhanced. Among strategies, the use of an optical resonant cavity was recently proposed, and graphene absorption enhancement was demonstrated, both, by theoretical and experimental studies. This paper summarizes our recent progress in graphene absorption enhancement by means of Si/SiO2-based Fabry-Perot filters fabricated by radiofrequency sputtering. Simulations and experimental achievements carried out during more than two years of investigations are reported here, detailing the technical expedients that were necessary to increase the single layer CVD graphene absorption first to 39% and then up to 84%. Graphene absorption increased when an asymmetric Fabry-Perot filter was applied rather than a symmetric one, and a further absorption increase was obtained when graphene was embedded in a reflective rather than a transmissive Fabry-Perot filter. Moreover, the effect of the incident angle of the electromagnetic radiation and of the polarization of the light was investigated in the case of the optimized reflective Fabry-Perot filter. Experimental challenges and precautions to avoid evaporation or sputtering induced damage on the graphene layers are described as well, disclosing some experimental procedures that may help other researchers to embed graphene inside PVD grown materials with minimal alterations.

Diffuse reflection in periodic arrayed disk metasurfaces

Metamaterials of metal-insulator-metal structures represent effective ways in manipulating light absorbance for photodetection, sensing, and energy harvesting etc. Most of the time, specular reflection has been used in characterizing resonances of metamaterials without considering diffuse scattering from their periodic subwavelength units. In this paper, we investigate diffuse reflection in metasurfaces made of periodic metallic disks in the mid-infrared region. Integrating sphere-based spectral measurements indicate that diffuse reflection is dominated by grating diffractions, which cause diffuse scattering in a spectral region with wavelengths less than that of the first order Rayleigh anomaly. The diffuse reflection is greatly enhanced by the metasurface resonance and exhibits a general increase towards shorter wavelengths, which not only causes a significant difference in evaluating the metamaterial resonant absorption efficiency but also a small blue-shift of the resonance frequency. These findings are helpful for designing and analyzing metamaterial resonant properties when diffuse scattering is taken into account.

Time-Resolved FDTD and Experimental FTIR Study of Gold Micropatch Arrays for Wavelength-Selective Mid-Infrared Optical Coupling

Infrared radiation reflection and transmission of a single layer of gold micropatch two-dimensional arrays, of patch length ∼1.0 μm and width ∼0.2 μm, have been carefully studied by a finite-difference time-domain (FDTD) method, and Fourier-transform infrared spectroscopy (FTIR). Through precision design of the micropatch array structure geometry, we achieve a significantly enhanced reflectance (85%), a substantial diffraction (10%), and a much reduced transmittance (5%) for an array of only 15% surface metal coverage. This results in an efficient far-field optical coupling with promising practical implications for efficient mid-infrared photodetectors. Most importantly we find that the propagating electromagnetic fields are transiently concentrated around the gold micropatch array in a time duration of tens of ns, providing us with a novel efficient near-field optical coupling.

Vanadium-dioxide microstructures with designable temperature-dependent thermal emission

We propose gold-vanadium dioxide microstructures for which the difference in thermally radiated power between the low and high temperature states can be tuned via structural design. We start by incorporating VO₂ in a gold-dielectric-gold waveguide to achieve a temperature-dependent mode effective index. We show that a cavity formed in this waveguide structure has a fundamental resonance wavelength that shifts with temperature. We calculate the thermal radiated power from the cavity at temperatures above and below the phase transition of VO₂ for wavelengths between 8 and 14 µm. We show that the difference in radiated power can be made positive, negative, or zero simply by adjusting the cavity length. Finally, we use our cavity to design thermally emissive metasurfaces with spatial emission patterns that can be inverted with temperature. Our emitters could serve as building blocks in the realization of metasurfaces enabling complex thermal radiation control.

All-Dielectric Color Filter with Ultra-Narrowed Linewidth

In this paper, a transmissive color filter with an ultra-narrow full width at half of the maximum is proposed. Exploiting a material with a high index of refraction and an extremely low extinction coefficient in the visible range allows the quality factor of the filter to be improved. Three groups of GaP/SiO₂ pairs are used to form a Distributed Brag reflector in a symmetrical Fabry-Pérot cavity. A band-pass filter which is composed of ZnS/SiO₂ pairs is also introduced to further promote the purity of the transmissive spectrum. The investigation manifests that a series of tuned spectrum with an ultra-narrow full width at half of the maximum in the full visible range can be obtained by adjusting the thickness of the SiO₂ interlayer. The full width at half of the maximum of the transmissive spectrum can reach 2.35 nm. Simultaneously, the transmissive efficiency in the full visible range can keep as high as 0.75. Our research provides a feasible and cost-effective way for realizing filters with ultra-narrowed linewidth.

Tunable absorber embedded with GST mediums and trilayer graphene strip microheaters

Investigation was made of the optical response of metal-dielectric stacks-based cavity structures embedded with graphene microheaters for the purpose of perfect absorption. The absorber configuration exploits the Ge2Sb2Te5 (GST) phase changing medium, and the effects of different parametric and operational conditions on the absorption spectra were explored. The refractive indices of GST layers can be manipulated by the external electrical pulses applied to microheaters. The amplitude and duration of electrical pulses define the crystallinity ratio of the used GST mediums. The results revealed achieving perfect absorption (> 99%) in the visible and infrared (IR) regimes of the electromagnetic spectrum upon incorporating two thin GST layers of different thicknesses (in the stack) in the amorphous state. The proposed configuration showed the capability of introducing independent transition state (amorphous and/or crystalline) for each GST layer-the visible regime could be extended to the IR regime, and the perfect absorption peak in the IR regime could be broadened and red-shifted. It is expected that the structure would find potential applications in active photonic devices, infrared imaging, detectors and tunable absorbers.

A Narrow-Band Multi-Resonant Metamaterial in Near-IR

We theoretically investigate a multi-resonant plasmonic metamaterial perfect absorber operating between 600 and 950 nm wavelengths. The presented device generates 100% absorption at two resonance wavelengths and delivers an ultra-narrow band (sub-20 nm) and high quality factor (Q=44) resonance. The studied perfect absorber is a metal–insulator–metal configuration where a thin MgF2 spacer is sandwiched between an optically thick gold layer and uniformly patterned gold circular nanodisc antennas. The localized and propagating nature of the plasmonic resonances are characterized and confirmed theoretically. The origin of the perfect absorption is investigated using the impedance matching and critical coupling phenomenon. We calculate the effective impedance of the perfect absorber and confirm the matching with the free space impedance. We also investigate the scattering properties of the top antenna layer and confirm the minimized reflection at resonance wavelengths by calculating the absorption and scattering cross sections. The excitation of plasmonic resonances boost the near-field intensity by three orders of magnitude which enhances the interaction between the metamaterial surface and the incident energy. The refractive index sensitivity of the perfect absorber could go as high as S=500 nm/RIU. The presented optical characteristics make the proposed narrow-band multi-resonant perfect absorber a favorable platform for biosensing and contrast agent based bioimaging.

Ultra-broadband metamaterial absorber from ultraviolet to long-wave infrared based on CMOS-compatible materials

Broadband absorption of electromagnetic waves in different wavelength regions is desired for applications ranging from highly efficient solar cells, waste heat harvesting, multi-color infrared (IR) detection to sub-ambient radiative cooling. Taper-shaped structures made up of alternating metal/dielectric multilayers offer the broadest absorption bandwidth so far, but face a trade-off between optical performance and material choice, i.e., those with the broadest bandwidth utilize exclusively CMOS-incompatible materials, hampering their large-scale applications. In this work, through careful examination of the unique material property of aluminum (Al) and zinc sulfide (ZnS), a sawtooth-like and a pyramid-like multilayer absorber is proposed, whose working bandwidth (0.2-15 µm) covers from ultraviolet (UV) all the way to long-wave infrared (LWIR) range, being compatible with CMOS technology at the same time. The working principle of broadband absorption is elucidated with effective hyperbolic metamaterial model plus the excitation of multiple slow-light modes. Absorption performance such as polarization and incidence-angle dependence are also investigated. The proposed Al-ZnS multilayer absorbers with ultra-broadband near-perfect absorption may find potential applications in infrared imaging and spectroscopy, radiative cooling, solar energy conversion, etc.

Wide-angle, wide-band, polarization-insensitive metamaterial absorber for thermal energy harvesting

We propose a wide-band metamaterial perfect absorber (MPA), using the coupling in the near-field of a quadruple split-ring resonator concentric with crossed ellipses. We designed the MPA with a metal–insulator-metal (MIM) structure for use in thermal energy harvesting. A gradient-based optimization approach was carried out to maximize the absorption of infrared (IR) radiation around 10 μm. Owing to the near-field coupling of resonators with optimal design parameters, the peaks of the absorption responses approach each other, thus broadening the overall bandwidth with almost unity absorptivity. The proposed design has a resonance at 10 μm resulting from magnetic polaritons (MPs) and thus maintains high absorption above 99% up to a range of incident-angles greater than 60° and exhibits a polarization-free behavior due to symmetry. When the optimal design was numerically examined to fabrication tolerances, it showed negligible sensitivities in the absorptivity with respect to design parameters. The strong electric field enhancement inside the split-ring gaps and between the ends of the cross arms and the surrounding ring enables designing MIM diodes to rectify the harvested thermal radiations at 288 K. MIM diodes can be built by the deposition of thin insulators to sit in these gaps. The MIM diode and MPA work together to harvest and rectify the incident IR radiation in a manner similar to the operation of rectennas. The MPA outperforms the traditional nano-antennas in impedance matching efficiency because of its higher resistance. Also, its dual-polarization reception capability doubles the rectenna efficiency. Our proposed MPA retained absorptivity more than 99% when coupled with MIM diodes whose resistances are in the range of 500 Ω–1 MΩ.

Three multispectral configurations of a snapshot kaleidoscope-based camera in long wavelength infrared spectral band

In the field of spectral imaging, numerous instruments use scanning-based technologies. However, the temporal dimension of these systems, whether to scan the spectrum or scan the scene, can be an issue for some applications. This is particularly the case when trying to observe and identify rapid temporal variations in a fixed scene or detecting objects of interest when moving. In this case, it is suitable to observe the desired spectral information of the scene simultaneously, and so-called snapshot systems have been thus investigated. In this paper, we study the ability of a kaleidoscope-based multiview camera to acquire multispectral information in the long wavelength infrared. Several strategies and technologies will be compared to add the spectral function inside the different blocks of a kaleidoscope-based camera: the front lens, the kaleidoscope, or the reimaging lens. The studied camera uses an uncooled infrared detector and thus must deal with the issue of having a large aperture.

Surface-plasmon-coupled optical force sensors based on metal-insulator-metal metamaterials with movable air gap

We proposed surface-plasmon-coupled optical force sensors based on metal–insulator–metal (MIM) metamaterials with a movable air gap as an insulator layer. The MIM metamaterial was composed of an air gap sandwiched by a metal nanodot array and a metal diaphragm, the resonant wavelength of which was red-shifted when the air gap was narrowed by applying a normal force. We designed and fabricated a prototype of the proposed sensor and confirmed that the MIM metamaterial could be used as a force sensor with larger sensitivity than a force sensor based on Fabry-Pérot interferometer (FPI).

Near-perfect broadband infrared metamaterial absorber utilizing nickel

We propose a thin, compact, broadband, polarization and angle insensitive metamaterial absorber based on a tungsten reflector, a silicon spacer, and a top pattern composed of a double square-like ring resonator utilizing nickel (Ni). In such a structure, a high absorption (above 80%) bandwidth ∼4.8µm from 3.52 up to 8.32 µm corresponding to the relative bandwidth ∼81% can be achieved with deeply subwavelength unit cell dimensions. Here the physical origin of the broadband absorption is associated with low Q-factor dipole modes of the top pattern inner and outer sides functioning as rectangular nanoantennas. Owing to the structural symmetry, the absorber shows a good incidence angle tolerance in the relatively wide range for both transverse electric and transverse magnetic polarizations. The effective parameters of the Ni-based absorber were retrieved using the constitutive effective medium theory, and the absorption characteristics of the effective medium and metamaterial were compared.

Graphene Plasmonics in Sensor Applications: A Review

Surface plasmon polaritons (SPPs) can be generated in graphene at frequencies in the mid-infrared to terahertz range, which is not possible using conventional plasmonic materials such as noble metals. Moreover, the lifetime and confinement volume of such SPPs are much longer and smaller, respectively, than those in metals. For these reasons, graphene plasmonics has potential applications in novel plasmonic sensors and various concepts have been proposed. This review paper examines the potential of such graphene plasmonics with regard to the development of novel high-performance sensors. The theoretical background is summarized and the intrinsic nature of graphene plasmons, interactions between graphene and SPPs induced by metallic nanostructures and the electrical control of SPPs by adjusting the Fermi level of graphene are discussed. Subsequently, the development of optical sensors, biological sensors and important components such as absorbers/emitters and reconfigurable optical mirrors for use in new sensor systems are reviewed. Finally, future challenges related to the fabrication of graphene-based devices as well as various advanced optical devices incorporating other two-dimensional materials are examined. This review is intended to assist researchers in both industry and academia in the design and development of novel sensors based on graphene plasmonics.

Critical Coupling of Visible Light Extends Hot-Electron Lifetimes for H2O2 Synthesis

Devices driven by above-equilibrium "hot" electrons are appealing for photocatalytic technologies, such as in situ H2O2 synthesis, but currently suffer from low (<1%) overall quantum efficiencies. Gold nanostructures excited by visible light generate hot electrons that can inject into a neighboring semiconductor to drive electrochemical reactions. Here, we designed and studied a metal-insulator-metal (MIM) structure of Au nanoparticles on a ZnO/TiO2/Al film stack, deposited through room-temperature, lithography-free methods. Light absorption, electron injection efficiency, and photocatalytic yield in this device are superior in comparison to the same stack without Al. Our device absorbs >60% of light at the Au localized surface plasmon resonance (LSPR) peak near 530 nm-a 5-fold enhancement in Au absorption due to critical coupling to an Al film. Furthermore, we show through ultrafast pump-probe spectroscopy that the Al-coupled samples exhibit a nearly 5-fold improvement in hot-electron injection efficiency as compared to a non-Al device, with the hot-electron lifetimes extending to >2 ps in devices photoexcited with fluence of 0.1 mJ cm-2. The use of an Al film also enhances the photocatalytic yield of H2O2 more than 3-fold in a visible-light-driven reactor. Altogether, we show that the critical coupling of Al films to Au nanoparticles is a low-cost, lithography-free method for improving visible-light capture, extending hot-carrier lifetimes, and ultimately increasing the rate of in situ H2O2 generation.

Achieving broadband absorption and polarization conversion with a vanadium dioxide metasurface in the same terahertz frequencies

We present the bifunctional design of a broadband absorber and a broadband polarization converter based on a switchable metasurface through the insulator-to-metal phase transition of vanadium dioxide. When vanadium dioxide is metal, the designed switchable metasurface behaves as a broadband absorber. This absorber is composed of a vanadium dioxide square, silica spacer, and vanadium dioxide film. Calculated results show that in the frequency range of 0.52-1.2 THz, the designed system can absorb more than 90% of the energy, and the bandwidth ratio is 79%. It is insensitive to polarization due to the symmetry, and can still work well even at large incident angles. When vanadium dioxide is an insulator, a terahertz polarizer is realized by a simple anisotropic metasurface. Numerical calculation shows that efficient conversion between two orthogonal linear polarizations can be achieved. Reflectance of a cross-polarized wave can reach 90% from 0.42 THz to 1.04 THz, and the corresponding bandwidth ratio is 85%. This cross-polarized converter has the advantages of wide angle, broad bandwidth, and high efficiency. So our design can realize bifunctionality of broadband absorption and polarization conversion between 0.52 THz and 1.04 THz. This architecture could provide one new way to develop switchable photonic devices and functional components in phase change materials.

Special Issue: New Horizon of Plasmonics and Metamaterials

Plasmonics and metamaterials are growing fields that consistently produce new technologies for controlling electromagnetic waves. Many important advances in both fundamental knowledge and practical applications have been achieved in conjunction with a wide range of materials, structures and wavelengths, from the ultraviolet to the microwave regions of the spectrum. In addition to this remarkable progress across many different fields, much of this research shares many of the same underlying principles, and so significant synergy is expected. This Special Issue introduces the recent advances in plasmonics and metamaterials and discusses various applications, while addressing a wide range of topics in order to explore the new horizons emerging for such research.

A Perfect Absorber Based on Similar Fabry-Perot Four-Band in the Visible Range

A simple metamaterial absorber is proposed to achieve near-perfect absorption in visible and near-infrared wavelengths. The absorber is composed of metal-dielectric-metal (MIM) three-layer structure. The materials of these three-layer structures are Au, SiO₂, and Au. The top metal structure of the absorber is composed of hollow three-dimensional metal rings regularly arranged periodically. The results show that the high absorption efficiency at a specific wavelength is mainly due to the resonance of the Fabry-Perot effect (FP) in the intermediate layer of the dielectric medium, resulting in the resonance light being trapped in the middle layer, thus improving the absorption efficiency. The almost perfect multiband absorption, which is independent of polarization angle and insensitivity of incident angle, lends the absorber great application prospects for filtering and optoelectronics.

Perfect Absorption Efficiency Circular Nanodisk Array Integrated with a Reactive Impedance Surface with High Field Enhancement

Infrared (IR) absorbers based on a metal–insulator–metal (MIM) have been widely investigated due to their high absorption performance and simple structure. However, MIM absorbers based on ultrathin spacers suffer from low field enhancement. In this study, we propose a new MIM absorber structure to overcome this drawback. The proposed absorber utilizes a reactive impedance surface (RIS) to boost field enhancement without an ultrathin spacer and maintains near-perfect absorption by impedance matching with the vacuum. The RIS is a metallic patch array on a grounded dielectric substrate that can change its surface impedance, unlike conventional metallic reflectors. The final circular nanodisk array mounted on the optimum RIS offers an electric field enhancement factor of 180 with nearly perfect absorption of 98% at 230 THz. The proposed absorber exhibits robust performance even with a change in polarization of the incident wave. The RIS-integrated MIM absorber can be used to enhance the sensitivity of a local surface plasmon resonance (LSPR) sensor and surface-enhanced IR spectroscopy.

Generalized circuit model for all-dielectric narrowband metasurface absorbers

All-dielectric metasurface absorbers have great potential in many scientific and technical applications. The emerging metasurfaces show strong and versatile capabilities in controlling absorptance, reflectance, and transmittance of electromagnetic waves. In this work, we propose and investigate all-dielectric metasurface absorbers with an equivalent circuit model. In the proposed circuit model, we satisfy the first Kerker condition. To verify the accuracy of the proposed model, the obtained results for an all-dielectric cubic metasurface absorber are compared with the existing experimental data. Moreover, using the proposed circuit model, we propose a hemisphere structure and compare the results of the proposed model with those of full-wave simulations. With this novel structure, we achieve higher absorptance and quality factor in comparison to a cubic one. Additionally, our proposed model reduces the calculation time and needs less memory compared to full-wave simulations. The results of the circuit model have an acceptable agreement with the experimental data and those of full-wave simulations. The proposed circuit model is simple yet general. It provides physical insight into the design and operation of various sub-wavelength structures in the broad frequency range, including THz and visible regions.

Incandescent Light Bulbs Based on a Refractory Metasurface

A thermal radiation light source, such as an incandescent light bulb, is considered a legacy light source with low luminous efficacy. However, it is an ideal energy source converting light with high efficiency from electric power to radiative power. In this work, we evaluate a thermal radiation light source and propose a new type of filament using a refractory metasurface to fabricate an efficient light bulb. We demonstrate visible-light spectral control using a refractory metasurface made of tantalum with an optical microcavity inserted into an incandescent light bulb. We use a nanoimprint method to fabricate the filament that is suitable for mass production. A 1.8 times enhancement of thermal radiation intensity is observed from the microcavity filament compared to the flat filament. Then, we demonstrate the thermal radiation control of the metasurface using a refractory plasmonic cavity made of hafnium nitride. A single narrow resonant peak is observed at the designed wavelength as well as the suppression of thermal radiation in wide mid-IR range under the condition of constant surface temperature.

Elimination of Unwanted Modes in Wavelength-Selective Uncooled Infrared Sensors with Plasmonic Metamaterial Absorbers using a Subtraction Operation

Wavelength- or polarization-selective uncooled infrared (IR) sensors have various applications, such as in fire detection, gas analysis, hazardous material recognition, biological analysis, and polarimetric imaging. The unwanted modes originating due to the absorption by the materials used in these sensors, other than plasmonic metamaterial absorbers (PMAs), cause serious issues by degenerating the wavelength or polarization selectivity. In this study, we demonstrate a method for eliminating these unwanted modes in wavelength- or polarization-selective uncooled IR sensors with various PMAs, using a subtraction operation and a reference pixel. The aforementioned sensors and the reference pixels were fabricated using a complementary metal oxide semiconductor and micromachining techniques. We fabricated the reference pixel with the same structure as the PMA sensors, except a flat mirror was formed on the absorber surface instead of PMAs. The spectral responsivity measurements demonstrated that single-mode detection can be achieved through the subtraction operation with the reference pixel. The method demonstrated in this study can be applied to any type of uncooled IR sensors to create high-performance wavelength- or polarization-selective absorbers capable of multispectral or polarimetric detection.

Ultrathin and Electrically Tunable Metamaterial with Nearly Perfect Absorption in Mid-Infrared

Metamaterials integrated with graphene exhibit tremendous freedom in tailoring their optical properties, particularly in the infrared region, and are desired for a wide range of applications, such as thermal imaging, cloaking, and biosensing. In this article, we numerically and experimentally demonstrate an ultrathin (total thickness < λ 0 / 15 ) and electrically tunable mid-infrared perfect absorber based on metal–insulator–metal (MIM) structured metamaterials. The Q-values of the absorber can be tuned through two rather independent parameters, with geometrical structures of metamaterials tuning radiation loss (Qr) of the system and the material loss (tanδ) to further change mainly the intrinsic loss (Qa). This concise mapping of the structural and material properties to resonant mode loss channels enables a two-stage optimization for real applications: geometrical design before fabrication and then electrical tuning as a post-fabrication and fine adjustment knob. As an example, our device demonstrates an electrical and on-site tuning of ~5 dB change in absorption near the perfect absorption region. Our work provides a general guideline for designing and realizing tunable infrared devices and may expand the applications of perfect absorbers for mid-infrared sensors, absorbers, and detectors in extreme spatial-limited circumstances.

Core-shell particles as efficient broadband absorbers in infrared optical range

We demonstrate that efficient broadband absorption of infrared radiation can be obtained with deeply subwavelength spherical dielectric particles covered by a thin metal layer. Considerations based on Mie theory and the quasi-static approximation reveala wide range of configuration parameters, within which the absorption cross section reaches the geometrical one and exceeds more than by order of magnitude the scattering cross section in the infrared spectrum. We show that the absorption is not only efficient but also broadband with the spectral width being close to the resonant wavelength corresponding to the maximum of the absorption cross section. We obtain a simple analytical expression for the absorption resonance that allows one to quickly identify the configuration parameters ensuring strong infrared absorption in a given spectral range. Relation between the absorption resonance and excitation of the short-range surface palsmon modes in the metal shell of particles is demonstrated and discussed. Our results can be used as practical guidelines for realization of efficient broadband infrared absorbers of subwavelength sizes desirable in diverse applications.

Large-Area Broadband Near-Perfect Absorption from a Thin Chalcogenide Film Coupled to Gold Nanoparticles

Perfect absorbers that can efficiently absorb electromagnetic waves over a broad spectral range are crucial for energy harvesting, light detection, and optical camouflage. Recently, perfect absorbers based on a metasurface have attracted intensive attention. However, high-performance metasurface absorbers in the visible spectra require strict fabrication tolerances, and this is a formidable challenge. Moreover, fabricating subwavelength meta-atoms requires a top-down approach, thus limiting their scalability and spectral applicability. Here, we introduce a plasmonic nearly perfect absorber that exhibits a measured polarization-insensitive absorptance of ∼92% across the spectral region from 400 to 1000 nm. The absorber is realized via a one-step self-assembly deposition of 50 nm gold (Au) nanoparticle (NP) clusters onto a 35 nm-thick Ge₂Sb₂Te₅ (GST225) chalcogenide film. An excellent agreement between the measured and theoretically simulated absorptance was found. The coalescence of the lossy GST225 dielectric layer and high density of localized surface plasmon resonance modes induced by the randomly distributed Au NPs play a vital role in obtaining the nearly perfect absorptance. The exceptionally high absorptance together with large-area high-throughput self-assembly fabrication demonstrates their potential for industrial-scale manufacturability and consequential widespread applications in thermophotovoltaics, photodetection, and sensing.

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

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

Recent Advances of Plasmonic Nanoparticles and their Applications

In the past half-century, surface plasmon resonance in noble metallic nanoparticles has been an important research subject. Recent advances in the synthesis, assembly, characterization, and theories of traditional and non-traditional metal nanostructures open a new pathway to the kaleidoscopic applications of plasmonics. However, accurate and precise models of plasmon resonance are still challenging, as its characteristics can be affected by multiple factors. We herein summarize the recent advances of plasmonic nanoparticles and their applications, particularly regarding the fundamentals and applications of surface plasmon resonance (SPR) in Au nanoparticles, plasmon-enhanced upconversion luminescence, and plasmonic chiral metasurfaces.

Independent Manipulating of Orthogonal-Polarization Terahertz Waves Using A Reconfigurable Graphene-Based Metasurface

Viewing the trend of miniaturization and integration in modern electronic device design, a reconfigurable multi-functional graphene-based metasurface is proposed in this paper. By virtue of the reconfigurability of reflection patterns, this metasurface is able to independently manipulate orthogonal linearly polarized terahertz wave. The building blocks of the proposed metasurface are series of graphene-strips-based unit-cells. Each unit-cell consists of two orthogonal graphene strips and a grounded substrate, which has anisotropic responses for each of orthogonal polarizations (x-polarized and y-polarized waves). The reflection phases of both x- and y-polarized waves can be controlled independently through separate electrical tuning. Based on the proposed metasurface, functionalities including beam splitting, beam deflecting, and linear-to-circular polarization converting using a shared aperture are numerically demonstrated and analyzed. Simulation results demonstrate excellent performance, which is consistent with the theorized expectations. This work paves the way for enhancing the miniaturization of modern electronic/optical devices and potentially has important applications in the next-generation information systems for communication, sensing, and imaging.

Independently Tunable Fano Resonances Based on the Coupled Hetero-Cavities in a Plasmonic MIM System

In this paper, based on coupled hetero-cavities, multiple Fano resonances are produced and tuned in a plasmonic metal-insulator-metal (MIM) system. The structure comprises a rectangular cavity, a side-coupled waveguide, and an upper-coupled circular cavity with a metal-strip core, used to modulate Fano resonances. Three Fano resonances can be realized, which originate from interference of the cavity modes between the rectangular cavity and the metal-strip-core circular cavity. Due to the different cavity-cavity coupling mechanisms, the three Fano resonances can be divided into two groups, and each group of Fano resonances can be well tuned independently by changing the different cavity parameters, which can allow great flexibility to control multiple Fano resonances in practice. Furthermore, through carefully adjusting the direction angle of the metal-strip core in the circular cavity, the position and lineshape of the Fano resonances can be easily tuned. Notably, reversal asymmetry takes place for one of the Fano resonances. The influence of the direction angle on the figure of merit (FOM) value is also investigated. A maximum FOM of 3436 is obtained. The proposed structure has high transmission, sharp Fano lineshape, and high sensitivity to change in the background refractive index. This research provides effective guidance to tune multiple Fano resonances, which has important applications in nanosensors, filters, modulators, and other related plasmonic devices.

3D-Printed Low-Cost Dielectric-Resonator-Based Ultra-Broadband Microwave Absorber Using Carbon-Loaded Acrylonitrile Butadiene Styrene Polymer

In this study, an ultra-broadband dielectric-resonator-based absorber for microwave absorption is numerically and experimentally investigated. The designed absorber is made of the carbon-loaded Acrylonitrile Butadiene Styrene (ABS) polymer and fabricated using the 3D printing technology based on fused deposition modeling with a quite low cost. Profiting from the fundamental dielectric resonator (DR) mode, the higher order DR mode and the grating mode of the dielectric resonator, the absorber shows an absorptivity higher than 90% over the whole ultra-broad operating band from 3.9 to 12 GHz. The relative bandwidth can reach over 100% and cover the whole C-band (4–8 GHz) and X-band (8–12 GHz). Utilizing the numerical simulation, we have discussed the working principle of the absorber in detail. What is more, the absorption performance under different incident angles is also simulated, and the results indicate that the absorber exhibits a high absorptivity at a wide angle of incidence. The advantages of low cost, ultra-broad operating band and a wide-angle feature make the absorber promising in the areas of microwave measurement, stealth technology and energy harvesting.

Wide Angle of Incidence-Insensitive Polarization-Independent THz Metamaterial Absorber for Both TE and TM Mode Based on Plasmon Hybridizations

An ultra-wide-angle THz metamaterial absorber (MA) utilizing sixteen-circular-sector (SCR) resonator for both transverse electric (TE) and transverse magnetic (TM) mode is designed and investigated numerically. At normal incidence, the absorptivity of the proposed MA is higher than 93.7% at 9.05 THz for different polarization angles, due to the rotational symmetry structure of the unit cell. Under oblique incidence, the absorptivity can still exceed 90%, even when the incident angle is up to 70° for both TE and TM mode. Especially, the frequency variation in TE mode is less than 0.25% for different incident angles from 0° to 70°. The electric field (E_z) distributions are used to explain the absorption mechanism. Numerical simulation results show that the high absorption with wide-angle independence stems from fundamental dipole resonance and gap surface plasmons. The broadband deep-infrared MA is also obtained by stacking three metal-dielectric layers. The designed MA has great potential in bolometric pixel elements, biomedical sensors, THz imaging, and solar cells.