A perfect absorber made of a graphene micro-ribbon metamaterial

Strong Laser Emission Modulation by Coherent Perfect Absorption Inside Complex-Coupled Distributed Feedback Laser Diodes

The proof-of-concept of the exploitation of Coherent Perfect Absorption (CPA) in electrically-injected distributed-feedback laser sources is reported. Capitalizing on the essence of CPA as "light extinction by light", an integrated laser-modulator scheme emerges. The key ingredient compared to conventional single-frequency laser diodes is a careful periodic in-phase modulation of both real and imaginary parts of the complex grating index profile that enables both single-frequency operation and 40 dB line purity at the Bragg frequency. It is shown that this combination is most apt for the operation of CPA as a modulation mechanism that respects the laser spectral purity. The specific proof-of-concept is based on an ultra-short external cavity formed by a metallic micro-mirror, whose role is to generate the second beam of more conventional CPA interferometric approaches. The implemented complex-coupled grating is compatible with existing industrial technologies and promising for real-life laser source applications. Furthermore, the concept can be directly transferred to other material platforms and other wavelengths ranging from terahertz to ultraviolet.

Actively tunable and switchable terahertz metamaterials with multi-band perfect absorption and polarization conversion

In this paper, we theoretically present and numerically demonstrate an actively tunable and switchable multi-functional metamaterial based on vanadium dioxide (VO2) and graphene in the terahertz region. When VO2 is in the metallic phase, the proposed metamaterial serves as a multi-band perfect absorber, which exhibits the characteristics of insensitive polarization and robust tolerance for variations of the incidence angle. When VO2 is in the insulator phase, the proposed metamaterial acts as a polarization converter, which can simultaneously achieve perfect linear-to-linear and linear-to-circular polarization conversions. The simulation results show the cross-polarization conversion rate can reach ∼100% at the frequency region from 6.09 to 6.43 THz as well as 8.15 THz. Moreover, the ellipticity of linear-to-circular polarization conversion reaches ±1 at frequencies of 5.75 and 8.34 THz, respectively, which means the linear polarization waves can be completely converted into circular polarization waves. The proposed metamaterial provides new insight for the design of optoelectronic devices with multi-functionality in the terahertz region.

Tunable optical differential operation based on graphene at a telecommunication wavelength

Optical differential operation based on the photonic spin Hall effect(SHE) has attracted extensive attention in image processing of edge detection, which has advantages of high speed, parallelism, and low power consumption. Here, we theoretically demonstrate tunable optical differential operation in a four-layered nanostructure of prism-graphene-air gap-substrate. It is shown that the spatial differentiation arises inherently from the photonic SHE. Furthermore, we find that the transverse spin-Hall shift induced by the photonic SHE changes dramatically near the Brewster angle with the incident angle increases at a telecommunication wavelength. Meanwhile, the Fermi energy of graphene and the thickness of the air gap can affect the transverse spin shift. Interestingly, we can easily adjust the Fermi energy of graphene in real time through external electrostatic field biasing, enabling fast edge imaging switching at a telecommunication wavelength. This may provide a potential way for future tunable spin-photonic devices, and open up more possible applications for artificial intelligence, such as target recognition, biomedical imaging, and edge detection.

Design of a Reconfigurable Ultra-Wideband Terahertz Polarization Rotator Based on Graphene Metamaterial

In this work, a reconfigurable ultra-wideband transmissive terahertz polarization rotator based on graphene metamaterial is proposed that can switch between two states of polarization rotation within a broad terahertz band by changing the Fermi level of graphene. The proposed reconfigurable polarization rotator is based on a two-dimensional periodic array of multilayer graphene metamaterial structure, which is composed of metal grating, graphene grating, silicon dioxide thin film, and a dielectric substrate. The graphene metamaterial can achieve high co-polarized transmission of a linearly polarized incident wave at the off-state of the graphene grating without applying the bias voltage. Once the specially designed bias voltage is applied to change the Fermi level of graphene, the polarization rotation angle of linearly polarized waves is switched to 45° by the graphene metamaterial at the on-state. The working frequency band with 45-degree linear polarized transmission remaining above 0.7 and the polarization conversion ratio (PCR) above 90% is from 0.35 to 1.75 THz, and the relative bandwidth reaches 133.3% of the central working frequency. Furthermore, even with oblique incidence at large angles, the proposed device retains high-efficiency conversion in a broad band. The proposed graphene metamaterial offers a novel approach for the design of a terahertz tunable polarization rotator and is expected to be applied in the applications of terahertz wireless communication, imaging, and sensing.

Triple-Band Surface Plasmon Resonance Metamaterial Absorber Based on Open-Ended Prohibited Sign Type Monolayer Graphene

This paper introduces a novel metamaterial absorber based on surface plasmon resonance (SPR). The absorber is capable of triple-mode perfect absorption, polarization independence, incident angle insensitivity, tunability, high sensitivity, and a high figure of merit (FOM). The structure of the absorber consists of a sandwiched stack: a top layer of single-layer graphene array with an open-ended prohibited sign type (OPST) pattern, a middle layer of thicker SiO₂, and a bottom layer of the gold metal mirror (Au). The simulation of COMSOL software suggests it achieves perfect absorption at frequencies of f_I = 4.04 THz, f_II = 6.76 THz, and f_III = 9.40 THz, with absorption peaks of 99.404%, 99.353%, and 99.146%, respectively. These three resonant frequencies and corresponding absorption rates can be regulated by controlling the patterned graphene's geometric parameters or just adjusting the Fermi level (E_F). Additionally, when the incident angle changes between 0~50°, the absorption peaks still reach 99% regardless of the kind of polarization. Finally, to test its refractive index sensing performance, this paper calculates the results of the structure under different environments which demonstrate maximum sensitivities in three modes: S_I = 0.875 THz/RIU, S_II = 1.250 THz/RIU, and S_III = 2.000 THz/RIU. The FOM can reach FOM_I = 3.74 RIU^-1, FOM_II = 6.08 RIU^-1, and FOM_III = 9.58 RIU^-1. In conclusion, we provide a new approach for designing a tunable multi-band SPR metamaterial absorber with potential applications in photodetectors, active optoelectronic devices, and chemical sensors.

Strongly suppressed diffuse scattering in periodic graphene metamaterials

As an emerging two-dimensional material, graphene offers an alternative material platform for exploring new metamaterial phenomena and device functionalities. In this work, we examine diffuse scattering properties in graphene metamaterials. We take periodic graphene nanoribbons as a representative example and show that diffuse reflection in graphene metamaterials as dominated by diffraction orders is restricted to wavelengths less than that of first-order Rayleigh anomaly, and is enhanced by plasmonic resonances in graphene nanoribbons, as similar to metamaterials made of noble metals. However, the overall magnitude of diffuse reflection in graphene metamaterial is less than 10^-2 due to the large period to nanoribbon size ratio and ultra-thin thickness of the graphene sheet, which suppress the grating effect from the structural periodicity. Our numerical results indicate that, in contrast to the cases of metallic metamaterials, diffuse scattering plays a negligible role in spectral characterization of graphene metamaterials in cases with large resonance wavelength to graphene feature size ratio, which corresponds to typical chemical vapor deposition (CVD)-grown graphene with relatively small Fermi energy. These results shed light on fundamental properties of graphene nanostructures and are helpful in designing graphene metamaterials for applications in infrared sensing, camouflaging, and photodetection, etc.

Thermal metasurface with tunable narrowband absorption from a hybrid graphene/silicon photonic crystal resonance

We report the design of a tunable, narrowband, thermal metasurface that employs a hybrid resonance generated by coupling a tunable permittivity graphene ribbon to a silicon photonic crystal. The gated graphene ribbon array, proximitized to a high quality factor Si photonic crystal supporting a guided mode resonance, exhibits tunable narrowband absorbance lineshapes (Q > 10,000). Actively tuned Fermi level modulation in graphene with applied gate voltage between high absorptivity and low absorptivity states gives rise to absorbance on/off ratios exceeding 60. We employ coupled-mode theory as a computationally efficient approach to elements of the metasurface design, demonstrating an orders of magnitude speedup over typical finite element computational methods.

THz broadband and dual-channel perfect absorbers based on patterned graphene and vanadium dioxide metamaterials

This paper proposes a novel and perfect absorber based on patterned graphene and vanadium dioxide hybrid metamaterial, which can not only achieve wide-band perfect absorption and dual-channel absorption in the terahertz band, but also realize their conversion by adjusting the temperature to control the metallic or insulating phase of VO₂. Firstly, the absorption spectrum of the proposed structure is analyzed without graphene, where the absorption can reach as high as 100% at one frequency point (f = 5.956 THz) when VO₂ is in the metal phase. What merits attention is that the addition of graphene above the structure enhances the almost 100% absorption from one frequency point (f = 5.956 THz) to a wide frequency band, in which the broadband width records 1.683 THz. Secondly, when VO₂ is the insulating phase, the absorption of the metamaterial structure with graphene outperforms better, and two high absorption peaks are formed, logging 100% and 90.7% at f₃ = 5.545 THz and f₄ = 7.684 THz, respectively. Lastly, the adjustment of the Fermi level of graphene from 0.8 eV to 1.1 eV incurs an obvious blueshift of the absorption spectra, where an asynchronous optical switch can be achieved at fK₁ = 5.782 THz and fK₂ = 6.898 THz. Besides, the absorber exhibits polarization sensitivity at f₃ = 5.545 THz, and polarization insensitivity at f₄ = 7.684 THz with the shift in the polarization angle of incident light from 0° to 90°. Accordingly, this paper gives insights into the new method that increases the high absorption width, as well as the great potential in the multifunctional modulator.

Mixed finite element numerical mode matching method for designing infrared broadband polarization-independent metamaterial absorbers

Conventional numerical methods have found widespread applications in the design of metamaterial structures, but their computational costs can be high due to complex three-dimensional discretization needed for large complex problems. In this work, we apply a recently developed numerical mode matching (NMM) method to design a black phosphorus (BP) absorber. NMM transforms a complex three-dimensional (3D) problem into 2D numerical eigenvalue problems plus a 1-D analytical propagation solution, thus it can save a lot of computational costs. BP is treated as a 2D surface and represented by the anisotropic surface conductance. With a realistic simulation study, we show that our method is more accurate and efficient than the standard finite element method (FEM). Our designed absorber can achieve an average absorption of 97.4% in the wavelength range of 15 to 23 μm under normal incidence. Then, we investigate the physical mechanism of the absorber, tuning the geometric parameters and electron doping to optimize the performance. In addition, the absorption spectra under oblique incidence and arbitrary polarization are studied. The results confirm that our absorber is polarization-independent and has high absorption at large incident angles. Our work validates the superiority of NMM and provides a new simulation platform for emerging metamaterial device design.

Metasurface Terahertz Perfect Absorber with Strong Multi-Frequency Selectivity

In this paper, we design a metasurface terahertz perfect absorber with multi-frequency selectivity and good incident angle compatibility using a double-squared open ring structure. Simulations reveal five selective absorption peaks located at 0-1.2 THz with absorption 94.50% at 0.366 THz, 99.99% at 0.507 THz, 95.65% at 0.836 THz, 98.80% at 0.996 THz, and 86.70% at 1.101 THz, caused by two resonant absorptions within the fundamental unit (fundamental mode of resonance absorption, FRA) and its adjacent unit (supermodel of resonance absorption, SRA) in the structure, respectively, when the electric field of the electromagnetic wave is incident perpendicular to the opening. The strong frequency selectivity at 0.836 THz with a Q-factor of 167.20 and 0.996 THz with a Q-factor of 166.00 is due to the common effect of the FRA and SRA. Then, the effect of polarized electromagnetic wave modes (TE and TM modes) at different angles of incidence (θ) and the size of the open rings on the device performance is analyzed. We find that for the TM mode, the absorption of the resonance peak changes only slightly at θ = 0-80°, which explains this phenomenon. The frequency shift of the absorption peaks caused by the size change of the open rings is described reasonably by an equivalent RLC resonant circuit. Next, by adjusting two-dimensional materials and photosensitive semiconductor materials embedded in the unit structure, the designed metasurface absorber has excellent tunable modulation. The absorption modulation depth (MD) reaches ≈100% using the conductivity of photosensitive semiconductor silicon (σ_SI-ps), indicating excellent control of the absorption spectrum. Our results can greatly promote the absorption of terahertz waves, absorption spectrum tunability, and frequency selectivity of devices, which are useful in the applications such as resonators, bio-detection, beam-controlled antennas, hyperspectral thermal imaging systems, and sensors.

Tunable high-quality-factor absorption in a graphene monolayer based on quasi-bound states in the continuum

A tunable graphene absorber, composed of a graphene monolayer and a substrate spaced by a subwavelength dielectric grating, is proposed and investigated. Strong light absorption in the graphene monolayer is achieved due to the formation of embedded optical quasi-bound states in the continuum in the subwavelength dielectric grating. The physical origin of the absorption with high quality factor is examined by investigating the electromagnetic field distributions. Interestingly, we found that the proposed absorber possesses high spatial directivity and performs similar to an antenna, which can also be utilized as a thermal emitter. Besides, the spectral position of the absorption peak can not only be adjusted by changing the geometrical parameters of dielectric grating, but it is also tunable by a small change in the Fermi level of the graphene sheet. This novel scheme to tune the absorption of graphene may find potential applications for the realization of ultrasensitive biosensors, photodetectors, and narrow-band filters.

Engineering van der Waals Materials for Advanced Metaphotonics

The outstanding chemical and physical properties of 2D materials, together with their atomically thin nature, make them ideal candidates for metaphotonic device integration and construction, which requires deep subwavelength light-matter interaction to achieve optical functionalities beyond conventional optical phenomena observed in naturally available materials. In addition to their intrinsic properties, the possibility to further manipulate the properties of 2D materials via chemical or physical engineering dramatically enhances their capability, evoking new science on light-matter interaction, leading to leaped performance of existing functional devices and giving birth to new metaphotonic devices that were unattainable previously. Comprehensive understanding of the intrinsic properties of 2D materials, approaches and capabilities for chemical and physical engineering methods, the resulting property modifications and novel functionalities, and applications of metaphotonic devices are provided in this review. Through reviewing the detailed progress in each aspect and the state-of-the-art achievement, insightful analyses of the outstanding challenges and future directions are elucidated in this cross-disciplinary comprehensive review with the aim to provide an overall development picture in the field of 2D material metaphotonics and promote rapid progress in this fast emerging and prosperous field.

Analysis of Electromagnetic Properties of New Graphene Partial Discharge Sensor Electrode Plate Material

Advanced sensing and measurement technology is the key to realizing the transparent power grid and electric internet of things. Meanwhile, sensors, as an indispensable part of the smart grid, can monitor, collect, process, and transmit various types of data information of the power system in real-time. In this way, it is possible to further control the power system. Among them, partial discharge (PD) sensors are of great importance in the fields of online monitoring of insulation condition, intelligent equipment control, and power maintenance of power systems. Therefore, this paper intends to focus on advanced sensing materials and study new materials for the improvement for partial discharge sensors. As two-dimensional material, graphene is introduced. The electromagnetic properties of graphene partial discharge sensor electrode plate material are analyzed theoretically. By studying the influence of different chemical potential, relaxation time, temperature, and frequency, we obtain the changing curve of conductivity, dielectric constant, and refractive index. A linear regression model based on the least-squares method was developed for the three electromagnetic properties. Finally, the simulation and experiment verified that the graphene partial discharge sensor has better absorption of the partial discharge signal. This study can apply to the design of graphene partial discharge sensors.

Mid-Infrared Sensor Based on Dirac Semimetal Coupling Structure

A multilayer structure based on Dirac semimetals is investigated, where long-range surface plasmon resonance (LRSPR) of a dielectric layer/Dirac semimetal/dielectric layer are coupled with surface plasmon polaritons (SPPs) on graphene to substantially improve the Goos−Hänchen (GH) shift of Dirac semimetals in the mid-infrared band. This has important implications for the study of mid-infrared sensors. We studied the reflection coefficient and phase of this multilayer structure using a generalized transport matrix. We established that subtle changes in the refractive index of the sensing medium and the Fermi energy of the Dirac semimetal significantly affected the GH shift. Our numerical simulations show that the sensitivity of the coupling structure is more than 2.7×107 λ/RIU, which can be used as a potential new sensor application. The novelty of this work is the design of a tunable, highly sensitive, and simple structured mid-infrared sensor that takes advantage of the excellent properties of Dirac semimetals.

Independently tunable multi-band terahertz absorber based on graphene sheet and nanoribbons

A multi-band terahertz (THz) absorber based on graphene sheet and nanoribbons is proposed and investigated. In the studied frequency range, five absorption peaks are observed, with four originate from lateral Fabry-Perot resonance (LFPR) and one originates from guided-mode resonance (GMR). The LFPR and GMR peaks behave differently when geometric parameters are adjusted, which makes independent tuning possible. When period increases, the GMR peak red shifts and the frequencies of LFPR peaks remain almost unchanged. On the contrary, as nanoribbon width increases, the frequency of GMR remains almost unchanged while that of LFPRs decrease significantly. With increasing top dielectric layer thickness, the LFPR peaks blue shift while the GMR peak red shifts. In addition, the absorber has the merit of multi-band high absorptivity and frequency stability under large angle oblique incidence. The proposed terahertz absorber may benefit the areas of medical imaging, sensing, non-destructive testing, THz communications and other applications.

Absorption and scattering in perfect thermal radiation absorber-emitter metasurfaces

Detailed spectral analysis of radiation absorption and scattering behaviors of metasurfaces was carried out via finite-difference time-domain (FDTD) photonic simulations. It revealed that, for typical metal-insulator-metal (MIM) nanodisc metasurfaces, absorbance and scattering cross-sections exhibit a ratio of σ_abs/σ_sca = 1 at the absorption peak spectral position. This relationship was likewise found to limit the attainable photo-thermal conversion efficiency in experimental and application contexts. By increasing the absorption due to optical materials, such as Cr metal nano-films typically used as an adhesion layer, it is possible to control the total absorption efficiency η = σ_abs/σ_sca and to make it the dominant extinction mechanism. This guided the design of MIM metasurfaces tailored for near-perfect-absorption and emission of thermal radiation. We present the fabrication as well as the numerical and experimental spectral characterisation of such optical surfaces.

Graphene optical modulators using bound states in the continuum

Graphene-based optical modulators have been widely investigated due to the high mobility and tunable permittivity of graphene. However, achieving a high modulation depth with a low insertion loss is challenging owing to low graphene-light interaction. To date, only waveguide-type modulators have been extensively studied to improve light-graphene interaction, and few free-space type modulators have been demonstrated in the optical communication wavelength range. In this study, we propose two graphene-based optical free-space type modulators in a simple silicon photonic crystal structure that supports bound states in the continuum. The designed modulator with an ultra-high quality factor from the bound states in the continuum achieves a high modulation depth (MD = 0.9972) and low insertion loss (IL = 0.0034) with a small Fermi level change at the optical communication wavelength. In addition, the proposed modulators support outstanding modulation performance in the normal chemical vapor deposition (CVD) graphene (mobility = 0.5 m²/Vs). We believe the scheme may pave the way for graphene-based optical active devices.

Moiré graphene nanoribbons: nearly perfect absorptions and highly efficient reflections with wide angles

The strong absorption and reflection from atomically thin graphene nanoribbons has been demonstrated over the past decade. However, due to the significant band dispersion of graphene nanoribbons, the angle of incident wave has remained limited to a very narrow range. Obtaining strong absorption and reflection with a wide range of incident angles from atomically thin graphene layers has remained an unsolvable problem. Here, we construct a tunable moiré superlattice composed of a pair of graphene nanoribbon arrays to achieve this goal. By designing the interlayer coupling between two graphene nanoribbon arrays with mismatched periods, the moiré flat bands and the localization of their eigen-fields was realized. Based on the moiré flat bands of graphene nanoribbons, highly efficient reflection and nearly perfect absorption was achieved with a wide range of incident angles. Even more interesting, is how these novel phenomena can be tuned through the adjustment of the graphene's Fermi energy, either electrostatically or chemically. Our designed moiré graphene nanoribbons suggest a promising platform to engineer moiré physics with tunable behaviors, and may have potential applications in the field of wide-angle absorbers and reflectors in the mid-infrared region.

Polarization and incidence insensitive analogue of electromagnetically induced reflection metamaterial with high group delay

In this work, we demonstrate an analogue of electromagnetically induced reflection (EIR) effect with hybrid structure consisting of a silica (SiO₂) square array layer embedded in graphene-dielectric-Au film constructed F-P cavity. It is shown that the SiO₂ square array and F-P cavity create transverse waveguide with high quality factor (Q-factor) and longitudinal F-P modes, and their destructive interference effectively forms the EIR-like effect, which benefits for obtaining high group delay. In addition, the C4 symmetric structure ensures the polarization-independent for this EIR-like effect. With high Q-factor at the reflection window, the ultra-high group delay as high as 245 ps can be obtained. This structure will be useful to develop the EIT-like devices with excellent performance such as high group delay, polarization and incident insensitivity, and environmental stability.

Tunable terahertz metasurface platform based on CVD graphene plasmonics

Graphene plasmonics provides a powerful means to extend the reach of metasurface technology to the terahertz spectral region, with the distinct advantage of active tunability. Here we introduce a comprehensive design platform for the development of THz metasurfaces capable of complex wavefront manipulation functionalities, based on ribbon-shaped graphene plasmonic resonators combined with metallic antennas on a vertical cavity. Importantly, this approach is compatible with the electrical characteristics of graphene grown by chemical vapor deposition (CVD), which can provide the required mm-scale dimensions unlike higher-mobility exfoliated samples. We present a single device structure that can be electrically reconfigured to enable multiple functionalities with practical performance metrics, including tunable beam steering and focusing with variable numerical aperture. These capabilities are promising for a significant impact in a wide range of THz technologies for sensing, imaging, and future wireless communications.

Polarization-Insensitive Broadband THz Absorber Based on Circular Graphene Patches

A polarization-insensitive broadband terahertz absorber based on single-layer graphene metasurface has been designed and simulated, in which the graphene metasurface is composed of isolated circular patches. After simulation and optimization, the absorption bandwidth of this absorber with more than 90% absorptance is up to 2 THz. The simulation results demonstrate that the broadband absorption can be achieved by combining the localized surface plasmon (LSP) resonances on the graphene patches and the resonances caused by the coupling between them. The absorption bandwidth can be changed by changing the chemical potential of graphene and the structural parameters. Due to the symmetrical configuration, the proposed absorber is completely insensitive to polarization and have the characteristics of wide angle oblique incidence that they can achieve broadband absorption with 70% absorptance in the range of incident angle from 0° to 50° for both TE and TM polarized waves. The flexible and simple design, polarization insensitive, wide-angle incident, broadband and high absorption properties make it possible for our proposed absorber to have promising applications in terahertz detection, imaging and cloaking objects.

Ultra-wideband graphene-based absorber in the terahertz regime based on elliptical slots and a complementary sinusoidal-patterned dielectric layer

A graphene-based absorber is presented based on elliptical slots and a complementary sinusoidal-patterned dielectric layer. The proposed absorber structure includes upper dielectric and metal film layers resulting in a Fabry-Perot cavity. The produced cavity enhances the confinement of electromagnetic waves. The presented absorber yields an absolute bandwidth of 4.02 THz (0.51-4.53 THz) considering an 0.88 absorbance level together with normal incidence. The presented structure benefits from an almost insensitive ultra-wideband absorbing performance as θ varies up to 50°and 60° for TE and TM polarizations, respectively. Moreover, the considered symmetry in the design procedure results in an almost insensitive structure with respect to the φ incident angle. Finally, the obtained 160% fractional bandwidth is much greater compared to mentioned previous works in literature.

Switchable terahertz metamaterial absorber with broadband absorption and multiband absorption

Based on the phase-transition property of vanadium dioxide (VO₂), a terahertz bifunctional absorber is proposed with switchable functionalities of broadband absorption and multiband absorption. When VO₂ is metal, the system is regarded as a broadband absorber, which is composed of VO₂ patch, topas spacer, and VO₂ film with metallic disks inserted. The system obtains a broadband absorption with absorptance >90% from 3.25 THz to 7.08 THz. Moreover, the designed broadband absorber has a stable performance within the incident angle range of 50°. When VO₂ is dielectric, multiband absorption with six peaks is realized in the designed system. Graphene and the metallic disk-shaped array play the dominant role in the mechanism of multiband absorption. Through changing the Fermi energy level of graphene, the performance of multiband absorption can be dynamically adjusted. Because of the switchable functionalities, the proposed design may have potential application in the fields of intelligent absorption and terahertz switch.

Large bandwidth and high-efficiency plasmonic quarter-wave plate

A large bandwidth and high-efficiency subwavelength quarter-wave plate (QWP) is an indispensable component of an integrated miniaturized optical system. The bandwidth of existing plasmonic quarter-wave plates with a transmission efficiency of more than 50% is less than 320 nm in the near-infrared band. In this paper, a metallic quarter-wave plate with a bandwidth of 600 nm (0.95-1.55 µm) and an average transmittance of more than 70% has been designed and shows excellent potential to be used in miniaturized optical polarization detection systems and as an optical data storage device. For TE mode incident waves, this miniaturized optical element can be equivalent to a Fabry-Pérot (FP) resonator. Meanwhile, for the TM mode incident wave, the transmission characteristics of this structure are controlled by gap surface plasmon polaritons (G-SPPs) existing in the symmetric metal/insulator/metal (MIM) configuration.

Infrared Plasmonic Sensing with Anisotropic Two-Dimensional Material Borophene

Borophene, a new member of the two-dimensional material family, has been found to support surface plasmon polaritons in visible and infrared regimes, which can be integrated into various optoelectronic and nanophotonic devices. To further explore the potential plasmonic applications of borophene, we propose an infrared plasmonic sensor based on the borophene ribbon array. The nanostructured borophene can support localized surface plasmon resonances, which can sense the local refractive index of the environment via spectral response. By analytical and numerical calculation, we investigate the influences of geometric as well as material parameters on the sensing performance of the proposed sensor in detail. The results show how to tune and optimize the sensitivity and figure of merit of the proposed structure and reveal that the borophene sensor possesses comparable sensing performance with conventional plasmonic sensors. This work provides the route to design a borophene plasmonic sensor with high performance and can be applied in next-generation point-of-care diagnostic devices.

Bandwidth-tunable THz absorber based on diagonally distributed double-sized VO2 disks

Terahertz absorbers combined with phase-changing VO₂ are a class of stealth materials with adjustable absorptance. However, such absorbers still suffer from insufficient absorption bandwidth. We propose a three-layer terahertz (THz) absorber, consisting of an array of diagonally distributed double-sized VO₂ disks on a silica-coated gold film. We find this structure can generate the superposition of three resonant absorption peaks to broaden the absorption band. The finite element simulation (FES) results show that the absorption bandwidth can be adjusted from 2.63 to 5.04 THz by simply changing the sizes of the VO₂ disks. In addition, the peak absorptance can be continuously regulated from 9.8% to 96% by varying the conductivity of VO₂. Finally, the absorber is polarization-insensitive and has wide-angle absorption. The wide absorption band and adjustable bandwidth of the absorbers have important applications potentially for intelligent stealth materials.

Recent progress in two-dimensional materials for terahertz protection

With the wide applications of terahertz (THz) devices in future communication technology, THz protection materials are essential to overcome potential threats. Recently, THz metamaterials (MMs) based on two-dimensional (2D) materials (e.g., graphene, MXenes) have been extensively investigated due to their unique THz response properties. In this review, THz protection theories are briefly presented first, including reflection loss and shielding mechanisms. Then, the research progress of graphene and other 2D material-based THz MMs and intrinsic materials are reviewed. MMs absorbers in the forms of single layer, multiple layers, hybrid and tunable metasurfaces show excellent THz absorbing performance. These studies provide a sufficient theoretical and practical basis for THz protection, and superior properties promised the wide application prospects of 2D MMs. Three-dimensional intrinsic THz absorbing materials based on porous and ordered 2D materials also show exceptional THz protection performance and effectively integrate the advantages of intrinsic properties and the structural characteristics of 2D materials. These special structures can optimize the surface impedance matching and enable multiple THz scatterings and electric transmission loss, which can realize high-efficiency absorption loss and active controllable protection performance in ultra-wide THz wavebands. Finally, the advantages and existing problems of current THz protection materials are summarized, and their possible future development and applications are prospected.

Strong Terahertz Absorption of Monolayer Graphene Embedded into a Microcavity

Terahertz reflection behaviors of metallic-grating-dielectric-metal (MGDM) microcavity with a monolayer graphene embedded into the dielectric layer are theoretically investigated. A tunable wideband reflection dip at about the Fabry–Pérot resonant frequency of the structure is found. The reflectance at the dip frequency can be electrically tuned in the range of 96.5% and 8.8%. Because of the subwavelength distance between the metallic grating and the monolayer graphene, both of the evanescent grating slit waveguide modes and the evanescent Rayleigh modes play key roles in the strong absorption by the graphene layer. The dependence of reflection behaviors on the carrier scattering rate of graphene is analyzed. A prototype MGDM-graphene structure is fabricated to verify the theoretical analysis. Our investigations are helpful for the developments of electrically controlled terahertz modulators, switches, and reconfigurable antennas based on the MGDM-graphene structures.

Investigation of acoustic plasmons in vertically stacked metal/dielectric/graphene heterostructures for multiband coherent perfect absorption

Coherent absorption, as the time-reversed counterpart to laser, has been widely proposed recently to flexibly modulate light-matter interactions in two-dimensional materials. However, the multiband coherent perfect absorption (CPA) in atomically thin materials still has been elusive. We exploit the multiband CPA in vertically stacked metal/dielectric/graphene heterostructures via ultraconfined acoustic plasmons which can reduce the photon wavelength by a factor of about 70 and thus enable multiple-order resonances on a graphene ribbon of finite width. Under the illumination of two counter-propagating coherent beams, the two-stage coupling scheme is used for exciting multispectral acoustic plasmon resonances on the heterostructure simultaneously, thereby contributing to the ultimate multiband CPA in the mid-infrared region. The strong dependence of the nearly linear dispersion of acoustic plasmons on the chemical potential in graphene and the separation between the metal and the graphene allows the tunability in spectral positions of absorption peaks. Intriguingly, the absorption of each resonant peak is continuously tuned by varying the relative amplitude of two counter-propagating beams, and even their phase difference, respectively. The maximum modulation depth of 4.46*105 is observed. The scattering matrix is employed to demonstrate the principle of CPA and the finite-difference time-domain (FDTD) simulations are used for elucidating the flexible tunability. More importantly, the multiband coherent absorber is robust to the incident angle, and thus undoubtedly benefits extensive applications on optoelectronic and engineering technology areas for modulators and optical switches.

Dynamic coherent perfect absorption in nonlinear metasurfaces

In this Letter, we propose a tunable coherent perfect absorber based on ultrathin nonlinear metasurfaces. A nonlinear metasurface is made of plasmonic nanoantennas coupled to an epsilon-near-zero material with a large optical nonlinearity. The coherent perfect absorption is achieved by controlling the relative phases of the input beams. Here, we show that the optical response of the nonlinear metasurface can be tuned from a complete to a partial absorption by changing the intensity of the pump beam. The proposed nonlinear metasurface can be used to design optically tunable thermal emitters, modulators, and sensors.

Dual-channel optical switch, refractive index sensor and slow light device based on a graphene metasurface

In this paper, we propose a graphene-based metasurface that exhibits multifunctions including tunable filter and slow-light which result from surface plasmon polaritons (SPPs) of graphene and plasmon induced transparency (PIT), respectively. The proposed metasurface is composed by two pairs of graphene nano-rings and a graphene nanoribbon. Each group of graphene rings is separately placed on both sides of the graphene nanoribbon. Adjusting the working state of the nanoribbon can realize the functional conversion of the proposed multifunctional metasurface. After that, in the state of two narrow filters, we put forward the application concept of dual-channel optical switch. Using phase modulation of PIT and flexible Fermi level of graphene, we can achieve tunable slow light. In addition, the result shows that the graphene-based metasurface as a refractive index sensor can achieve a sensitivity of 13670 nm/RIU in terahertz range. These results enable the proposed device to be widely applied in tunable optical switches, slow light, and sensors.

Broadband/multiband absorption through surface plasmon engineering in graphene-wrapped nanospheres

In this paper, a thin film constructed by a periodic assembly of graphene-wrapped particles with spherical geometry has been proposed as a polarization-insensitive reconfigurable perfect absorber. The performance of the proposed structure is based on the cooperative excitation of the quadrupole localized surface plasmons on graphene shells. By sweeping the quality of graphene shells, it is recognized that the low-quality graphene material is the best choice for the absorber design. Moreover, the effect of graphene chemical potential and periodicity of the particles on the absorptivity of the structure is investigated. The physical mechanism of performance is clarified by investigating the excited localized surface plasmon resonances. In addition, the angle-independent behavior up to around 60 degrees for both transverse electric (TE) and transverse magnetic (TM) waves is proved. Interestingly, by engineering the substrate height, our proposed absorber exhibits dynamic broadband performance due to the impedance matching and multiband absorption by enhancing the Fabry-Perot resonances of a micrometer-sized substrate. The possibility of attaining a similar static broadband response by stacking multiple layers is also proved. Our proposed sub-wavelength absorber can be suitable for novel optoelectronic devices due to its simple geometry.

Ultrawideband terahertz absorber with a graphene-loaded dielectric hemi-ellipsoid

We herein present a high-performance ultrawideband terahertz absorber with a silicon hemi-ellipsoid (SHE) on a monolayer graphene that is separated by a dielectric spacer from a bottom metal reflector. The constitution of the absorber, which includes dielectric-mode structures and unstructured monolayer graphene, can minimize undesired optical losses in metals and avoid graphene processing. The absorber achieved an ultrawide absorption bandwidth from 2 THz to more than 10 THz with an average absorption of 95.72%, and the relative bandwidth is 133%. The excellent absorption properties are owing to the presence of graphene and the shape morphing of the SHE, in which multiple discrete graphene plasmon resonances (GPRs) and continuous multimode Fabry-Perot resonances (FPRs) can be excited. By coupling the GPRs and FPRs, the absorption spectrum is extended and smoothed to realize an ultrawideband absorber. The incident angular insensitivity within 50° of the absorber is discussed. The results will shed light on the better performance of terahertz trapping, imaging, communication and detection.

Plasmonic wavy surface for ultrathin semiconductor black absorbers

In this work, we propose and demonstrate a near-unity light absorber in the ultra-violet to near-infrared range (300-1100 nm) with the average efficiency up to 97.7%, suggesting the achievement of black absorber. The absorber consists of a wavy surface geometry, which is formed by the triple-layer of ITO (indium tin oxide)-Ge (germanium)-Cu (copper) films. Moreover, the minimal absorption is even above 90% in the wide wavelength range from 300 nm to 1015 nm, suggesting an ultra-broadband near-perfect absorption window covering the main operation range for the conventional semiconductors. Strong plasmonic resonances and the near-field coupling effects located in the spatially geometrical structure are the key contributions for the broadband absorption. The absorption properties can be well maintained during the tuning of the polarization and incident angles, indicating the high tolerance in complex electromagnetic surroundings. These findings pave new ways for achieving high-performance optoelectronic devices based on the light absorption over the full-spectrum energy gap range.

Tunable Infrared Metamaterial Emitter for Gas Sensing Application

We present an on-chip tunable infrared (IR) metamaterial emitter for gas sensing applications. The proposed emitter exhibits high electrical-thermal-optical efficiency, which can be realized by the integration of microelectromechanical system (MEMS) microheaters and IR metamaterials. According to the blackbody radiation law, high-efficiency IR radiation can be generated by driving a Direct Current (DC) bias voltage on a microheater. The MEMS microheater has a Peano-shaped microstructure, which exhibits great heating uniformity and high energy conversion efficiency. The implantation of a top metamaterial layer can narrow the bandwidth of the radiation spectrum from the microheater to perform wavelength-selective and narrow-band IR emission. A linear relationship between emission wavelengths and deformation ratios provides an effective approach to meet the requirement at different IR wavelengths by tailoring the suitable metamaterial pattern. The maximum radiated power of the proposed IR emitter is 85.0 µW. Furthermore, a tunable emission is achieved at a wavelength around 2.44 µm with a full-width at half-maximum of 0.38 µm, which is suitable for high-sensitivity gas sensing applications. This work provides a strategy for electro-thermal-optical devices to be used as sensors, emitters, and switches in the IR wavelength range.

A Polarization-Insensitive and Wide-Angle Terahertz Absorber with Ring-Porous Patterned Graphene Metasurface

A broadband terahertz (THz) absorber, based on a graphene metasurface, which consists of a layer of ring-porous patterned structure array and a metallic mirror separated by an ultrathin SiO₂ dielectric layer, is proposed and studied by numerical simulation. The simulated results show that the absorptivity of the absorber reaches 90% in the range of 0.91–1.86 THz, and the normalized bandwidth of the absorptivity is 68.6% under normal incidence. In the simulation, the effects of the geometric parameters of the structure on the absorption band have been investigated. The results show that the absorber is insensitive to the incident polarization angle for both transverse electric (TE) and transverse magnetic (TM) under normal incidence. In addition, the absorber is not sensitive to oblique incidence of the light source under TE polarization conditions, and has an approximately stable absorption bandwidth at the incident angle from 0° to 50°. The absorption band can be adjusted by changing the bias voltage of the graphene Fermi level without varying the nanostructure. Furthermore, we propose that a two-layer graphene structure with the same geometric parameters is separated by a dielectric layer of appropriate thickness. The simulated results show that the absorptivity of the two-layer absorber reaches 90% in the range of 0.83-2.04 THz and the normalized bandwidth of the absorptivity is 84.3% under normal incidence. Because of its excellent characteristics based on graphene metamaterial absorbers, it has an important application value in the field of subwavelength photonic devices.

Machine learning and evolutionary algorithm studies of graphene metamaterials for optimized plasmon-induced transparency

Machine learning and optimization algorithms have been widely applied in the design and optimization for photonics devices. We briefly review recent progress of this field of research and show data-driven applications, including spectrum prediction, inverse design and performance optimization, for novel graphene metamaterials (GMs). The structure of the GMs is well-designed to achieve the wideband plasmon induced transparency (PIT) effect, which can be theoretically demonstrated by using the transfer matrix method. Some traditional machine learning algorithms, including k nearest neighbour, decision tree, random forest and artificial neural networks, are utilized to equivalently substitute the numerical simulation in the forward spectrum prediction and complete the inverse design for the GMs. The calculated results demonstrate that all algorithms are effective and the random forest has advantages in terms of accuracy and training speed. Moreover, evolutionary algorithms, including single-objective (genetic algorithm) and multi-objective optimization (NSGA-II), are used to achieve the steep transmission characteristics of PIT effect by synthetically taking many different performance metrics into consideration. The maximum difference between the transmission peaks and dips in the optimized transmission spectrum reaches 0.97. In comparison to previous works, we provide a guidance for intelligent design of photonics devices based on machine learning and evolutionary algorithms and a reference for the selection of machine learning algorithms for simple inverse design problems.

Broadband terahertz absorber with a flexible, reconfigurable performance based on hybrid-patterned vanadium dioxide metasurfaces

An actively tunable broadband terahertz absorber is numerically demonstrated, which consists of four identical vanadium dioxide (VO₂) square loops and a metal ground plane separated by a dielectric spacer. Simulation results show that an excellent absorption bandwidth of 90% terahertz absorptance reaches as wide as 2.45 THz from 1.85 to 4.3 THz under normal incidence. By changing the conductivity of VO₂, an approximately perfect amplitude modulation is realized with the absorptance dynamically tuned from 4% to 100%. This absorption performance is greatly improved compared with previously reported VO₂-based absorbers. The physical mechanisms of a single absorption band and the perfect absorption are elucidated by the wave-interference theory and the impedance matching theory, respectively. Field distributions are further discussed to explore the physical origin of this absorber. In addition, it also has the advantages of polarization insensitivity and wide-angle absorption. The proposed absorber may have many promising applications in the terahertz range such as modulator, sensor, cloaking and optic-electro switches.

Thermal manipulation of plasmons in atomically thin films

Nanoscale photothermal effects enable important applications in cancer therapy, imaging and catalysis. These effects also induce substantial changes in the optical response experienced by the probing light, thus suggesting their application in all-optical modulation. Here, we demonstrate the ability of graphene, thin metal films, and graphene/metal hybrid systems to undergo photothermal optical modulation with depths as large as >70% over a wide spectral range extending from the visible to the terahertz frequency domains. We envision the use of ultrafast pump laser pulses to raise the electron temperature of graphene during a picosecond timescale in which its mid-infrared plasmon resonances undergo dramatic shifts and broadenings, while visible and near-infrared plasmons in the neighboring metal films are severely attenuated by the presence of hot graphene electrons. Our study opens a promising avenue toward the active photothermal manipulation of the optical response in atomically thin materials with potential applications in ultrafast light modulation.

Broadband near-perfect terahertz absorber in single-layered and non-structured graphene loaded with dielectrics

In this work, we have done an extensive study on broadband near-perfect absorbers consisting of single-layered and non-structured graphene loaded with periodical arrays of dielectric bricks, including square and elliptical bricks. We also propose and investigate circular cylinder, rectangular brick, and racecourse dielectric structure. Moreover, the calculated ${z}$z component of the electric field enables us to understand the physical mechanism of resonance absorption. Furthermore, we also studied and proposed a new absorber with periodical arrays of stepped rectangle dielectric structure. We could achieve a broadband absorption from 1.6 to 4.2 THz, with a bandwidth of 2.6 THz and absorption over 90%. The proposed absorber is also tunable; the tunability of the terahertz (THz) broadband absorber is achieved via changing the external gate voltage to modify the Fermi energy of graphene. Also, we compared the results and absorption spectra of different dielectric structures. This THz metamaterial structure can be used in different THz applications such as cloaking, sensing, detection, and imaging.

A Triple-Band Hybridization Coherent Perfect Absorber Based on Graphene Metamaterial

In this paper, a triple-band hybridization coherent perfect absorber based on graphene metamaterial is proposed, which consists of graphene concentric nanorings with different sizes and a metallic mirror separated by SiO2 layer. Based on the finite-difference time-domain (FDTD) solution, triple-band coherent perfect absorption is achieved at frequencies from 0.6 THz to 1.8 THz, which results from the surface plasmon resonance hybridization. The wavelength of the absorption peak can be rapidly changed by varying the Fermi level of graphene. Most importantly, the wavelength of the absorption peak can be independently tuned by varying the Fermi level of the single graphene nanoring. Moreover, the triple hybridization perfect absorber is angle-insensitive because of the perfect symmetry structure of the graphene nanorings. Therefore, our results may widely inspire optoelectronic and micro-nano applications, such as cloaking, tunable sensor, etc.

A Broadband Tunable Terahertz Metamaterial Absorber Based on Single-Layer Complementary Gammadion-Shaped Graphene

We present a simple design of a broadband tunable metamaterial absorber (MMA) in the terahertz (THz) region, which consists of a single layer complementary gammadion-shaped (CGS) graphene sheet and a polydimethylsiloxane (PDMS) dielectric substrate placed on a continuous metal film. The Fermi energy level (E_f) of the graphene can be modulated dynamically by the applied DC bias voltage, which enables us to electrically control the absorption performance of the proposed MMA flexibly. When E_f = 0.8 eV, the relative bandwidth of the proposed MMA, which represents the frequency region of absorption beyond 90%, can reaches its maximal value of 72.1%. Simulated electric field distributions reveal that the broadband absorption mainly originates from the excitation of surface plasmon polaritons (SPPs) on the CGS graphene sheet. Furthermore, the proposed MMA is polarization-insensitive and has wide angles for both transverse-electric (TE) and transverse-magnetic (TM) waves in the broadband frequency range. The broadband absorption capacity of the designed MMA can be effectively adjusted by varying the Fermi energy level of graphene. Lastly, the absorbance of the MMA can be adjusted from 42% to 99.1% by changing the E_f from 0 eV to 0.8 eV, which is in agreement with the theoretical calculation by using the interference 41theory. Due to its simple structure and flexible tunability, the proposed MMA has potential application prospects in tunable filtering, modulators, sensing, and other multispectral devices.

Multi-bit dielectric coding metasurface for EM wave manipulation and anomalous reflection

In this paper, a multi-bit dielectric reflective metasurface is presented for control of electromagnetic (EM) wave scattering and anomalous reflection. The unit cell is designed to act as a 1-, 2-, and 3-bit coding metasurface to attain better control of EM waves. For the 3-bit coding metasurface, the eight digital states have phase responses of 0°, 45°, 90°, 135°, 180°, 225°, 270°, and 315°. The top layer of the proposed metasurface consists of high permittivity material to realize a high Q factor. The proposed multi-bit coding metasurface can reflect the incident EM wave to the desired angle with more than 93% power efficiency. For radar cross section reduction applications, the discrete water cycle algorithm is utilized to obtain an optimal coding matrix for the unit cell arrangement, leading to better diffusion-like scattering, dispersion of the EM wave in all directions, and hence minimal specular reflection. The simulation and experimental results verify that the proposed metasurface is a suitable candidate for control of EM wave scattering and anomalous reflection.

Ultra-wideband terahertz metamaterial absorber based on Snowflake Koch Fractal dielectric loaded graphene

In this paper, an ultra-wideband terahertz metamaterial absorber is introduced based on a Snowflake Koch Fractal (SKF) dielectric loaded on a sheet of graphene. Instead of multilayered-graphene conventional structures, a single-layered non-structured graphene absorber is presented based on gradient width modulation and cavity method. The structure of the absorber is composed of four layers, which are upper SKF dielectric and metal film layer form two mirrors of an asymmetric Fabry-Perot cavity to confine terahertz electromagnetic (EM) waves. Full wave simulations demonstrate that the proposed structure is highly efficient whereas a 161% fractional bandwidth of over 0.9 absorbance is achieved under normal incident wave considering both TE and TM polarizations. The proposed structure is polarization insensitive yielding the same absorbance for both TE and TM polarizations. The absorbance and bandwidth of the structure is almost independent of altering the incident angle θ up to 60° and 30° for TM and TE polarizations, respectively. Avoiding graphene processing and simple shape geometry are the interesting advantages of this structure resulting in feasible fabrication. The proposed structure provides much greater absorbance bandwidth in comparison to previous works.

Terahertz metamaterial beam splitters based on untraditional coding scheme

Terahertz waves have attracted considerable research interest in recent years because of their potential applications in diverse fields. As an important device to control terahertz waves, beam splitters with greater flexibility and higher degrees of freedom are highly desirable. In order to obtain higher degrees of freedom in beam splitting, 2-bit or higher-bit coding elements are usually introduced into metamaterial beam splitters based on the coding theory. In this work, a new "offset" coding scheme using only the 1-bit coding elements of "0" and "1" is presented, and the period of coding for beam splitting can be a non-integer multiple of the length of a single unit rather than only its integer multiples. Therefore, more beam-splitting degrees of freedom can be obtained, and the design strategy is experimentally verified. We believe that the new coding scheme will also be of significance in radar cross section reduction and flexible wave control.

A Dual-Band Terahertz Absorber with Two Passbands Based on Periodic Patterned Graphene

In this paper, a dual-band terahertz absorber with two passbands is proposed. The absorber is composed of periodic patterned graphene arrays on the top of a SiO2 substrate and a frequency selective surface (FSS) on the bottom of the substrate. The simulated results indicate that there are two absorption bands (absorption greater than 90%) ranging from 0.54 to 0.84 THz and 2.13 to 2.29 THz. It is almost transparent to incident waves (transmission greater than 50%) below 0.19 THz and between 1.3 and 1.67 THz with a center frequency of 1.49 THz. The absorber has a good tolerance to the transverse electric (TE) and transverse magnetic (TM) polarized wave oblique incidence, and the transmission rate of the passbands remains greater than 50% within 70 degrees. Moreover, the absorption rate of the absorber can be tuned by the chemical potential of graphene. The structure with absorption and transmission properties has potential applications in filtering, sensing, detecting and antenna stealth.

Hybrid graphene metasurface for near-infrared absorbers

We experimentally demonstrated an amorphous graphene-based metasurface yielding near-infrared super absorber characteristic. The structure is obtained by alternatively combining magnetron-sputtering deposition and graphene transfer coating fabrication techniques. The thickness constraint of the physical vapor-deposited amorphous metallic layer is unlocked and as a result, the as-fabricated graphene-based metasurface absorber achieves near-perfect absorption in the near-infrared region with an ultra-broad spectral bandwidth of 3.0 µm. Our experimental characterization and theoretical analysis further point out that the strong light-matter interaction observed is caused by localized surface plasmon resonance of the metal film's particle-like surface morphology. In addition to the enhanced light absorption characteristics, such an amorphous metasurface can be used for surface-enhanced Raman scattering applications. Meanwhile, the proposed graphene-based metasurface relies solely on CMOS-compatible, low cost and large-area processing, which can be flexibly scaled up for mass production.

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.

Stress transfer at the nanoscale on graphene ribbons of regular geometry

The knowledge of the mechanism of stress transfer from a polymer matrix to a 2-dimensional nano-inclusion such as a graphene flake is of paramount importance for the design and the production of effective nanocomposites. For efficient reinforcement the shape of the inclusion must be accurately controlled since the axial stress transfer from matrix to the inclusion is affected by the axial-shear coupling observed upon loading of a flake of irregular geometry. Herein, we study true axial phenomena on regular- exfoliated-graphene micro-ribbons which are perfectly aligned to the loading direction. We exploit the strain sensitivity of vibrational wave numbers in order to map point-by-point the strain built up along the length of graphene. By considering the balance of shear-to-axial forces, we identify the shear stress at the interface and develop a universal inverse-length parameter that governs the stress transfer process at the nanoscale. An important parameter that has come out of this approach is the prediction and measurement of the transfer length that is required for efficient stress in these systems.