Graphene in a photonic metamaterial

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.

Ultra-thin light-weight laser-induced-graphene (LIG) diffractive optics

The realization of hybrid optics could be one of the best ways to fulfill the technological requirements of compact, light-weight, and multi-functional optical systems for modern industries. Planar diffractive lens (PDL) such as diffractive lenses, photonsieves, and metasurfaces can be patterned on ultra-thin flexible and stretchable substrates and be conformally attached on top of arbitrarily shaped surfaces. In this review, we introduce recent research works addressed to the design and manufacturing of ultra-thin graphene optics, which will open new markets in compact and light-weight optics for next-generation endoscopic brain imaging, space internet, real-time surface profilometry, and multi-functional mobile phones. To provide higher design flexibility, lower process complexity, and chemical-free process with reasonable investment cost, direct laser writing (DLW) of laser-induced-graphene (LIG) is actively being applied to the patterning of PDL. For realizing the best optical performances in DLW, photon-material interactions have been studied in detail with respect to different laser parameters; the resulting optical characteristics have been evaluated in terms of amplitude and phase. A series of exemplary laser-written 1D and 2D PDL structures have been actively demonstrated with different base materials, and then, the cases are being expanded to plasmonic and holographic structures. The combination of these ultra-thin and light-weight PDL with conventional bulk refractive or reflective optical elements could bring together the advantages of each optical element. By integrating these suggestions, we suggest a way to realize the hybrid PDL to be used in the future micro-electronics surface inspection, biomedical, outer space, and extended reality (XR) industries.

2D Material Infrared Photonics and Plasmonics

Two-dimensional (2D) materials including graphene, transition metal dichalcogenides, black phosphorus, MXenes, and semimetals have attracted extensive and widespread interest over the past years for their many intriguing properties and phenomena, underlying physics, and great potential for applications. The vast library of 2D materials and their heterostructures provides a diverse range of electrical, photonic, mechanical, and chemical properties with boundless opportunities for photonics and plasmonic devices. The infrared (IR) regime, with wavelengths across 0.78 μm to 1000 μm, has particular technological significance in industrial, military, commercial, and medical settings while facing challenges especially in the limit of materials. Here, we present a comprehensive review of the varied approaches taken to leverage the properties of the 2D materials for IR applications in photodetection and sensing, light emission and modulation, surface plasmon and phonon polaritons, non-linear optics, and Smith-Purcell radiation, among others. The strategies examined include the growth and processing of 2D materials, the use of various 2D materials like semiconductors, semimetals, Weyl-semimetals and 2D heterostructures or mixed-dimensional hybrid structures, and the engineering of light-matter interactions through nanophotonics, metasurfaces, and 2D polaritons. Finally, we give an outlook on the challenges in realizing high-performance and ambient-stable devices and the prospects for future research and large-scale commercial applications.

Molecular Plasmonics with Metamaterials

Molecular plasmonics, the area which deals with the interactions between surface plasmons and molecules, has received enormous interest in fundamental research and found numerous technological applications. Plasmonic metamaterials, which offer rich opportunities to control the light intensity, field polarization, and local density of electromagnetic states on subwavelength scales, provide a versatile platform to enhance and tune light-molecule interactions. A variety of applications, including spontaneous emission enhancement, optical modulation, optical sensing, and photoactuated nanochemistry, have been reported by exploiting molecular interactions with plasmonic metamaterials. In this paper, we provide a comprehensive overview of the developments of molecular plasmonics with metamaterials. After a brief introduction to the optical properties of plasmonic metamaterials and relevant fabrication approaches, we discuss light-molecule interactions in plasmonic metamaterials in both weak and strong coupling regimes. We then highlight the exploitation of molecules in metamaterials for applications ranging from emission control and optical modulation to optical sensing. The role of hot carriers generated in metamaterials for nanochemistry is also discussed. Perspectives on the future development of molecular plasmonics with metamaterials conclude the review. The use of molecules in combination with designer metamaterials provides a rich playground both to actively control metamaterials using molecular interactions and, in turn, to use metamaterials to control molecular processes.

Detection of cancer biomarkers CA125 and CA199 via terahertz metasurface immunosensor

The cancer biomarkers including AFP, CEA, CA199 and CA125, are of great importance in the diagnosis, prognostic prediction and recurrence monitoring of malignancies. However, in clinical practical applications, most tumor cancer biomarkers are lack of sensitivity and specificity. In this study, we propose a terahertz (THz) metasurface (MS) immunosensor coupled with gold nanoparticles (AuNPs), which have good biocompatibility and high specific surface area for biomarkers. Firstly, we added AuNPs to the surface of the sensor. And then, the surface is modified with Anti-CA125 or Anti-CA199 to improve the sensitivity and specificity to the target antigen. The biosensor was fabricated using a surface micromachining process and characterized by a THz-time-domain spectroscopy (TDS) system. The sensitivity of the resonance frequency of the sensor to the refractive index was 65 GHz/RIU (refractive index unit). The detection performance of the THz immunosensor was also verified with different concentrations of CA125 and CA199. The experimental results showed that the frequency shift of the resonance peak was linearly related to the concentration of CA125 and CA199. The detection limits for both CA125 and CA199 are 0.01 U/ml, which is better than that of other common methods. Finally, serum samples were collected and detected to explore whether this method is suitable for clinical detection. The results are consistent with the results of antigen recognition. This study proves that the practicability of the THz immunosensor, which potentially provides important techniques and equipment for improving the sensitivity and specificity of cancer biomarkers.

A Review: The Functional Materials-Assisted Terahertz Metamaterial Absorbers and Polarization Converters

When metamaterial structures meet functional materials, what will happen? The recent rise of the combination of metamaterial structures and functional materials opens new opportunities for dynamic manipulation of terahertz wave. The optical responses of functional materials are greatly improved based on the highly-localized structures in metamaterials, and the properties of metamaterials can in turn be manipulated in a wide dynamic range based on the external stimulation. In the topical review, we summarize the recent progress of the functional materials-based metamaterial structures for flexible control of the terahertz absorption and polarization conversion. The reviewed devices include but are not limited to terahertz metamaterial absorbers with different characteristics, polarization converters, wave plates, and so on. We review the dynamical tunable metamaterial structures based on the combination with functional materials such as graphene, vanadium dioxide (VO2) and Dirac semimetal (DSM) under various external stimulation. The faced challenges and future prospects of the related researches will also be discussed in the end.

Enhanced light-matter interaction in two-dimensional transition metal dichalcogenides

Two-dimensional (2D) transition metal dichalcogenide (TMDC) materials, such as MoS₂, WS₂, MoSe₂, and WSe₂, have received extensive attention in the past decade due to their extraordinary electronic, optical and thermal properties. They evolve from indirect bandgap semiconductors to direct bandgap semiconductors while their layer number is reduced from a few layers to a monolayer limit. Consequently, there is strong photoluminescence in a monolayer (1L) TMDC due to the large quantum yield. Moreover, such monolayer semiconductors have two other exciting properties: large binding energy of excitons and valley polarization. These properties make them become ideal materials for various electronic, photonic and optoelectronic devices. However, their performance is limited by the relatively weak light-matter interactions due to their atomically thin form factor. Resonant nanophotonic structures provide a viable way to address this issue and enhance light-matter interactions in 2D TMDCs. Here, we provide an overview of this research area, showcasing relevant applications, including exotic light emission, absorption and scattering features. We start by overviewing the concept of excitons in 1L-TMDC and the fundamental theory of cavity-enhanced emission, followed by a discussion on the recent progress of enhanced light emission, strong coupling and valleytronics. The atomically thin nature of 1L-TMDC enables a broad range of ways to tune its electric and optical properties. Thus, we continue by reviewing advances in TMDC-based tunable photonic devices. Next, we survey the recent progress in enhanced light absorption over narrow and broad bandwidths using 1L or few-layer TMDCs, and their applications for photovoltaics and photodetectors. We also review recent efforts of engineering light scattering, e.g., inducing Fano resonances, wavefront engineering in 1L or few-layer TMDCs by either integrating resonant structures, such as plasmonic/Mie resonant metasurfaces, or directly patterning monolayer/few layers TMDCs. We then overview the intriguing physical properties of different van der Waals heterostructures, and their applications in optoelectronic and photonic devices. Finally, we draw our opinion on potential opportunities and challenges in this rapidly developing field of research.

Polarization-insensitive graphene photodetectors enhanced by a broadband metamaterial absorber

Graphene, combined with plasmonic nanostructures, shows great promise for achieving desirable photodetection properties and functionalities. Here, we theoretically proposed and experimentally demonstrated a graphene photodetector based on the metamaterial absorber in the visible and near-infrared wavebands. The experimental results show that the metamaterial-based graphene photodetector (MGPD) has achieved up to 3751% of photocurrent enhancement relative to an antennasless graphene device at zero external bias. Furthermore, the polarization-independent of photoresponse has resulted from the polarization-insensitive absorption of symmetric square-ring antennas. Moreover, the spectral-dependent photocurrent enhancement, originated from the enhanced light-trapping effect, was experimentally confirmed and understood by the simulated electric field profiles. The design contributes to the development of high-performance graphene photodetectors and optoelectronic devices.

Current Modulation of Plasmonic Nanolasers by Breaking Reciprocity on Hybrid Graphene-Insulator-Metal Platforms

A hybrid graphene-insulator-metal (GIM) platform is proposed with a supported surface plasmon polariton (SPP) wave that can be manipulated by breaking Lorentz reciprocity. The ZnO SPP nanowire lasers on the GIM platforms are demonstrated up to room temperature to be actively modulated by applying external current to graphene, which transforms the cavity mode from the standing to propagation wave pattern. With applying 100 mA external current, the laser threshold increases by ≈100% and a 1.2 nm Doppler shift is observed due to the nonreciprocal propagation characteristic. The nanolaser performance also depends on the orientation of the nanowire with respect to the current flow direction. The GIM platform can be a promising platform for integrated plasmonic system functioning laser generation, modulation, and detection.

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.

Realization of a near-infrared active Fano-resonant asymmetric metasurface by precisely controlling the phase transition of Ge2Sb2Te5

A metasurface is one of the most effectual platforms for the manipulation of complex optical fields. One of the current challenges in the field is to develop active or reconfigurable functionalities to extend its operation band which is limited by its intrinsic resonant nature. Here we demonstrate a kind of active Fano-resonant asymmetric metasurface in the near-infrared (NIR) region with heterostructures made of a layer of asymmetric split-ring resonators and a thin layer of phase-change material (PCM). In the asymmetric metasurface, significant tunability in the frequency, Q-factor and strength of the Fano resonance are all achieved by precisely controlling the phase transition of the contained PCM Ge₂Sb₂Te₅ (GST), together with changing the geometric asymmetry of the split-ring resonators. Moreover, we provide a complete transition process of the optical properties for GST and an optimized modulation on the active Fano-resonant metasurface. Our approach to dynamically control a Fano-resonant metasurface paves the way to realizing various active photonic meta-devices involving PCM.

Graphene Plasmonic Fractal Metamaterials for Broadband Photodetectors

Metamaterials have recently established a new paradigm for enhanced light absorption in state-of-the-art photodetectors. Here, we demonstrate broadband, highly efficient, polarization-insensitive, and gate-tunable photodetection at room temperature in a novel metadevice based on gold/graphene Sierpinski carpet plasmonic fractals. We observed an unprecedented internal quantum efficiency up to 100% from the near-infrared to the visible range with an upper bound of optical detectivity of 10¹¹ Jones and a gain up to 10⁶, which is a fingerprint of multiple hot carriers photogenerated in graphene. Also, we show a 100-fold enhanced photodetection due to highly focused (up to a record factor of |E/E₀| ≈ 20 for graphene) electromagnetic fields induced by electrically tunable multimodal plasmons, spatially localized in self-similar fashion on the metasurface. Our findings give direct insight into the physical processes governing graphene plasmonic fractal metamaterials. The proposed structure represents a promising route for the realization of a broadband, compact, and active platform for future optoelectronic devices including multiband bio/chemical and light sensors.

Graphene-based hyperbolic metamaterial as a switchable reflection modulator

A tunable graphene-based hyperbolic metamaterial is designed and numerically investigated in the mid-infrared frequencies. Theoretical analysis proves that by adjusting the chemical potential of graphene from 0.2 eV to 0.8 eV, the reflectance can be blue-shifted up to 2.3 µm. Furthermore, by modifying the number of graphene monolayers in the hyperbolic metamaterial stack, we are able to shift the plasmonic resonance up to 3.6 µm. Elliptic and type II hyperbolic dispersions are shown for three considered structures. Importantly, a blue/red-shift and switching of the reflectance are reported at different incident angles in TE/TM modes. The obtained results clearly show that graphene-based hyperbolic metamaterials with reversibly controlled tunability may be used in the next generation of nonlinear tunable and reversibly switchable devices operating in the mid-IR range.

Reconfigurable Radiation Pattern of Planar Antenna Using Metamaterial for 5G Applications

In this research, a reconfigurable metamaterial (MM) structure was designed using a millimeter-wave (MMW) band with two configurations that exhibit different refractive indices. These two MM configurations are used to guide the antenna's main beam in the desired direction in the 5th generation (5G) band of 28 GHz. The different refractive indices of the two MM configurations created phase change for the electromagnetic (EM) wave of the antenna, which deflected the main beam. A contiguous squares resonator (CSR) is proposed as an MM structure to operate at MMW band. The CSR is reconfigured using three switches to achieve two MM configurations with different refractive indices. The simulation results of the proposed antenna loaded by MM unit cells demonstrate that the radiation beam is deflected by angles of +30° and -27° in the E-plane, depending on the arrangement of the two MM configurations on the antenna substrate. Furthermore, these deflections are accompanied by gain enhancements of 1.9 dB (26.7%) and 1.5 dB (22.4%) for the positive and negative deflections, respectively. The reflection coefficients of the MM antenna are kept below -10 dB for both deflection angles at 28 GHz. The MM antennas are manufactured and measured to validate the simulated results.

Design of a Tunable Absorber Based on Active Frequency-Selective Surface for UHF Applications

An ultrathin tunable absorber for the ultrahigh frequency (UHF) band is presented in this paper. The absorber is a single-layer structure based on the topology of a Salisbury screen, in which the conventional resistive layer is replaced by an active frequency-selective surface (AFSS) loaded with resistors and varactors. The reflectivity response of the absorber can be controlled by adjusting the reverse bias voltage for the varactors, which is verified by both simulated and measured results. The experimental results show that the reflectivity response of the absorber can be modulated below −10 dB over a frequency band ranging from 415 to 822 MHz. The total thickness of the absorber, 10 mm, is equivalent to only λ/72 of the lower limit frequency. The absorbing mechanism for the designed absorber is illustrated by simulating the volume loss density distributions. A detailed analysis is also carried out on the basis of these parameters, such as the AFSS shape, resistor, thickness of the foam, thickness and permittivity of the dielectric substrate, and incident angles, which contribute to the reflectivity of the AFSS absorber.

Plasmonic Nanolasers Enhanced by Hybrid Graphene-Insulator-Metal Structures

Graphene is a two-dimensional (2D) structure that creates a linear relationship between energy and momentum that not only forms massless Dirac fermions with extremely high group velocity but also exhibits a broadband transmission from 300 to 2500 nm that can be applied to many optoelectronic applications, such as solar cells, light-emitting devices, touchscreens, ultrafast photodetectors, and lasers. Although the plasmonic resonance of graphene occurs in the terahertz band, graphene can be combined with a noble metal to provide a versatile platform for supporting surface plasmon waves. In this study, we propose a hybrid graphene-insulator-metal (GIM) structure that can modulate the surface plasmon polariton (SPP) dispersion characteristics and thus influence the performance of plasmonic nanolasers. Compared with values obtained when graphene is not used on an Al template, the propagation length of SPP waves can be increased 2-fold, and the threshold of nanolasers is reduced by 50% when graphene is incorporated on the template. The GIM structure can be further applied in the future to realize electrical control or electrical injection of plasmonic devices through graphene.

A Review of THz Modulators with Dynamic Tunable Metasurfaces

Terahertz (THz) radiation has received much attention during the past few decades for its potential applications in various fields, such as spectroscopy, imaging, and wireless communications. To use terahertz waves for data transmission in different application systems, the efficient and rapid modulation of terahertz waves is required and has become an in-depth research topic. Since the turn of the century, research on metasurfaces has rapidly developed, and the scope of novel functions and operating frequency ranges has been substantially expanded, especially in the terahertz range. The combination of metasurfaces and semiconductors has facilitated both new opportunities for the development of dynamic THz functional devices and significant achievements in THz modulators. This paper provides an overview of THz modulators based on different kinds of dynamic tunable metasurfaces combined with semiconductors, two-dimensional electron gas heterostructures, superconductors, phase-transition materials, graphene, and other 2D material. Based on the overview, a brief discussion with perspectives will be presented. We hope that this review will help more researchers learn about the recent developments and challenges of THz modulators and contribute to this field.

Coexistence of two graphene-induced modulation effects on surface plasmons in hybrid graphene plasmonic nanostructures

Integrating gate-tunable graphene with plasmonic nanostructures or metamaterials offers a great potential in achieving dynamic control of plasmonic response. While remarkable progress has been made in realizing efficient graphene-induced modulations of plasmon resonances, a full picture of graphene-plasmon interactions and the consequent deep understanding on graphene-enabled tuning mechanism remain largely unexplored. Here, we theoretically identify, for the first time, two distinct modulation effects that can coexist in graphene-based plasmonic nanostructure: graphene can influence the plasmon resonances by either acting as equivalent nanocircuit elements or effectively altering their excitation environment, leading to totally different tuning behaviors. A general dependency of tuning features on the graphene-induced impedance, irrespective of structure geometries, is established when graphene serves as nanocircuit elements. We demonstrate that these two modulation effects can be dynamically controlled by appropriately integrating graphene with plasmonic nanostructures, which provide an active window for efficient modulation of surface plasmons. Our findings may pave the way towards realizing dynamic control of plasmonic response, which holds great potential applications in graphene-based active nanoplasmonic devices.

High-speed amplitude modulator with a high modulation index based on a plasmonic resonant tunable metasurface

High-speed optical amplitude modulation is important for optical communication systems and sensors. Moreover, nano-optical modulators are important for developing optical-communication-aided high-speed parallel-operation processors and micro-biomedical sensors for inside-blood-capillary examinations or microsurgery operations. In this paper, we have designed a plasmonic resonant tunable metasurface with barium titanate (BTO) as a nanoscale optical modulator with a high modulation index and high speed. The BTO operated well in the VIS and near-IR ranges, enabling tunable optical devices with zero dispersion and high speed. The results obtained by rigorous finite-element method simulations have shown that the hypothesized device has good potential for fast modulation in related applications, e.g., modulators in nano-optical systems, nano-optical switches and nanosensors.

Nonlinear Optics with 2D Layered Materials

2D layered materials (2DLMs) are a subject of intense research for a wide variety of applications (e.g., electronics, photonics, and optoelectronics) due to their unique physical properties. Most recently, increasing research efforts on 2DLMs are projected toward the nonlinear optical properties of 2DLMs, which are not only fascinating from the fundamental science point of view but also intriguing for various potential applications. Here, the current state of the art in the field of nonlinear optics based on 2DLMs and their hybrid structures (e.g., mixed-dimensional heterostructures, plasmonic structures, and silicon/fiber integrated structures) is reviewed. Several potential perspectives and possible future research directions of these promising nanomaterials for nonlinear optics are also presented.

Thermally induced tunability of a terahertz metamaterial by using a specially designed nematic liquid crystal mixture

The search for new low loss nematic liquid crystal mixtures with enhanced birefringence and low temperature of nematic-to-isotropic phase transition plays a pivotal role in a development of new applications in the emerging field of thermally tunable metamaterials. Here we maximize thermally induced tunability of a terahertz metamaterial by using a specially designed nematic liquid crystal mixture. It is shown that the resonant response of a metamaterial device can be effectively tuned both in terms of its magnitude and wavelength with the spectral tunability approaching the theoretical limit of 8 GHz. Electromagnetic simulations confirm our tests and match the experimental observations well. The suggested approach opens new routes for next-generation soft-matter-based filtering and sensing components and devices.

The effect of graphene on surface plasmon resonance of metal nanoparticles

Two transparent graphene–metal nanoparticle (NP) hybrid schemes, namely Au NPs covered by graphene layers and Au NPs encapsulated by graphene layers, are presented and the effect of graphene on the localized surface plasmon resonance of metal NPs is systematically investigated.

Beam switching and bifocal zoom lensing using active plasmonic metasurfaces

Compact nanophotonic elements exhibiting adaptable properties are essential components for the miniaturization of powerful optical technologies such as adaptive optics and spatial light modulators. While the larger counterparts typically rely on mechanical actuation, this can be undesirable in some cases on a microscopic scale due to inherent space restrictions. Here, we present a novel design concept for highly integrated active optical components that employs a combination of resonant plasmonic metasurfaces and the phase-change material Ge₃Sb₂Te₆. In particular, we demonstrate beam switching and bifocal lensing, thus, paving the way for a plethora of active optical elements employing plasmonic metasurfaces, which follow the same design principles.

2D Materials for Optical Modulation: Challenges and Opportunities

Owing to their atomic layer thickness, strong light-material interaction, high nonlinearity, broadband optical response, fast relaxation, controllable optoelectronic properties, and high compatibility with other photonic structures, 2D materials, including graphene, transition metal dichalcogenides and black phosphorus, have been attracting increasing attention for photonic applications. By tuning the carrier density via electrical or optical means that modifies their physical properties (e.g., Fermi level or nonlinear absorption), optical response of the 2D materials can be instantly changed, making them versatile nanostructures for optical modulation. Here, up-to-date 2D material-based optical modulation in three categories is reviewed: free-space, fiber-based, and on-chip configurations. By analysing cons and pros of different modulation approaches from material and mechanism aspects, the challenges faced by using these materials for device applications are presented. In addition, thermal effects (e.g., laser induced damage) in 2D materials, which are critical to practical applications, are also discussed. Finally, the outlook for future opportunities of these 2D materials for optical modulation is given.

Double-resonant enhancement of third-harmonic generation in graphene nanostructures

Intriguing and unusual physical properties of graphene offer remarkable potential for advanced, photonics-related technological applications, particularly in the area of nonlinear optics at the deep-subwavelength scale. In this study, we use a recently developed numerical method to illustrate an efficient mechanism that can lead to orders of magnitude enhancement of the third-harmonic generation in graphene diffraction gratings. In particular, we demonstrate that by taking advantage of the geometry dependence of the resonance wavelength of localized surface-plasmon polaritons of graphene ribbons and discs one can engineer the spectral response of graphene gratings so that strong plasmonic resonances exist at both the fundamental frequency and third-harmonic (TH). As a result of this double-resonant mechanism for optical near-field enhancement, the intensity of the TH can be increased by more than six orders of magnitude.This article is part of the themed issue 'New horizons for nanophotonics'.

Electrical tuning of the polarization state of light using graphene-integrated anisotropic metasurfaces

Plasmonic metasurfaces have been employed for moulding the flow of transmitted and reflected light, thereby enabling numerous applications that benefit from their ultra-thin sub-wavelength format. Their appeal is further enhanced by the incorporation of active electro-optic elements, paving the way for dynamic control of light's properties. In this paper, we realize a dynamic polarization state generator using a graphene-integrated anisotropic metasurface (GIAM) that converts the linear polarization of the incident light into an elliptical one. This is accomplished by using an anisotropic metasurface with two principal polarization axes, one of which possesses a Fano-type resonance. A gate-controlled single-layer graphene integrated with the metasurface was employed as an electro-optic element controlling the phase and intensity of light polarized along the resonant axis of the GIAM. When the incident light is polarized at an angle to the resonant axis of the metasurface, the ellipticity of the reflected light can be dynamically controlled by the application of a gate voltage. Thus accomplished dynamic polarization control is experimentally demonstrated and characterized by measuring the Stokes polarization parameters. Large changes of the ellipticity and the tilt angle of the polarization ellipse are observed. Our measurements show that the tilt angle can be changed from positive values through zero to negative values while keeping the ellipticity constant, potentially paving the way to rapid ellipsometry and other characterization techniques requiring fast polarization shifting.This article is part of the themed issue 'New horizons for nanophotonics'.

Dynamical tuning between nearly perfect reflection, absorption, and transmission of light via graphene/dielectric structures

Exerting well-defined control over the reflection (R), absorption (A), and transmission (T) of electromagnetic waves is a key objective in quantum optics. To this end, one often utilizes hybrid structures comprised of elements with different optical properties in order to achieve features such as high R or high A for incident light. A desirable goal would be the possibility to tune between all three regimes of nearly perfect reflection, absorption, and transmission within the same device, thus swapping between the cases R → 1, A → 1, and T → 1 dynamically. We here show that a dielectric interfaced with a graphene layer on each side allows for precisely this: by tuning only the Fermi level of graphene, all three regimes can be reached in the THz regime and below. Moreover, we show that the inclusion of cylindrical defects in the system offers a different type of control of the scattering of electromagnetic waves by means of the graphene layers.

Dielectric-Like Behavior of Graphene in Au Plasmon Resonator

Graphene has proven to be a promising conductive layer in fabricating optical plasmon resonators on insulator substrate using electron beam lithography and has the potential to construct electrically controlled active plasmon resonators. In this study, we investigate the effect of graphene on plasmon resonance using graphene and Au plasmon resonator system as a model at visible and near-infrared wavelength. Our experiment data show that the presence of graphene does not weaken and annihilate the plasmon resonance peaks, instead it predominantly makes the peaks redshift, which is similar to the behavior of depositing SiO₂ film on Au plasmon resonators. This fact indicates that graphene predominantly exhibits dielectric-like behavior at visible and near-infrared wavelength, which can be attributed to the low carrier density in graphene compared with metals.

The effect of silver nanoparticles/graphene-coupled TiO2 beads photocatalyst on the photoconversion efficiency of photoelectrochemical hydrogen production

In this work, a novel configuration of the photoelectrochemical hydrogen production device is demonstrated. It is based on TiO2 beads as the primary photoanode material with the addition of a heterostructure of silver nanoparticles/graphene. The heterostructure not only caters to a great improvement in light harvesting efficiency (LHE) but also enhances the charge collection efficiency. For LHE, the optimized cell based on TiO2 beads/Ag/graphene shows a 47% gain as compared to the cell having a photoanode of commercial P25 TiO2 powders. For the charge collection efficiency, there is a pronounced improvement of an impressive value of 856%. The reason for the improvement in light absorption is attributed to either the light scattering of TiO2 beads or the surface plasmonic resonance on the Ag nanoparticles/graphene. The photoconversion efficiency (PCE) of the resulting cells is also presented and discussed. The PCE of the TiO2 beads/Ag/graphene cell is approximately 2.5 times than that of pure P25 cell.

Family of graphene-assisted resonant surface optical excitations for terahertz devices

The majority of the proposed graphene-based THz devices consist of a metamaterial that can optically interact with graphene. This coupled graphene-metamaterial system gives rise to a family of resonant modes such as the surface plasmon polariton (SPP) modes of graphene, the geometrically induced SPPs, also known as the spoof SPP modes, and the Fabry-Perot (FP) modes. In the literature, these modes are usually considered separately as if each could only exist in one structure. By contrast, in this paper, we show that even in a simple metamaterial structure such as a one-dimensional (1D) metallic slit grating, these modes all exist and can potentially interact with each other. A graphene SPP-based THz device is also fabricated and measured. Despite the high scattering rate, the effective SPP resonances can still be observed and show a consistent trend between the effective frequency and the grating period, as predicted by the theory. We also find that the excitation of the graphene SPP mode is most efficient in the terahertz spectral region due to the Drude conductivity of graphene in this spectral region.

Monolayer graphene sensing enabled by the strong Fano-resonant metasurface

Recent advances in graphene photonics reveal promising applications in the technologically important terahertz spectrum, where graphene-based active terahertz metamaterial modulators have been experimentally demonstrated. However, the sensitivity of the atomically thin graphene monolayer towards sharp Fano resonant terahertz metasurfaces remains unexplored. Here, we demonstrate thin-film sensing of the graphene monolayer with a high quality factor terahertz Fano resonance in metasurfaces consisting of a two-dimensional array of asymmetric resonators. A drastic change in the transmission amplitude of the Fano resonance was observed due to strong interactions between the monolayer graphene and the tightly confined electric fields in the capacitive gaps of the Fano resonator. The deep-subwavelength sensing of the atomically thin monolayer graphene further highlights the extreme sensitivity of the resonant electric field excited at the dark Fano resonance, allowing the detection of an analyte that is λ/1 000 000 thinner than the free space wavelength.

Graphene-based extremely wide-angle tunable metamaterial absorber

We investigate the absorption properties of graphene-based anisotropic metamaterial structures where the metamaterial layer possesses an electromagnetic response corresponding to a near-zero permittivity. We find that through analytical and numerical studies, near perfect absorption arises over an unusually broad range of beam incidence angles. Due to the presence of graphene, the absorption is tunable via a gate voltage, providing dynamic control of the energy transmission. We show that this strongly enhanced absorption arises due to a coupling between light and a fast wave-mode propagating along the graphene/metamaterial hybrid.

Strong modulation of plasmons in Graphene with the use of an Inverted pyramid array diffraction grating

An optical device configuration allowing efficient electrical tuning of surface plasmon wavelength and absorption in a suspended/conformal graphene film is reported. An underlying 2-dimensional array of inverted rectangular pyramids greatly enhances optical coupling to the graphene film. In contrast to devices utilising 1D grating or Kretchman prism coupling configurations, both s and p polarization can excite plasmons due to symmetry of the grating structure. Additionally, the excited high frequency plasmon mode has a wavelength independent of incident photon angle allowing multidirectional coupling. By combining analytical methods with Rigorous Coupled-Wave Analysis, absorption of plasmons is mapped over near infrared spectral range as a function of chemical potential. Strong control over both plasmon wavelength and strength is provided by an ionic gel gate configuration. 0.04eV change in chemical potential increases plasmon energy by 0.05 eV shifting plasmon wavelength towards the visible, and providing enhancement in plasmon absorption. Most importantly, plasmon excitation can be dynamically switched off by lowering the chemical potential and moving from the intra-band to the inter-band transition region. Ability to electrically tune plasmon properties can be utilized in applications such as on-chip light modulation, photonic logic gates, optical interconnect and sensing applications.

Hybrid graphene plasmonic waveguide modulators

The unique optical and electronic properties of graphene make possible the fabrication of novel optoelectronic devices. One of the most exciting graphene characteristics is the tunability by gating which allows one to realize active optical devices. While several types of graphene-based photonic modulators have already been demonstrated, the potential of combining the versatility of graphene with subwavelength field confinement of plasmonic waveguides remains largely unexplored. Here we report fabrication and study of hybrid graphene-plasmonic waveguide modulators. We consider several types of modulators and identify the most promising one for telecom applications. The modulator working at the telecom range is demonstrated, showing a modulation depth of >0.03 dB μm(-1) at low gating voltages for an active device area of just 10 μm(2), characteristics which are already comparable to those of silicon-based waveguide modulators while retaining the benefit of further device miniaturization. Our proof-of-concept results pave the way towards on-chip realization of efficient graphene-based active plasmonic waveguide devices for optical communications.

Coherent and Tunable Terahertz Radiation from Graphene Surface Plasmon Polarirons Excited by Cyclotron Electron Beam

Terahertz (THz) radiation can revolutionize modern science and technology. To this date, it remains big challenges to develop intense, coherent and tunable THz radiation sources that can cover the whole THz frequency region either by means of only electronics (both vacuum electronics and semiconductor electronics) or of only photonics (lasers, for example, quantum cascade laser). Here we present a mechanism which can overcome these difficulties in THz radiation generation. Due to the natural periodicity of 2π of both the circular cylindrical graphene structure and cyclotron electron beam (CEB), the surface plasmon polaritions (SPPs) dispersion can cross the light line of dielectric, making transformation of SPPs into radiation immediately possible. The dual natural periodicity also brings significant excellences to the excitation and the transformation. The fundamental and hybrid SPPs modes can be excited and transformed into radiation. The excited SPPs propagate along the cyclotron trajectory together with the beam and gain energy from the beam continuously. The radiation density is enhanced over 300 times, up to 10(5) W/cm(2). The radiation frequency can be widely tuned by adjusting the beam energy or chemical potential. This mechanism opens a way for developing desired THz radiation sources to cover the whole THz frequency regime.

Enhanced transmission modulation based on dielectric metasurfaces loaded with graphene

We present a hybrid graphene/dielectric metasurface design to achieve strong tunable and modulated transmission at near-infrared (near-IR) frequencies. The proposed device is constituted by periodic pairs of asymmetric silicon nanobars placed over a silica substrate. An one-atom-thick graphene sheet is positioned over the all-dielectric metasurface. The in-plane electromagnetic fields are highly localized and enhanced with this metasurface due to its very low Ohmic losses at near-IR wavelengths. They strongly interact with graphene. Sharp Fano-type transmission spectrum is obtained at the resonant frequency of this hybrid configuration due to the cancelation of the electric and magnetic dipole responses at this frequency point. The properties of the graphene monolayer flake can be adjusted by tuning its Fermi energy or chemical potential, leading to different doping levels and, equivalently, material parameters. As a result, the Q-factor and the Fano-type resonant transmission spectrum of the proposed hybrid system can be efficiently tuned and controlled due to the strong light-graphene interaction. Higher than 60% modulation in the transmission coefficient is reported at near-IR frequencies. The proposed hybrid graphene/dielectric nanodevice has compact footprint, fast speed, and can be easily integrated to the current CMOS technology. These features would have promising applications to near-IR tunable filters, faster optical interconnects, efficient sensors, switches, and amplitude modulators.

Active Chiral Plasmonics

Active control over the handedness of a chiral metamaterial has the potential to serve as key element for highly integrated polarization engineering approaches, polarization sensitive imaging devices, and stereo display technologies. However, this is hard to achieve as it seemingly involves the reconfiguration of the metamolecule from a left-handed into a right-handed enantiomer and vice versa. This type of mechanical actuation is intricate and usually neither monolithically realizable nor viable for high-speed applications. Here, enabled by the phase change material Ge3Sb2Te6 (GST-326), we demonstrate a tunable and switchable mid-infrared plasmonic chiral metamaterial in a proof-of-concept experiment. A large tunability range of the circular dichroism response from λ = 4.15 to 4.90 μm is achieved, and we experimentally demonstrate that the combination of a passive bias-type chiral layer with the active chiral metamaterial allows for switchable chirality, that is, the reversal of the circular dichroism sign, in a fully planar, layered design without the need for geometrical reconfiguration. Because phase change materials can be electrically and optically switched, our designs may open up a path for highly integrated mid-IR polarization engineering devices that can be modulated on ultrafast time scales.

Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems

We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.

Plasmon modes in graphene: status and prospect

Plasmons in graphene have unusual properties and offer promising prospects for plasmonic applications covering a wide frequency range, ranging from terahertz up to the visible. Plasmon modes have been recently studied in both free-standing and supported graphene. Here, we review plasmons in graphene with particular emphasis on plasmonic excitations in epitaxial graphene and on the influence of the underlying substrate on the screening processes. Although the theoretical comprehension of plasmons in supported graphene is still incomplete, several experimental results provide clues regarding the nature of plasmonic excitations in graphene on metals and semiconductors. Plasmon in graphene can be tuned by chemical doping and gating potentials. We show through selected examples that the adsorbates can be used to tune the plasmon frequency, while the intercalation of chemical species allows the decoupling of the graphene sheet from the substrate to recover the plasmon dispersion of pristine graphene. Finally, we also report intriguing effects due to many-body interaction, such as the excitations generated by electron-electron coupling (magnetoplasmons) and the composite modes arising from the coupling of plasmons with phonons and with charge carriers.

High-contrast terahertz wave modulation by gated graphene enhanced by extraordinary transmission through ring apertures

Gate-controllable transmission of terahertz (THz) radiation makes graphene a promising material for making high-speed THz wave modulators. However, to date, graphene-based THz modulators have exhibited only small on/off ratios due to small THz absorption in single-layer graphene. Here we demonstrate a ∼50% amplitude modulation of THz waves with gated single-layer graphene by the use of extraordinary transmission through metallic ring apertures placed right above the graphene layer. The extraordinary transmission induced ∼7 times near-filed enhancement of THz absorption in graphene. These results promise complementary metal-oxide-semiconductor compatible THz modulators with tailored operation frequencies, large on/off ratios, and high speeds, ideal for applications in THz communications, imaging, and sensing.

Electrical modulation of fano resonance in plasmonic nanostructures using graphene

Pauli blocking of interband transistions gives rise to tunable optical properties in single layer graphene (SLG). This effect is exploited in a graphene-nanoantenna hybrid device where Fano resonant plasmonic nanostructures are fabricated on top of a graphene sheet. The use of Fano resonant elements enhances the interaction of incident radiation with the graphene sheet and enables efficient electrical modulation of the plasmonic resonance. We observe electrically controlled damping in the Fano resonances occurring at approximately 2 μm, and the results are verified by full-wave 3D finite-element simulations. Our approach can be used for development of next generation of tunable plasmonic and hybrid nanophotonic devices.

Graphene's potential in materials science and engineering

Materials have become an indispensable part of our modern life, which was tailored such as good mechanical, electrical, thermal properties, establish the basis and fundamentals and the governing rules for every modern technology.

Enhanced light-matter interactions in graphene-covered gold nanovoid arrays

The combination of graphene with noble-metal nanostructures is currently being explored for strong light-graphene interactions enhanced by plasmons. We introduce a novel hybrid graphene-metal system for studying light-matter interactions with gold-void nanostructures exhibiting resonances in the visible range. Enhanced coupling of graphene to the plasmon modes of the nanovoid arrays results in significant frequency shifts of the underlying plasmon resonances, enabling 30% enhanced absolute light absorption by adding a monolayer graphene and up to 700-fold enhancement of the Raman response of the graphene. These new perspectives enable us to verify the presence of graphene on gold-void arrays, and the enhancement even allows us to accurately quantify the number of layers. Experimental observations are further supported by numerical simulations and perturbation-theory analysis. The graphene gold-void platform is beneficial for sensing of molecules and placing Rhodamine 6G (R6G) dye molecules on top of the graphene; we observe a strong enhancement of the R6G Raman fingerprints. These results pave the way toward advanced substrates for surface-enhanced Raman scattering (SERS) with potential for unambiguous single-molecule detection on the atomically well-defined layer of graphene.

Low-bias active control of terahertz waves by coupling large-area CVD graphene to a terahertz metamaterial

We propose an hybrid graphene/metamaterial device based on terahertz electronic split-ring resonators directly evaporated on top of a large-area single-layer CVD graphene. Room temperature time-domain spectroscopy measurements in the frequency range from 250 GHz to 2.75 THz show that the presence of the graphene strongly changes the THz metamaterial transmittance on the whole frequency range. The graphene gating allows active control of such interaction, showing a modulation depth of 11.5% with an applied bias of 10.6 V. Analytical modeling of the device provides a very good qualitative and quantitative agreement with the measured device behavior. The presented system shows potential as a THz modulator and can be relevant for strong light-matter coupling experiments.

Alternative plasmonic materials: beyond gold and silver

Materials research plays a vital role in transforming breakthrough scientific ideas into next-generation technology. Similar to the way silicon revolutionized the microelectronics industry, the proper materials can greatly impact the field of plasmonics and metamaterials. Currently, research in plasmonics and metamaterials lacks good material building blocks in order to realize useful devices. Such devices suffer from many drawbacks arising from the undesirable properties of their material building blocks, especially metals. There are many materials, other than conventional metallic components such as gold and silver, that exhibit metallic properties and provide advantages in device performance, design flexibility, fabrication, integration, and tunability. This review explores different material classes for plasmonic and metamaterial applications, such as conventional semiconductors, transparent conducting oxides, perovskite oxides, metal nitrides, silicides, germanides, and 2D materials such as graphene. This review provides a summary of the recent developments in the search for better plasmonic materials and an outlook of further research directions.

An all-optical, non-volatile, bidirectional, phase-change meta-switch

Non-volatile, bidirectional, all-optical switching in a phase-change metamaterial delivers high-contrast transmission and reflection modulation at near- to mid-infrared wavelengths in device structures down to ≈1/27 of a wavelength thick.