Graphene Plasmons Research Articles

Wake effect in interactions of ions with graphene-sapphire-graphene composite system

We study the wake effect in a graphene-Al 2 O 3 -graphene composite system induced by an external charged particle moving parallel to it by using the dynamic polarization function of graphene within the random phase approximation for its π electrons described as Dirac's fermions and by using a local dielectric function for the bulk sapphire (aluminum oxide, Al 2 O 3 ). We explore the effects of variation of the particle speed, its distance from the top graphene layer, the thickness of the Al 2 O 3 layer, the damping rate of plasmons in graphene, and the doping density (i.e., Fermi energy) of graphene on the wake potential. For the velocity of the charged particle below the threshold for excitations of the Dirac plasmon in graphene, given by its Fermi velocity v F , strong effects are observed due to variation of the particle distance, while for the velocity of the charged particle above v F strong effects are observed due to varying the thickness of the Al 2 O 3 layer, as well as due to plasmon damping of graphene's π electrons, and graphene doping. • We present the wake effect in graphene-Al 2 O 3 -graphene due to moving charge particle. • Particle velocity v is taken to be below and above Fermi velocity in graphene v F . • For v < v F wake is dominated by Fuchs-Kliewer phonons regardless of the Al 2 O 3 thickness. • Fuchs-Kliewer phonons are weakened by Landau damping as doping of graphene increases. • For v F < v < 3 v F wake is dominated by hybridization of Dirac plasmons in graphene layers.

Coupled plasmonic graphene wires: theoretical study including complex frequencies and field distributions of bright and dark surface plasmons

Theoretical research on localized surface plasmons (LSPs) supported by a structure formed by two parallel dielectric wires with a circular cross section wrapped with a graphene sheet has an impact in the practical realm. Here, LSPs are represented in the form of an infinite series of cylindrical multipole partial waves linked to each of the graphene wires. To obtain the kinematics (complex eigenfrequencies) and dynamic characteristics (field distributions) of the LSPs, we consider the analytic extension to the complex plane of the solution to the boundary value problem. The lower frequency LSP group is formed by four branches, two of them corresponding to bright modes and the others to dark modes. By tuning the chemical potential of the graphene covers so that the dimer symmetry breaks, these dark modes are transformed into resonant observables that can be excited by plane wave incidence.

Resolved Infrared Spectroscopy of Aqueous Molecules Employing Tunable Graphene Plasmons in an Otto Prism.

Real-time and in situ detection of aqueous solution is essential for bioanalysis and chemical reactions. However, it is extremely challenging for infrared microscopic measurement because of the large background of water absorption. Here, we proposed a wideband-tunable graphene plasmonic infrared biosensor to detect biomolecules in an aqueous environment, employing attenuated total reflection in an Otto prism configuration and tightly confined plasmons in graphene nanoribbons. Benefiting from the graphene plasmonic electric field enhancement, such a biosensor is able to identify the molecular chemical fingerprints without the interference of water absorption. As a proof of concept, the recombinant protein AG and goat anti-mouse immunoglobulin G (IgG) are used as the sensing analytes, of which the vibrational modes (1669 and 1532 cm-1) are very close to the OH-bending mode of water (1640 cm-1). Simulation results show that the fingerprints of protein molecules in the water environment can be selectively enhanced. Therefore, the water absorption is successfully suppressed so that two protein modes can be resolved by sweeping graphene Fermi energy in a wide waveband. By further optimizing the incident angle and graphene mobility to improve the mode energy of graphene plasmons, maximum enhancement factors of 112 and 130 can be achieved for amide I and II bands. Our work provides an effective approach for the highly sensitive and selective in situ identification of aqueous-phase molecular fingerprints in fields of healthcare, food safety, and biochemical sensing.

Enhancing Graphene Plasmonic Device Performance via its Dielectric Environment

Graphene plasmons provide a compelling avenue toward chip-scale dynamic tuning of infrared light. Dynamic tunability emerges through controlled alterations in the optical properties of the system defining graphene's plasmonic dispersion. Typically, electrostatic induced alterations of the carrier concentration in graphene working in conjunction with mobility have been considered the primary factors dictating plasmonic tunability. We find here that the surrounding dielectric environment also plays a primary role, dictating not just the energy of the graphene plasmon but so too the magnitude of its tuning and spectral width. To arrive at this conclusion, poles in the imaginary component of the reflection coefficient are used to efficiently survey the effect of the surrounding dielectric on the tuning of the graphene plasmon. By investigating several common polar materials, optical phonons (i.e., the Reststrahlen band) of the dielectric substrate are shown to appreciably affect not only the plasmon's spectral location but its tunability, and its resonance shape as well. In particular, tunability is maximized when the resonances are spectrally distant from the Reststrahlen band, whereas sharp resonances (i.e., high-$Q$) are achievable at the band's edge. These observations both underscore the necessity of viewing the dielectric environment in aggregate when considering the plasmonic response derived from two-dimensional materials and provide heuristics to design dynamically tunable graphene-based infrared devices.

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.

"Fast" Plasmons Propagating in Graphene Plasmonic Waveguides with Negative Index Metamaterial Claddings.

We propose the monolayer graphene plasmonic waveguide (MGPW), which is composed of graphene core sandwiched by two graphene metamaterial (GMM) claddings and investigate the properties of plasmonic modes propagating in the waveguide. The effective refraction index of the GMMs claddings takes negative (or positive) at the vicinity of the Dirac-like point in the band structure. We show that when the effective refraction index of the GMMs is positive, the plasmons travel forward in the MGPW with a positive group velocity (vg > 0, vp > 0). In contrast—for the negative refraction index GMM claddings—a negative group velocity of the fundamental mode (vg < 0, vp > 0) appears in the proposed waveguide structure when the core is sufficiently narrow. A forbidden band appears between the negative and positive group velocity regions, which is enhanced gradually as the width of the core increases. On the other hand, one can overcome this limitation and even make the forbidden band disappear by increasing the chemical potential difference between the nanodisks and the ambient graphene of the GMM claddings. The proposed structure offers a novel scheme of on-chip electromagnetic field and may find significant applications in the future high density plasmonic integrated circuit technique.

Graphene Plasmonics: Fully Atomistic Approach for Realistic Structures.

We demonstrate that the plasmonic properties of realistic graphene and graphene-based materials can effectively and accurately be modeled by a novel, fully atomistic, yet classical, approach, named ωFQ. Such a model is able to reproduce all plasmonic features of these materials and their dependence on shape, dimension, and fundamental physical parameters (Fermi energy, relaxation time, and two-dimensional electron density). Remarkably, ωFQ is able to accurately reproduce experimental data for realistic structures of hundreds of nanometers (∼370k atoms), which cannot be afforded by any ab initio method. Also, the atomistic nature of ωFQ permits the investigation of complex shapes, which can hardly be dealt with by exploiting widespread continuum approaches.

Bistable Terahertz Switch Designed by Integration of a Graphene Plasmonic Crystal into Fabry-Perot Resonator

An appropriately designed periodic lattice in graphene can cause plasmonic modes, absorbing a specific THz frequency band, minimizing transmission. A suitable Fabry-Perot cavity, on the other hand, can resonate at another desired frequency in the nearby THz band, enhancing the transmittance. Integrating, these two types of structures into a THz switch, in which the peaks of the plasmonic absorption and the Fabry-Perot resonance band play the role of the OFF and ON states, yielding a bistable switch with a very high output due to the resonance and a high extinction (ON/OFF) ratio. By placing a patterned with an array of circular micro-holes with a plasmonic absorption band centered about 4.1 THz in the middle of a Fabry-Perot vertical cavity of resonance band centered about 3.9 THz, we have designed a bistable THz switch with 87% transmission in the ON state and the extinction ratio of 23 dB. The optical bistability, in this switch, is the consequence of graphene nonlinear property invoked by the enhanced optical intensity due to the cavity resonance, adding an intensity-dependent term to the graphene surface conductivity.

Nanopolaritonic second-order topological insulator based on graphene plasmons

Ultrastrong confinement, long lifetime, and gate-tunability of graphene plasmon polaritons (GPPs) motivate wide-ranging efforts to develop GPP-based active nanophotonic platforms. Incorporation of topological phenomena into such platforms will ensure their robustness as well as enrich their capabilities as promising test beds of strong light–matter interactions. A recently reported approach suggests an experimentally viable platform for topological graphene plasmonics by introducing nanopatterned gates—metagates. We propose a metagate-tuned GPP platform emulating a second-order topological crystalline insulator. The metagate imprints its crystalline symmetry onto graphene by modulating its chemical potential via field-effect gating. Depending on the gate geometry and applied voltage, the resulting two-dimensional crystal supports either one-dimensional chiral edge states or zero-dimensional midgap corner states. The proposed approach to achieving the hierarchy of nontrivial topological invariants at all dimensions lower than the dimension of the host material paves the way to utilizing higher-order topological effects for on-chip and ultracompact nanophotonic waveguides and cavities.

Nonlocal effects and enhanced nonreciprocity in current-driven graphene systems

A graphene sheet biased with a drift electric current offers a tantalizing opportunity to attain unidirectional, backscattering-immune, and subwavelength light propagation, as proposed in [T. A. Morgado, M. G. Silveirinha, ACS Photonics 5(11), 4253 (2018)]. Here, we investigate in detail the impact of the intrinsic nonlocal response of graphene in the dispersion characteristics of the current-driven plasmons supported by single-layer and double-layer graphene systems. It is theoretically shown that even though the nonlocal effects weaken the spectral asymmetry of the plasmons dispersion, the studied platforms can support unidirectional backscattering-immune guided modes. Our analysis also confirms that the drift-current bias can effectively pump the graphene plasmons and enhance the propagation distance. Moreover, it is shown that the nonreciprocity and optical isolation can be boosted by pairing two drift-current biased graphene sheets due to the enhanced radiation drag by the drifting electrons.

Hydrodynamic Terahertz Plasmons and Electron Sound in Graphene with Spatial Dispersion

The intrinsic transverse-magnetic modes of graphene with spatial dispersion are investigated theoretically in the hydrodynamic regime in a terahertz frequency range. It is revealed that spatial dispersion is caused by the diffusion electron-transport mechanism due to the spreading of electron-concentration gradients under the effect of pressure in an electron liquid. The cases of a screened and unscreened plasmon as well as electron sound are considered.

Ultrathin multi-band coherent perfect absorber in graphene with high-contrast gratings.

High-contrast gratings (HCGs) can be designed as a resonator with high-quality factor and surface-normal emission, which are excellent characters for designing optical devices. In this work, we combine HCGs with plasmonic graphene structure to achieve an ultrathin five-band coherent perfect absorber (CPA). The presented CPA can achieve multi- and narrow-band absorption with high intensity under a relatively large incident angle. The good agreement between theoretical analysis and numerical simulated results demonstrates that our proposed HCGs-based structure is feasible to realize CPA. Besides, by dynamically adjusting the Fermi energy of graphene, we realize the active tunability of resonance frequency and absorption intensity simultaneously. Benefitting from the combination of HCGs and the one-atom thickness of graphene, the proposed device possesses an extremely thin feature. Our work proposes a novel method to manipulate coherent perfect absorption and is helpful to design tunable multi-band and ultrathin absorbers.

Confined transverse-electric graphene plasmons in negative refractive-index systems

Transverse electric graphene plasmons are generally weakly confined in the direction perpendicular to the graphene plane. They are featured by a skin depth δ, namely the penetration depth of their evanescent fields into the surrounding environment, much larger than the wavelength λ in free space (e.g., δ > 10λ). The weak spatial confinement of transverse electric graphene plasmons is now the key drawback that limits their practical applications. Here we report the skin depth of TE graphene plasmons can be largely decreased down to the subwavelength scale (e.g., δ < λ/10) in negative refractive-index environments. The underlying mechanism originates from the different existence conditions for TE graphene plasmons in negative and positive refractive-index environments. To be specific, their existence in negative (positive) refractive-index environments requires Im(σs) > 0 (Im(σs) < 0) and lies in the frequency range of ħω/μc < 1.667 (ħω/μc > 1.667), where σs and μc are the surface conductivity and chemical potential of monolayer graphene, respectively.

High-Q dual-band graphene absorbers by selective excitation of graphene plasmon polaritons: Circuit model analysis

This article presents the design of two dual-band graphene-based absorbers for terahertz frequencies. The absorbers are composed of two-dimensional (2D) arrays of ribbons and disks printed on a ground plane backed dielectric spacer. The design is based on the excitation of a specific plasmon polariton of the graphene patterned array at each resonance band. An analytical circuit model is used to derive closed-form relations for the geometrical parameters of the absorber and graphene parameters. The graphene patterned array appears as a surface admittance made of an infinite parallel array of series RLC branches. Each branch is equivalent to a graphene plasmon polariton (GPP) providing a distinct resonance mode. The design procedure is based on selectively exciting the first two GPPs. This means that two of the parallel RLC branches are selectively used in the circuit model analysis. The results obtained using the analytical solution are compared with the full-wave simulations in HFSS. The agreement between the results validates the developed design method.

Understanding the Electromagnetic Response of Graphene/Metallic Nanostructures Hybrids of Different Dimensionality

Plasmonic excitations such as surface-plasmon-polaritons (SPPs) and graphene-plasmons (GPs), carry large momenta and are thus able to confine electromagnetic fields to small dimensions. This property makes them ideal platforms for subwavelength optical control and manipulation at the nanoscale. The momenta of these plasmons are even further increased if a scheme of metal-insulator-metal and graphene-insulator-metal are used for SPPs and GPs, respectively. However, with such large momenta, their far-field excitation becomes challenging. In this work, we consider hybrids of graphene and metallic nanostructures and study the physical mechanisms behind the interaction of far-field light with the supported high momenta plasmon modes. While there are some similarities in the properties of GPs and SPPs, since both are of the plasmon-polariton type, their physical properties are also distinctly different. For GPs we find two different physical mechanism related to either GPs confined to isolated cavities, or large area collective grating couplers. Strikingly, we find that although the two systems are conceptually different, under specific conditions they can behave similarly. By applying the same study to SPPs, we find a different physical behavior, which fundamentally stems from the different dispersion relations of SPPs as compared to GPs. Furthermore, these hybrids produce large field enhancements that can also be electrically tuned and modulated making them the ideal candidates for a variety of plasmonic devices.

Electric Control of Spin‐Orbit Coupling in Graphene‐Based Nanostructures with Broken Rotational Symmetry

AbstractSpin and orbital angular momenta of light are important degrees of freedom in nanophotonics which control light propagation, optical forces, and information encoding. Here, it is shown that graphene‐supported plasmonic nanostructures with broken rotational symmetry provide a surprising spin to orbital angular momentum conversion, which can be continuously controlled by changing the electrochemical potential of graphene. Upon resonant illumination by a circularly polarized plane wave, a polygonal array of indium‐tin‐oxide nanoparticles on a graphene sheet generates the scattered field carrying electrically‐tunable orbital angular momentum. This unique photonic spin‐orbit interaction occurs due to the strong coupling between graphene plasmon polaritons and localized surface plasmons of the nanoparticles and leads to the controlled directional excitation of graphene plasmons. The tuneable spin‐orbit conversion paves the way for high‐rate information encoding in optical communications, electric steering functionalities in optical tweezers, and nanorouting of higher‐dimensional entangled photon states.

Flexible control of absorption enhancement of circularly polarized light via square graphene disks

A narrowband absorber consisting of periodically patterned square graphene disks (SGDs) is proposed to achieve flexible control of the absorption enhancement of circularly polarized light (CPL) in the far-infrared region. It is shown that absorption of CPL can be enhanced by utilizing the double-cavity enhancement of edge graphene plasmons (EGPs) of the SGDs in both x and y directions. Perfect light absorption can be achieved by minimizing the reflectance through perfect impedance matching and simultaneously eliminating the transmittance by the metallic substrate. By using the Fabry-Pérot (F-P) cavity model with a linear fitting method, the location of the absorption peak of CPL can be well estimated. The location of the absorption peak can be modulated by changing the Fermi level of graphene, while it can be kept almost the same even though the structural parameters such as period and the thickness of the dielectric spacer are significantly altered. Furthermore, by integrating multi-sized SGDs into the unit cell of the structure, multiple absorption channels of CPL with good absorption performances can be realized. As examples, two and three absorption channels with high peak absorptivity are demonstrated via double and triple SGDs, respectively.

Graphene sensing nanostructure for exact graphene layers identification at terahertz frequency

Abstract In this paper, we present a new graphene plasmonic sensing structure and use the finite difference time domain method and also a graphene superconducting properties for counting the number of n-graphene layer, where 1 ≤ n ≤ 5. From the intensity change of the reflection of graphene plasmons excited at the boundary of graphene/white graphene layer, we can exactly estimate the number of graphene layers. The structure considered here supports surface plasmons whose dispersion properties can be harnessed by applying an electrical field to the graphene. By utilizing this feature, efficient coupling of the incident light to the surface plasmons of the structure is used for counting the number of graphene layer. Therefore, single layer graphene can be distinguished from double- and few-layer by efficient coupling condition. Also, the effects of the substrate of graphene on the reflection of the transverse electric and magnetic graphene plasmons are investigated. By applying a chemical potential of v g = 0.4 eV, a compact sensor with the footprint of 0.6 nm × 10 μ m × 20 μ m is achieved. Finally, experimental setup is used to verify and validate this numerical model with good results.

Graphene plasmonic nanoresonators/graphene heterostructures for efficient room-temperature infrared photodetection

High-performance infrared (IR) photodetectors made by low dimensional materials promise a wide range of applications in communication, security and biomedicine. Moreover, light-harvesting effects based on novel plasmonic materials and their combinations with two-dimensional (2D) materials have raised tremendous interest in recent years, as they may potentially help the device complement or surpass currently commercialized IR photodetectors. Graphene is a particularly attractive plasmonic material because graphene plasmons are electrically tunable with a high degree of electromagnetic confinement in the mid-infrared (mid-IR) to terahertz regime and the field concentration can be further enhanced by forming nanostructures. Here, we report an efficient mid-IR room-temperature photodetector enhanced by plasmonic effect in graphene nanoresonators (GNRs)/graphene heterostructure. The plasmon polaritons in GNRs are size-dependent with strong field localization. Considering that the size and density of GNRs are controllable by chemical vapor deposition method, our work opens a cost-effective and scalable pathway to fabricate efficient IR optoelectronic devices with wavelength tunability.

Surface plasmon dispersion and modes on the graphene metasurface with periodical ribbon arrays

Graphene plasmonics on the structured metasurface demonstrate many exotic properties which can combine novel nanomaterials and well-established plasmonics, providing unique opportunities to develop a series of novel photonic, plasmonic and optoelectronic devices across a wideband spectrum. Dispersion theory and its propagating characteristics of surface plasmon polaritons (SSPs) mode along the graphene metasurface can provide a powerful guidance to design related devices and systems. In this paper, the fundamental dispersion theory and the numerical studies of graphene SSPs (GSPs) on a graphene metasurface i.e. periodical ribbon arrays which are bounded by a superstrate and substrate dielectric are presented. The dispersion expression of GSPs is deduced and revealed by a modal expansion method combined with periodical boundary conditions on the structure. According to this fully analytical dispersion expression of SSPs mode on the graphene metasurface, the dispersion characteristics, propagation loss and field profiles of SSPs mode with different graphene material parameters (e.g. graphene ribbon width and chemical potential) and bounded dielectric mediums are studied and analyzed in detail in terahertz (THz) band. Moreover, the dynamical tunable dispersion characteristics of SSPs mode on the graphene metasurface via electrostatic gating of a ground metal plate can be readily obtained by applying a graphene biased voltage model to this analytical dispersion theory. The presented studies on the dispersion theory of the graphene metasurface provide an analytical method to understand the propagation characteristics of SSPs mode on the structure. Besides, the calculation results on the structure can also be used to design some novel graphene-based optoelectronic and plasmonic devices with planar gradient-index distributions such as couplers, tunable focused lens and enhanced radiation sources in THz band.