Graphene Plasmon Modes Research Articles

Efficient guiding mid-infrared waves with graphene-coated nanowire based plasmon waveguides

A graphene-coated nanowire (GCNW) based plasmon waveguide is proposed to achieve outstanding guiding performance in the mid-infrared band. The modal properties of the fundamental graphene plasmon mode and modal dependence on the elliptical nanowire size, thickness and width of the low-index dielectric layer, chemical potential of graphene and the operating frequency are revealed in detail by using finite element method. Results show that the low-index layer has a significant influence on modal properties, and a propagation length over 13 μm along with an extremely small normalized mode size (~10−6) could be obtained simultaneously, which is more than two orders of magnitude smaller compared with free-standing GCNW and GCNW dimers. Besides, study on crosstalk characteristics between the adjacent structures indicates the proposed waveguide exhibits very low crosstalk. Due to these excellent transmission characteristics, the proposed configuration could be used to fabricate various functional photonic devices for the future nanoscale photonic integrated circuits.

Low voltage, high modulation depth graphene THz modulator employing Fabry–Perot resonance in a metal/dielectric/graphene sandwich structure

In this article, a novel THz modulator for modulating signals in the frequency range of 0.5–10 THz is designed and numerically investigated with finite difference time domain method. The structure is composed of a graphene layer sandwiched between two complementary metallic gratings. The proposed structure has Fabry–Perot like behavior at which the resonant condition depends on the geometrical parameters of the device and graphene plasmonic mode characteristics. A major advantage of the presented modulator is its capability to function in various regimes such as resonant and non-resonant as well as transmission and reflection. Based on the proposed structure, a broadband modulator for THz signals in the frequency range of 0.5–3 THz is proposed with modulation depth of 90% (36%) at 0.5 THz (3 THz). Insertion loss of the modulator is 1.2 dB. Electro-optic bandwidth is more than 2 GHz depending on the device surface area. In addition to this, several low voltage and relatively narrowband modulators are designed either at central frequencies of 4.5, 5.5, and 6.5 THz with modulation depth of more than 75% and insertion loss of less than 2.5 dB at their central frequency.

Mid-infrared Gas Sensing Using Graphene Plasmons Tuned by Reversible Chemical Doping

Highly confined plasmon modes in nanostructured graphene can be used to detect tiny quantities of biological and gas molecules. In biosensing, a specific biomarker can be concentrated close to grap...

Modal properties of a cylindrical graphene-coated nanowire deposited on a hexagonal boron nitride substrate.

Due to complementary chemical and optical characteristics, structural integration of graphene and hexagonal boron nitride (hBN) can lead to a promising platform for development of novel plasmonic devices. In this paper, we numerically investigate the modal behavior of a cylindrical graphene-coated nanowire (GNW) deposited on a thin hBN (GNW-hBN) substrate in the mid-infrared range. Our studies revealed that GNW-hBN can support hybridized plasmon-phonon modes in the upper reststrahlen band of hBN, which mainly originates from the strong coupling between plasmon modes in GNW and phonon modes in hBN. The characteristics of these hybrid modes can be effectively tuned by changing the chemical potential of graphene, hBN thickness, and gap distance between GNW and hBN. According to the results, by choosing smaller gap distances and tuning the chemical potential of graphene, GNW-hBN can exhibit a fundamental mode (m=0, where m is the azimuthal mode number) with higher effective index such that Real(neff) varies from 131.2-62.3 when the hBN thickness changes from 2-20nm. In addition, the presence of an hBN slab can break the azimuthal symmetry of the high-order graphene plasmon modes (m≥1) in the GNW-hBN structure.

Plasmon modes in graphene — GaAs heterostructures at finite temperature

This paper is to investigate the dispersion relation and decay rate of plasmon modes in a double layer system made of monolayer graphene (MLG) and infinite GaAs quantum well at finite temperature within the generalized random-phase-approximation and taking into account the 2DEG layer-thickness and the inhomogeneity of the background dielectric. Calculations demonstrate that when the quantum well width increases, the acoustic (AC) plasmon frequency decreases dramatically, but the optical (OP) one seems unchanged. In addition, the results also illustrate that the temperature and separated distance affect significantly both AC and OP plasmon modes of the system. Finally, the dielectric of the background acts strongly on the OP plasmon curve while carrier density in two layers and exchange-correlation effects only lead to remarkable changes for the acoustic one.

Graphene-coated nanowire dimers for deep subwavelength waveguiding in mid-infrared range.

In this paper, we show that the graphene-coated nanowire dimers could enable outstanding waveguiding performance in the mid-infrared range. The propagating properties of the fundamental graphene plasmon mode and their dependence on the nanowire radius, gap distance, nanowire permittivity and chemical potential of graphene are revealed in detail and compared with the graphene-coated circular nanowire. By improving the geometric parameters and the surface conductivity of graphene, the propagation length could reach about 9 μm, which is larger than that of the graphene-coated circular nanowire plasmon mode. Meanwhile, the effective mode area is only 10-4A0, which is one order of magnitude smaller than that of the graphene-coated circular nanowire plasmon mode. Theoretically, the propagation length could be further enhanced by increasing the chemical potential. Besides, the proposed graphene-coated nanowire dimers show quite good fabrication tolerance. The manipulation of mid-infrared waves at the deep subwavelength scale using graphene plasmons may offer potential applications in tunable integrated nanophotonic devices and infrared sensing.

Plasmon spectrum of graphene monolayer on substrate

Plasmon modes in monolayer graphene on substrate are analyzed taking into account the thickness of graphene and substrate material layer in the evaluation of Coulomb potential. It is shown that plasmon mode in graphene monolayer has linear dispersion in contrast to multilayer graphene in long-wavelength limit. The slope of plasmon spectrum is determined by the thickness and dielectric constant of substrate. Obtained results are in good agreement with experimental data and other theoretical considerations.

Split graphene nano-disks with tunable, multi-band, and high-Q plasmon modes

We propose a new approach to directly tune spectral Q-factor of the graphene, which has remained challenging in these years. The remarkable tuning of optical properties is reported by splitting graphene disk resonators. In contrast to the single mode of the circular graphene disk, the split graphene nano-disks produce new emerged plasmonic resonant modes, which can be further modified by tuning structural parameters. Such split graphene nano-disks can not only strengthen the multispectral light-graphene interactions drastically but also improve the spectral Q-factor naturally. The spectral Q-factor shows an enhancement of 230–360% for the proposed split graphene nano-disks in comparison with that of the conventional graphene plasmonic mode. With increasing Fermi energy, the tri-band spectrum shows a further narrowing behavior for the resonant bandwidth. A maximal enhancement ∼400% for the spectral Q-factor is obtained. Moreover, the graphene plasmonic properties can be artificially modulated by polarization states of the illumination. This study will open up an avenue for effectively operating graphene photonic devices in the infrared range and pave a way for high-Q multispectral plasmonic graphene structures.

Modulated Resonant Transmission of Graphene Plasmons Across a λ /50 Plasmonic Waveguide Gap

We theoretically demonstrate the nontrivial transmission properties of a graphene-insulator-metal waveguide segment of deeply subwavelength scale. We show that, at midinfrared frequencies, the graphene-covered segment allows for the resonant transmission through the graphene-plasmon modes as well as the nonresonant transmission through background modes, and that these two pathways can lead to a strong Fano interference effect. The Fano interference enables a strong modulation of the overall optical transmission with a very small change in graphene Fermi level. By engineering the waveguide junction, it is possible that the two transmission pathways perfectly cancel each other out, resulting in a zero transmittance. We theoretically demonstrate the transmission modulation from 0% to 25% at 7.5-µm wavelength by shifting the Fermi level of graphene by a mere 15 meV. In addition, the active region of the device is more than 50 times shorter than the free-space wavelength. Thus, the reported phenomenon is of great advantage to the development of on-chip plasmonic devices.

Tunable Fano Resonance Based Mode Interference in Waveguide-Cavity-Graphene Hybrid Structure

In this paper, a graphene strip is introduced into a metal-insulator-metal (MIM)-integrated square cavity hybrid structure; the transmission spectra are theoretically investigated by the finite different time domain (FDTD) methods. An asymmetric Fano resonance dip that has high figure of merit (FOM) value appears in the transmission band. According to the multimode interference coupled mode theory (MICMT) analytical method, the Fano resonance originates from the coherent coupling between TM10 cavity magnetic mode and graphene plasmonic resonance electric mode. The center wavelength, full width at half maximum (FWHM), and FOM value of the Fano resonance can be tuned dynamically by altering the Fermi level of the graphene. Through breaking the symmetry of the hybrid structure or introducing double graphene strips with different Fermi level into hybrid structure, double Fano resonance are realized. This study can provide some theoretical basis and design reference for designing ultrahigh sensitivity plasmonic sensor.

Experimental study of graphene-based plasmonic crystals in a strong-coupling regime

We have fabricated a large-area graphene-based transistor with top metal grating, which represents plasmonic crystal containing two-dimensional electron system gated by periodic metal grating. Using Fourier-transform infrared spectroscopy, we have studied the graphene plasmon modes in transmission spectra, which were efficiently excited by the incident light with the perpendicular polarization to metal grating. We have researched the dependence of resonance frequency of graphene plasmons on the bottom gate voltage. Here we demonstrate that excited graphene plasmons are strongly coupled to top metal grating and have the linear dispersion. These results not only prove the previous theoretical works but also demonstrate the possibility of significant reducing photon wavelength which can be controlled by modulating the carrier density of graphene via electrical gating. This result is important for design of highly effective tunable terahertz and far-infrared detectors.

Role of electron back action on photons in hybridizing double-layer graphene plasmons with localized photons

In this paper, we deal with the electromagnetic coupling between an incident surface-plasmon-polariton wave and relativistic electrons in two graphene layers. Our previous investigation was limited to single-layer graphene (Iurov et al 2017 Phys. Rev. B 96 081408). However, the present work, is both an expanded and extended version of this previous Phys. Rev. B paper after having included very detailed theoretical formalisms and extensive comparisons of results from either one or two graphene layers embedded in a dielectric medium. The additional retarded Coulomb interaction between two graphene layers will compete with the coupling between the single graphene layer and the surface of a conductor. Consequently, some distinctive features, such as triply-hybridized absorption peaks and a new acoustic-like graphene plasmon mode within the anticrossing region, have been found for the double-layer graphene system. Physically, our theory is self-consistent, in comparison with a commonly adopted perturbative theory, for studying hybrid light-plasmon modes and the electron back action on photons. Instead of usual radiation or grating-deflection field coupling, a surface-plasmon-polariton localized field coupling is introduced with completely different dispersion relations for radiative (small wave numbers) and evanescent (large wave numbers) field modes. Technically, the exactly calculated effective scattering matrix for this theory can be employed to construct an effective-medium theory in order to improve the accuracy of the well-known finite-difference time-domain method for solving Maxwell’s equations numerically. Practically, the predicted triply-hybridized absorption peaks can excite polaritons only, giving rise to a possible polariton-condensation based laser.

Channel hybrid plasmonic modes in dielectric-loaded graphene groove waveguides

Dielectric-loaded graphene groove waveguide (DLGGW) structure is designed and the channel hybrid plasmonic modes are investigated in the terahertz domain. The proposed structure could effectively suppress the mode field confinement with relative low transmission loss due to the strong coupling between the dielectric mode and the channel graphene plasmonic mode. A typical propagation length is 37.8μm, and optical field is confined into an ultra-small area of approximately 52μm2 at 1.5 THz. By changing the size of cylinder, the angle of the graphene groove, and the thickness of dielectric coating layer, the compromise between confinement and loss could be balanced flexibly. Unlike plasmons in noble metals, the propagation loss in DLGGW structure could be tuned by the graphene conductivity. Moreover, because of the imaginary part of graphene conductivity being positive, the proposed waveguide can propagate in a large terahertz frequency regime. This waveguide shows great applications in optical integrated devices, such as detectors, sensors and spectroscopy.

Mode energy of graphene plasmons and its role in determining the local field magnitudes.

We theoretically study the mode energy of graphene plasmons and its fundamental role in determining the local field magnitudes. While neglecting the magnetic field energy of the mode, we derive a concise expression for the total mode energy, which is independent on the details of the mode field distributions and valid for both propagating and localized modes. We find that the mean square of the local electric fields of a graphene plasmonic mode scales linearly with the light absorption rate of the mode and the electron relaxation time of graphene. The possible strategies for improving the local field magnitudes of graphene plasmons are also discussed. Our theoretical analysis presented here may benefit the design of various graphene-based optical and optoelectronic devices for light-harvesting or energy conversion.

Mechanism of propagating graphene plasmons excitation for tunable infrared photonic devices.

The mechanism of propagating graphene plasmons excitation using a nano-grating and a Fabry-Pérot cavity as the optical coupling components is studied. It is demonstrated that the system could be well described within the temporal coupled mode theory using two phenomenological parameters, namely, the intrinsic loss rate and the coupling rate of a graphene plasmonic mode, and their analytical expressions are derived. It is found that the intrinsic loss rate is solely determined by the electron relaxation time of graphene, while independent of the field distributions of the modes. Such result originates from the negligible magnetic field energy of the graphene plasmonic mode. The coupling rate is governed by the optical coupling components parameters, and varies periodically with the Fabry-Pérot cavity length. By modulating the two rates, quality factors and absorption rates can be adjusted. Furthermore, it is revealed that low refractive index of the Fabry-Pérot cavity material is vital to the enlargement of tunable band, and the underlying physics is discussed. Such plasmon excitation configuration is insensitive to light incident angle and could serve as a platform for many tunable infrared photonic device, such as surface-enhanced infrared absorption spectroscopies, infrared detectors and modulators.

Tunable Plasmon Induced Transparency in a Metallodielectric Grating Coupled With Graphene Metamaterials

A novel metallodielectric grating coupled with graphene metamaterials (MGCGM) structure is proposed to achieve the tunable plasmon induced transparency (PIT) effect. Two different mechanisms are employed to explain the PIT effect in the reflection spectrum: one is originated from the destructive interference between graphene plasmonic modes and the hybrid mode in the dielectric layer, the other is related to the absorption of the graphene surface plasmons. The simulated results indicate high tunability in the amplitude and the operating wavelength of the PIT effect can be obtained by controlling the Fermi levels of graphene ribbons. Besides, double PITs are demonstrated in this scheme by altering the occupation ratio of the graphene-based ribbon grating. Compared with previous reports, the MGCGM structure is much easier to fabricate and the tunable PIT effect has significant applications in optical modulator, switching, absorber, sensor, slow light, etc.

Plasmon modes in graphene–GaAs heterostructures

We investigate the plasmon dispersion relation and damping rate of collective excitations in a double-layer system consisting of bilayer graphene and GaAs quantum well, separated by a distance d, at zero temperature with no interlayer tunneling. We use the random-phase-approximation dielectric function and take into account the nonhomogeneity of the dielectric background of the system. We show that the plasmon frequencies and damping rates depend considerably on interlayer correlation parameters, electron densities and dielectric constants of the contacting media.

Effects of optical polarization on hybridization of radiative and evanescent field modes

A back action from Dirac electrons in graphene on the hybridization of radiative and evanescent fields is found as an analogy to Newton's third law. Here, the back action appears as a localized polarization field which greatly modifies an incident surface-plasmon-polariton (SPP) field. This yields a high sensitivity to local dielectric environments and provides a scrutiny tool for molecules or proteins selectively bounded with carbons. A scattering matrix is shown with varied frequencies nearby the surface-plasmon (SP) resonance for the increase, decrease and even a full suppression of the polarization field, which enables accurate effective-medium theories to be constructed for Maxwell-equation finite-difference time-domain methods. Moreover, double peaks in the absorption spectra for hybrid SP and graphene-plasmon modes are significant only with a large conductor plasma frequency, but are overshadowed by a round SPP peak at a small plasma frequency as the graphene is placed close to conductor surface. These resonant absorptions facilitate the polariton-only excitations, leading to polariton condensation for a threshold-free laser.

Low-dimensional gap plasmons for enhanced light-graphene interactions

Graphene plasmonics has become a highlighted research area due to the outstanding properties of deep-subwavelength plasmon excitation, long relaxation time, and electro-optical tunability. Although the giant conductivity of a graphene layer enables the low-dimensional confinement of light, the atomic scale of the layer thickness is severely mismatched with optical mode sizes, which impedes the efficient tuning of graphene plasmon modes from the degraded light-graphene overlap. Inspired by gap plasmon modes in noble metals, here we propose low-dimensional hybrid graphene gap plasmon waves for large light-graphene overlap factor. We show that gap plasmon waves exhibit improved in-plane and out-of-plane field concentrations on graphene compared to those of edge or wire-like graphene plasmons. By adjusting the chemical property of the graphene layer, efficient and linear modulation of hybrid graphene gap plasmon modes is also achieved. Our results provide potential opportunities to low-dimensional graphene plasmonic devices with strong tunability.

The Efficient Mixed FEM With the Impedance Transmission Boundary Condition for Graphene Plasmonic Waveguides

A mixed finite-element method with an impedance transmission boundary condition (ITBC) is proposed to solve the graphene plasmonic modes. The new variational formulation combines the Gauss’ law with the transverse components of vectorial Helmholtz equation and models the field penetration through a thin graphene sheet by ITBC. The second-order edge-based vector LT/QN basis functions are applied to expand the transverse components of the electric field, and the nodal-based scalar basis functions are employed to discretize its longitudinal component. Numerical results on some designed graphene-based waveguides clearly demonstrate that the proposed method is efficient and accurate for the determination of graphene plasmonic modes.