Graphene-coated Nanowires Research Articles

Study on properties of plasmonic waveguide of graphene-coated nanotube with a dielectric substrate

A hybrid plasmonic waveguide structure composed of graphene-coated nanotube with a dielectric substrate is proposed in this paper. Transmission properties of the fundamental mode are studied by the finite element method (FEM). The results reveal that the mode transmission properties are greatly dependent on the nanotube radius and internal–external diameter ratio, gap distance, nanotube permittivity, as well as chemical potential of graphene. Meanwhile, it is compared with the graphene-coated nanowires with a dielectric substrate (Hajati and Hajati in Plasmonics, 2018. https://doi.org/10.1007/s11468-017-0524-2). By optimizing the parameters, the structure could achieve long-range propagation with the propagation length about 12.56 um. Moreover, it could realize deep-subwavelength confinement with the normalized mode area only ~ 10–7 in the mid-infrared (mid-IR). This structure may offer a certain theoretical basis for integrated nanophotonic devices to achieve long-distance transmission in the deep-subwavelength range.

Study of modal properties in graphene-coated nanowires integrated with substrates

In this paper, we show that graphene-coated nanowire integrated with a semi-elliptical dielectric substrate could enable excellent subwavelength transmission performance in the mid-infrared range. The fundamental graphene plasmon mode behaviors on the thickness of silica layer, ratio of long- to short-axis of the ellipse, nanowire radius, and chemical potential of graphene are revealed in detail. By improving the geometric parameters and the surface conductivity of graphene, results show that deep-subwavelength modal field size and long propagation could be achieved. Our findings may promote the applications of graphene-coated nanowires in tunable nanoscale photonic devices and sensors.

Graphene-Coated Nanowire Waveguides and Their Applications.

In recent years, graphene-coated nanowires (GCNWs) have attracted considerable research interest due to the unprecedented optical properties of graphene in terahertz (THz) and mid-infrared bands. Graphene plasmons in GCNWs have become an attractive platform for nanoscale applications in subwavelength waveguides, polarizers, modulators, nonlinear devices, etc. Here, we provide a comprehensive overview of the surface conductivity of graphene, GCNW-based plasmon waveguides, and applications of GCNWs in optical devices, nonlinear optics, and other intriguing fields. In terms of nonlinear optical properties, the focus is on saturable absorption. We also discuss some limitations of the GCNWs. It is believed that the research of GCNWs in the field of nanophotonics will continue to deepen, thus laying a solid foundation for its practical application.

基于衬底的涂覆石墨烯层的三角形纳米线亚波长传输特性研究

基于衬底的涂覆石墨烯层的三角形纳米线亚波长传输特性研究

Tunable photonic-like modes in graphene-coated nanowires.

In this study, the dispersion equations of a graphene-coated nanowire (GN) are solved. It is found that in this waveguide, besides the surface plasmon polaritons (SPPs), there is another branch of guided modes, called photonic-like modes. The propagation distances of the photonic-like modes can be five orders of magnitude longer than those of the SPPs. Moreover, they can be modulated in the range of 10-4 to 1 m by changing the chemical potential of graphene. In particular, the mode field distributions remain nearly unchanged during the modulation. Based on the analysis performed using COMSOL Multiphysics, we further demonstrated that the propagation losses of the photonic-like modes are dependent on not only the chemical potential of graphene, but also the mode power proportion inside graphene. The photonic-like modes have tremendous potential to be used in optical switches, modulators, and switches in magnetic fields at the nanoscale.

Topological plasmonic modes in graphene-coated nanowire arrays

We investigate the topological edge modes of surface plasmonic polaritons (SPPs) in a two-dimensional waveguide array composed of graphene-coated nanowires. The system experiences topological phase transition by tuning the inter-layer spacing between adjacent unit cells. The topological modes emerge at the edge of topological non-trivial nanowire arrays when the intra-layer spacing is larger than the inter-layer spacing. Thanks to the strong confinement of graphene SPPs, the modal wavelength of topological edge modes can be squeezed as small as 1/20 of incident wavelength. The study provides a promising approach to realizing robust light transport beyond diffraction limit.

Assembly of graphene-coated nickel nanowires and their catalytic performance

ABSTRACTThe investigation on the catalysts for reduction of 4-nitrophenol (4-NP) into 4-aminophenol (4-AP) with NaBH4 have captured intensive interest. Nevertheless, the materials with low expenditure and outstanding catalytic performances are difficult to obtain. Herein, a simple and efficient strategy, which was assisted with external magnetic field, had been proposed for preparation of graphene-coated nickel nanowires (NiNWs/G) nanocomposite magnetic catalyst. The whole fabrication procedure was facile without harsh preparation conditions like high-pressure, high-temperature, strong acid and expensive templates. The NiNWs/G could be easily and quickly prepared in aqueous solution under mild conditions, and the external magnetic force was adopted to induce the unidirectional self-assembly of metal crystals on graphene oxide to form NiNWs/G with aspect ratio more than 50. The possible formation mechanism suggesting that the nanoparticles would aggregate along with direction of the magnetization axis because the dipole magnetic moments of them are in line under magnetic induction lines. The characterization results confirmed the chemical component and morphology of the nanocomposite. The ferromagnetism is determined by hysteresis loop of NiNWs/G. More importantly, the nanocomposite exhibited superior catalytic performance toward reduction of 4-nitrophenol. Therefore, such nanocomposite will be a promising candidate for water treatment and catalyst industry.

涂覆石墨烯的三根电介质纳米线波导的模式特性

涂覆石墨烯的三根电介质纳米线波导的模式特性

Tunable Fano resonances in the decay rates of a pointlike emitter near a graphene-coated nanowire

Based on the Lorenz-Mie theory, we derive analytical expressions of radiative and nonradiative transition rates for different orientations of a point dipole emitter in the vicinity of an infinitely long circular cylinder of arbitrary radius. Special attention is devoted to the spontaneous decay rate of a dipole emitter near a subwavelength-diameter nanowire coated with a graphene monolayer. We show that plasmonic Fano resonances associated with light scattering by graphene-coated nanowires appear in the Purcell factor as a function of transition wavelength. Furthermore, the Fano line shape of transition rates can be tailored and electrically tuned by varying the distance between emitter and cylinder and by modulating the graphene chemical potential, where the Fano asymmetry parameter is proportional to the square root of the chemical potential. This gate-voltage-tunable Fano resonance leads to a resonant enhancement and suppression of light emission in the far-infrared range of frequencies. This result could be explored in applications involving ultrahigh-contrast switching for spontaneous emission in specifically designed tunable plasmonic nanostructures.

Multiple invisibility regions induced by symmetry breaking in a trimer of subwavelength graphene-coated nanowires.

Electromagnetic response is studied for clusters of subwavelength graphene-coated nanowires illuminated by a linearly polarized plane wave in the terahertz frequency range. The solution of the scattering problem is obtained with the Lorenz-Mie theory and multiple cylinder scattering formalism. The results show that normalized scattering cross-sections of nanowire clusters can be drastically changed by the symmetry breaking introduced into the cluster's design. This effect is due to excitation of dark modes and is observed only for the incident wave of TE polarization.

Graphene-coated nanowires with drop-shaped cross section for the low loss propagation of THz waves with sub-micron mode widths

Both the strong mode confinement and the low propagation loss are very important for the design of highly integrated THz devices, but they are often contradictory to each other and difficult to be achieved at the same time. In this paper, we study the inside-dominant graphene surface plasmon polaritons (GSPPs) propagating on the graphene-coated nanowires with drop-shaped cross section using COMSOL multiphysics. We find that the THz GSPPs achieve not only the sub-micron mode width but also the long propagation length. Furthermore, the inside-dominant GSPPs have very high energy utilization efficiency, because almost all the mode power is inside the nanowire. These excellent characteristics make the mode very useful in the nanophotonics, bio-photonics and highly integrated THz circuits.

Tunable invisibility cloaking by using isolated graphene-coated nanowires and dimers

We investigate, both theoretically and numerically, a graphene-coated nano-cylinder illuminated by a plane electromagnetic wave in the far-infrared range of frequencies. We have derived an analytical formula that enables fast evaluation of the spectral window with a substantial reduction in scattering efficiency for a sufficiently thin cylinder. This polarization-dependent effect leads to tunable resonant invisibility that can be achieved via modification of graphene chemical potential monitored by the gate voltage. A multi-frequency cloaking mechanism based on dimer coated nanowires is also discussed in detail.

Interaction between graphene-coated nanowires revisited with transformation optics.

The interaction between graphene-coated nanostructures provides interesting optical properties not found in isolated graphene plasmonic structures. However, full-analytical solutions, which can provide deep physical insights underlying the hybrid graphene plasmonic systems, are difficult to achieve. In this Letter, we deploy the theory of transformation optics to study the plasmonic interactions between two dielectric-core-graphene-shell nanowires. The scattering and absorption spectra as well as the field distributions are derived analytically. We find that the interaction between two graphene-coated nanowires results in polarization-independent multi-frequency Fano dips, which show a broadband red shift of bonding modes and a blue shift of anti-bonding modes when the nanowires approach each other. The analytical tool presented here offers a rigorous study of graphene plasmonic compound and can be extended to treat more complicated cases.

Graphene-coated nanowires with a drop-shaped cross section for 10 nm confinement and 1 mm propagation.

Strong confinement and long-range propagation of electromagnetic energy are longed for when designing efficient miniaturized photonic devices. Here, a graphene-coated nanowire with a drop-shaped cross section is proposed for guiding graphene surface plasmon polaritons to demonstrate an extremely long propagation length (1mm) with ultra-strong mode confinement (10nm), which results from the distinctive mode field distribution caused by both the top and bottom arcs of the waveguide. The combination of nanoconcentration and long-range propagation makes the waveguide very useful in nanophotonics, bio-photonics, and highly integrated photonic circuits.

Plasmonic characteristics of two vertically coupled graphene-coated nanowires integrated with substrate.

In this paper, we theoretically investigate the guiding properties of two vertically coupled cylindrical graphene-coated nanowires (GNWs) integrated with a thin high-index dielectric substrate. We show that the plasmonic properties of the proposed structure can be effectively modulated by changing the thickness of the dielectric substrate, the nanowire radius, the gap distance between the substrate and GNWs, and the Fermi level of graphene. Through optimizing the geometric and structural parameters, a surface plasmon mode with high optical performance and low propagation loss can be achieved in the studied structure. Compared with the plasmon mode guided in a single GNW over substrate, a highly improved figure of merit with nearly two-fold electric field enhancement can be yielded in the double GNW over substrate counterpart.

Nonlinear coupling in graphene-coated nanowires.

We propose and analyze nonlinear coupler based on a pair of single mode graphene-coated nanowires. Nonlinear wave interactions in such structure are analyzed by the coupled mode equations derived from the unconjugated Lorentz reciprocity theorem. We show that the routing of plasmons in the proposed structure can be controlled by the input power due to the third order nonlinear response of graphene layer. Our findings show that graphene nonlinearity can be exploited in tunable nanoplasmonic circuits based on low-loss, edgeless cylindrical graphene waveguides.

Gate-Programmable Electro-Optical Addressing Array of Graphene-Coated Nanowires with Sub-10 nm Resolution

The rapid development of highly integrated photonic circuits has been driving electro-optic (EO) devices to increasingly compact sizes, with the perspective of being able to control light at the nanoscale. However, tunability with spatial resolution below 10 nm scale with conventional approaches, such as metallic nanowires, remains a challenge. Here, we show a graphene-coated nanowire system aiming at beam spatial modulation at a deeply subwavelength scale. By analytically and numerically investigating the eigenmodal properties of this system, we found that beam power can propagate along either a swinging or a helical path in the hybrid nanowire. In particular, the period of the swing beam and the chirality and period of the helix beam can be flexibly controlled by tuning the chemical potential of graphene via the gate voltage. Significantly, due to its good modal confinement, such a beam can be independently manipulated even in the presence of another nanowire at a separation of 40 nm, which opens a real...

Second harmonic generation in graphene-coated nanowires.

We study second harmonic generation in a pair of graphene-coated nanowires. We show that the phase matching condition for harmonic generation can be engineered in a wide range of frequencies by tuning the spacing between graphene nanowires. We derive coupled mode equations describing the process of second harmonic generation using an unconjugated Lorentz reciprocity theorem. We show that the highest harmonic generation efficiency can be achieved by phase matching the fundamental mode with the two lowest order symmetric modes at the second harmonic frequency. Despite losses in graphene, we predict that the efficiency can be further enhanced by reducing the radius of nanowires due to larger mode overlap and lower propagation loss.

Plasmonically induced magnetic field in graphene-coated nanowires.

In this Letter, we investigate a magnetic field induced by guiding plasmonic modes in graphene-coated nanowire via an inverse Faraday effect. Magnetic field distribution for different plasmonic modes has been calculated. It has been shown that a magnetic field has a vortex-like distribution for some plasmonic modes. The possibility of producing magnetic field distribution that rotates along the nanowire axis and periodically depends on azimuthal angle has been demonstrated.