Graphene plasmon waveguiding and hybridization in individual and paired nanoribbons

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.

Analytical model for plasmon modes in graphene-coated nanowire

An analytical model for plasmon modes in graphene-coated dielectric nanowire is presented. Plasmon modes could be classified by the azimuthal field distribution characterized by a phase factor exp(imφ) in the electromagnetic field expression and eigen equation of dispersion relation for plasmon modes is derived. The characteristic of plasmon modes could be tuned by changing nanowire radius, dielectric permittivity of nanowire and chemical potential of graphene. The proposed model provides a fast insight into the mode behavior of graphene-coated nanowire, which would be useful for applications based on graphene plasmonics in cylindrical waveguide.

Edge-reflection phase directed plasmonic resonances on graphene nano-structures

The phase of graphene plasmon upon edge-reflection plays a crucial role on determining the spectral properties of graphene structures. In this article, by using the full-wave simulation, we demonstrate that the mid-infrared graphene plasmons are nearly totally reflected at the boundary together with a phase jump of approximately 0.27π, regardless of the environments surrounding it. Appling this phase pickup, a Fabry-Perot model is formulated that can predict accurately the resonant wavelengths of graphene nano-ribbons. Furthermore, we find that the magnitude of the phase jump will either increase or reduce when two neighboring coplanar graphene sheets couple with each other. This could be used to explain the red-shift of resonant wavelength of periodic ribbon arrays with respect to an isolated ribbon. We provide a straightforward way to uncover the phase jump of graphene plasmons that would be helpful for designing and engineering graphene resonators and waveguides as well as their associated applications.

Magneto-plasmonics in graphene-dielectric sandwich

In this paper, dispersion properties and field distributions of surface magneto plasmons (SMPs) in double-layer graphene structures at room temperature are studied. It is found that, the dispersion curves of both symmetric and antisymmetric SMPs modes split into several branches/bands when a magnetic field is applied perpendicularly to the graphene surface. Surprisingly, the lowest energy SMP band has anomalous dependence on the applied magnetic field, different to the other higher bands. In addition, the symmetric and antisymmetric modes can be decoupled if the two graphene layers possess different properties, such as different Fermi energies. Furthermore, electric components of the surface modes which are parallel to the graphene surfaces but perpendicular to the propagation direction (i.e. the transverse-electric mode) are no longer zero caused by the Lorentz force on the free electrons.

Terahertz science and technology of carbon nanomaterials

The diverse applications of terahertz (THz) radiation and its importance to fundamental science makes finding ways to generate, manipulate and detect THz radiation one of the key areas of modern applied physics. One approach is to utilize carbon nanomaterials, in particular, single-wall carbon nanotubes and graphene. Their novel optical and electronic properties offer much promise to the field of THz science and technology. This article describes the past, current, and future of THz science and technology of carbon nanotubes and graphene. We will review fundamental studies such as THz dynamic conductivity, THz nonlinearities and ultrafast carrier dynamics as well as THz applications such as THz sources, detectors, modulators, antennas and polarizers.

Frequency-tunable circular polarization beam splitter using a graphene-dielectric sub-wavelength film

Manipulating the circular polarization of light is of great importance in chemistry and biology, as chiral molecules exhibit different physiological properties when exposed to different circularly polarized waves. Here we suggest a graphene/dielectric-stacked structure, which has both the properties of an epsilon-near-zero material and the high Hall conductivity of graphene. The proposed sub-wavelength structure demonstrates efficient manipulation of circular polarization properties of light. In a quite broad frequency range and at a large oblique incidence angle, the present magnetically active structure is transparent for one circularly polarized wave, and opaque for another. Such an effect can be further tuned by changing the magnitude of the applied magnetic field and chemical potential of graphene.

Phase-resolved surface plasmon interferometry of graphene

The surface plasmon polaritons (SPPs) of graphene reflect the microscopic spatial variations of underlying electronic structure and dynamics. Here, we excite and image the graphene SPP response in phase and amplitude by near-field interferometry. We develop an analytic cavity model that can self-consistently describe the SPP response function for edge, grain boundary, and defect SPP reflection and scattering. The derived SPP wave vector, damping, and carrier mobility agree with the results from more complex models. Spatial variations in the Fermi level and associated variations in dopant concentration reveal a nanoscale spatial inhomogeneity in the reduced conductivity at internal boundaries. The additional SPP phase information thus opens a new degree of freedom for spatial and spectral graphene SPP tuning and modulation for optoelectronics applications.

Dominant mode control of a graphene-embedded hybrid plasmonic resonator for a tunable nanolaser

A graphene-embedded tunable plasmonic nanodisk resonator operating at near-infrared wavelength range is proposed, in which a certain resonant mode among multiple whispering-gallery modes (WGMs) can be selected as a dominant mode by modulating the Fermi level of the graphene. Our theoretical investigation reveals that the dominant mode selection mechanism in the proposed resonator is governed by the figure-of-merit (FOM) of the one-dimensional (1D) waveguide of the resonator's vertical structure, which is defined as a propagation length to mode size ratio. As the conductivity of the graphene changes with a gating voltage, the wavelength dependence of the FOM changes and a WGM closest to the maximum FOM wavelength is selected. Partial tuning of the selected dominant mode is incurred by the change of the effective index of the 1D waveguide. This novel mode selection mechanism of the proposed resonator can be adopted to realize an optically pumped tunable nanolaser with a wide wavelength tuning range.

Plasmonic excitations of 1D metal-dielectric interfaces in 2D systems: 1D surface plasmon polaritons

Surface plasmon-polariton (SPP) excitations of metal-dielectric interfaces are a fundamental light-matter interaction which has attracted interest as a route to spatial confinement of light far beyond that offered by conventional dielectric optical devices. Conventionally, SPPs have been studied in noble-metal structures, where the SPPs are intrinsically bound to a 2D metal-dielectric interface. Meanwhile, recent advances in the growth of hybrid 2D crystals, which comprise laterally connected domains of distinct atomically thin materials, provide the first realistic platform on which a 2D metal-dielectric system with a truly 1D metal-dielectric interface can be achieved. Here we show for the first time that 1D metal-dielectric interfaces support a fundamental 1D plasmonic mode (1DSPP) which exhibits cutoff behavior that provides dramatically improved light confinement in 2D systems. The 1DSPP constitutes a new basic category of plasmon as the missing 1D member of the plasmon family: 3D bulk plasmon, 2DSPP, 1DSPP, and 0D localized SP.

Plasmonics in Dirac systems: from graphene to topological insulators

Recent developments in the emerging field of plasmonics in graphene and other Dirac systems are reviewed and a comprehensive introduction to the standard models and techniques is given. In particular, we discuss intrinsic plasmon excitation of single and bilayer graphene via hydrodynamic equations and the random phase approximation, but also comment on double and multilayer structures. Additionally, we address Dirac systems in the retardation limit and also with large spin–orbit coupling including topological insulators. Finally, we summarize basic properties of the charge, current and photon linear response functions in an appendix.

Plasmon resonances in a stacked pair of graphene ribbon arrays with a lateral displacement

We find that a stacked pair of graphene ribbon arrays with a lateral displacement can excite plasmon waveguide mode in the gap between ribbons, as well as surface plasmon mode on graphene ribbon surface. When the resonance wavelengthes of plasmon waveguide mode and surface plasmon mode are close to each other, there is a strong electromagnetic interaction between the two modes, and then they contribute together to transmission dip. The plasmon waveguide mode resonance can be manipulated by the lateral displacement and longitudinal interval between arrays due to their influence on the manner and strength of electromagnetic coupling between two arrays. The findings expand our understanding of electromagnetic resonances in graphene-ribbon array structure and may affect further engineering of nanoplasmonic devices and metamaterials.

Plasmonic extinction of gated graphene nanoribbon array analyzed by a scaled uniform Fermi level

A uniform Fermi level profile is typically assumed in the analysis of a gated graphene nanoribbon, whose Fermi level is actually nonuniform in the experimental measurements. Here, we show that the uniform Fermi level has to be downshifted when it is used to analyze a backgated graphene nanoribbon array (GNRA). The plasmonic extinction behaviors of the GNRAs are perfectly preserved by assuming properly scaled uniform Fermi levels. The scaling factor is independent of the average value of the actual Fermi level profile, but it is a function of the ratio of the nanoribbon width to the distance of the nanoribbons from the backgate. This study facilitates the data postprocessing in the experiments, and may be helpful for analyzing the electron behaviors in GNRAs.

Graphene-based tunable plasmonic Bragg reflector with a broad bandwidth

We propose and numerically analyze a plasmonic Bragg reflector formed in a graphene waveguide. The results show that the graphene plasmonic Bragg reflector can produce a broadband stopband that can be tuned over a wide wavelength range by a small change in the Fermi energy level of graphene. By introducing a defect into the Bragg reflector, we can achieve a Fabry-Perot-like microcavity with a quality factor of 50 for the defect resonance mode formed in the stopband. The proposed Bragg reflector could be used as a broadband ultrafast tunable integrated filter and a broadband modulator. In addition, the defect microcavity may find applications in graphene-based resonators.

Surface plasmon modes in graphene wedge and groove waveguides

Surface plasmon modes at terahertz-infrared waveband in subwavelength graphene wedge and groove waveguides are investigated, which can be categorized into perfect electric conductor and perfect magnetic conductor symmetric modes with different propagation characteristics. The electromagnetic near-fields are localized strongly in different regions for these two kinds of modes. Moreover, these modes can be interpreted by the folded graphene ribbon modes. The brim width of the waveguides and the Fermi energy of the graphene strongly influence the dispersion and propagation distances of the plasmon modes, which can be used for tuning the plasmon modes in graphene wedge and groove waveguides efficiently.

Exciting graphene surface plasmon polaritons through light and sound interplay

We propose a concept that allows for efficient excitation of surface plasmon spolaritons (SPPs) on a thin graphene sheet located on a substrate by an incident electromagnetic field. Elastic vibrations of the sheet, which are generated by a flexural wave, act as a grating that enables the electromagnetic field to couple to propagating graphene SPPs. This scheme permits fast on-off switching of the SPPs and dynamic tuning of their excitation frequency by adjusting the vibration frequency (grating period). Potential applications include single molecule detection and enhanced control of SPP trajectories via surface wave patterning of graphene metasurfaces. Analytical calculations and numerical experiments demonstrate the practical applicability of the proposed concept.

Plane wave excitation-detection of non-resonant plasmons along finite-width graphene strips

An approach to couple free-space waves and non-resonant plasmons propagating along graphene strips is proposed based on the periodic modulation of the graphene strip width. The solution is technologically very simple, scalable in frequency, and provides customized coupling angle and intensity. Moreover, the coupling properties can be dynamically controlled at a fixed frequency via the graphene electrical field effect, enabling advanced and flexible plasmon excitation-detection strategies. We combine a previously derived scaling law for graphene strips with leaky-wave theory borrowed from microwaves to achieve rigorous and efficient modeling and design of the structure. In particular we analytically derive its dispersion, predict its coupling efficiency and radiated field structure, and design strip configurations able to fulfill specific coupling requirements. The proposed approach and developed methods are essential to the recent and fundamental problem of the excitation-detection of non-resonant plasmons propagating along a continuous graphene strip, and could pave the way to smart all-graphene sensors and transceivers.

Tuning of surface plasmon polaritons beat length in graphene directional couplers

We investigate the tuning of the coupling of surface plasmon polaritons between two spatially separated graphene layers. We demonstrate that by slightly changing the chemical potential, a graphene coupler can switch from the bar to the cross state; as a consequence, the coupling coefficient in such structures can be easily controlled by means of an applied electrical signal.

A dynamically reconfigurable Fano metamaterial through graphene tuning for switching and sensing applications

We report on a novel electrically tunable hybrid graphene-gold Fano resonator. The proposed metamaterial consists of a square graphene patch and a square gold frame. The destructive interference between the narrow- and broadband dipolar surface plasmons, which are induced respectively on the surfaces of the graphene patch and the gold frame, leads to the plasmonic equivalent of electromagnetically induced transparency (EIT). The response of the metamaterial is polarization independent due to the symmetry of the structure and its spectral features are shown to be highly controllable by changing a gate voltage applied to the graphene patch. Additionally, effective group index of the device is retrieved and is found to be very high within the EIT window suggesting its potential use in slow light applications. Potential outcomes such as high sensing ability and switching at terahertz frequencies are demonstrated through numerical simulations with realistic parameters.

Analysis of graphene TE surface plasmons in the terahertz regime

Unlike common metals, graphene can support transverse electric (TE) surface modes when the imaginary part of its conductivity is negative. We have theoretically investigated and numerically simulated plasmonic properties of graphene TE surface plasmons (SPs) in the terahertz regime. The influence of the external magnetic field, gate voltage and temperature as the tuning schemes of the SPs have been investigated. The results show that graphene TE modes can be realized by tuning the magnetic fields or gate voltage. If the permeability of the dielectrics on both sides of the graphene layer differs enough, the graphene TE modes can still be achieved. The work presented here has the potential for application to graphene-based plasmonic devices in photonics and optoelectronics, such as sensors, polarizers and modulators.

Fast convergent Fourier modal method for the analysis of periodic arrays of graphene ribbons

Li's Fourier factorization rules [J. Opt. Soc. Am. A13, 1870 (1996)] should be applied to achieve a fast convergence rate in the analysis of diffraction gratings with the Fourier modal method. I show, however, that Li's inverse rule cannot be applied for periodic patterns of graphene when the conventional boundary condition is used. I derive an approximate boundary condition in which a nonzero but sufficiently small height is assumed for the boundary. The proposed boundary condition enables us to apply the inverse rule, leading to a significantly improved convergence rate. A periodic array of graphene ribbons is in fact a special type of finite-conductivity strip grating, and thus the proposed approach is also applicable to these kinds of structures.

Graphene-based plasmonic switches at near infrared frequencies

The concept, analysis, and design of series switches for graphene-strip plasmonic waveguides at near infrared frequencies are presented. Switching is achieved by using graphene's field effect to selectively enable or forbid propagation on a section of the graphene strip waveguide, thereby allowing good transmission or high isolation, respectively. The electromagnetic modeling of the proposed structure is performed using full-wave simulations and a transmission line model combined with a matrix-transfer approach, which takes into account the characteristics of the plasmons supported by the different graphene-strip waveguide sections of the device. The performance of the switch is evaluated versus different parameters of the structure, including surrounding dielectric media, electrostatic gating and waveguide dimensions.

Scattering of graphene plasmons by defects in the graphene sheet

A theoretical study is presented on the scattering of graphene surface plasmons (GSPs) by defects in the graphene sheet they propagate in. These defects can be either natural (as domain boundaries, ripples, and cracks, among others) or induced by an external gate. The scattering is shown to be governed by an integral equation, derived from a plane wave expansion of the fields, which in general must be solved numerically, but it provides useful analytical results for small defects. Two main cases are considered: smooth variations of the graphene conductivity (characterized by a Gaussian conductivity profile) and sharp variations (represented by islands with different conductivity). In general, reflection largely dominates over radiation out of the graphene sheet. However, in the case of sharply defined conductivity islands, there are some values of island size and frequency where the reflectance vanishes and, correspondingly, the radiation out-of-plane is the main scattering process. For smooth defects, the reflectance spectra present a single maximum at the condition k(p)a ≈ √2, where k(p) is the GSP wavevector and a is the spatial width of the defect. In contrast, the reflectance spectra of sharp defects present periodic oscillations with period k(p)′a, where k(p)′ is the GSP wavelength inside the defect. Finally, the case of cracks (gaps in the graphene conductivity) is considered, showing that the reflectance is practically unity for gap widths larger than one-tenth of the GSP wavelength.

Magnetoplasmons in quasineutral epitaxial graphene nanoribbons

We present an infrared transmission spectroscopy study of the inter-Landau-level excitations in quasineutral epitaxial graphene nanoribbon arrays. We observed a substantial deviation in energy of the L(0(-1)) → L(1(0)) transition from the characteristic square root magnetic-field dependence of two-dimensional graphene. This deviation arises from the formation of an upper-hybrid mode between the Landau-level transition and the plasmon resonance. In the quantum regime, the hybrid mode exhibits a distinct dispersion relation, markedly different from that expected for conventional two-dimensional systems and highly doped graphene.

Highly confined tunable mid-infrared plasmonics in graphene nanoresonators

Single-layer graphene has been shown to have intriguing prospects as a plasmonic material, as modes having plasmon wavelengths ~20 times smaller than free space (λp ~ λ0/20) have been observed in the 2-6 THz range, and active graphene plasmonic devices operating in that regime have been explored. However there is great interest in understanding the properties of graphene plasmons across the infrared spectrum, especially at energies exceeding the graphene optical phonon energy. We use infrared microscopy to observe the modes of tunable plasmonic graphene nanoresonator arrays as small as 15 nm. We map the wavevector-dependent dispersion relations for graphene plasmons at mid-infrared energies from measurements of resonant frequency changes with nanoresonator width. By tuning resonator width and charge density, we probe graphene plasmons with λp ≤ λ0/100 and plasmon resonances as high as 310 meV (2500 cm(-1)) for 15 nm nanoresonators. Electromagnetic calculations suggest that the confined plasmonic modes have a local density of optical states more than 10(6) larger than free space and thus could strongly increase light-matter interactions at infrared energies.

Ultrahigh refractive index sensitivity of TE-polarized electromagnetic waves in graphene at the interface between two dielectric media

The behavior of the TE and TM electromagnetic waves in graphene at the interface between two semi-infinite dielectric media is studied. The dramatic influence on the TE waves propagation even at very small changes in the optical contrast between the two dielectric media is predicted. Frequencies of the TE waves are found to lie only in the window determined by the contrast. We consider this effect in connection with the design of graphene-based optical gas sensor. Near the frequency, where the imaginary part of the conductivity of graphene becomes zero, ultrahigh refractive index sensitivity and very low detection limit are revealed. The considered graphene-based optical gas sensor outperforms characteristics of modern volume refractive index sensors by several orders of magnitude.

A 3D tunable and multi-frequency graphene plasmonic cloak

We demonstrate the possibility of cloaking three-dimensional objects at multi-frequencies in the far-infrared part of the spectrum. The proposed cloaking mechanism exploits graphene layers wrapped around the object to be concealed. Graphene layers are doped via a variable external voltage difference permitting continuous tuning of the cloaking frequencies. Particularly, two configurations are investigated: (i) Only one graphene layer is used to suppress the scattering from a dielectric sphere. (ii) Several of these layers biased at different gate voltages are used to achieve a multi-frequency cloak. These frequencies can be set independently. The proposed cloak's functionality is verified by near- and far-field computations. By considering geometry and material parameters that are realizable by practical experiments, we contribute to the development of graphene based plasmonic applications that may find use in disruptive photonic technologies.

Mid-infrared tunable optical polarization converter composed of asymmetric graphene nanocrosses

We present a mid-IR highly tunable optical polarization converter composed of asymmetric graphene nanocrosses. It can convert linearly polarized light to circularly and elliptically polarized light or exhibit a giant optical activity at different wavelengths. The transmitted wavelength and polarization states can also be dynamically tuned by varying the Fermi energy of graphene, without reoptimizing and refabricating the nanostructures. This offers a further step in developing a controllable polarization converter.

Tunable molecular plasmons in polycyclic aromatic hydrocarbons

We show that chemically synthesized polycyclic aromatic hydrocarbons (PAHs) exhibit molecular plasmon resonances that are remarkably sensitive to the net charge state of the molecule and the atomic structure of the edges. These molecules can be regarded as nanometer-sized forms of graphene, from which they inherit their high electrical tunability. Specifically, the addition or removal of a single electron switches on/off these molecular plasmons. Our first-principles time-dependent density-functional theory (TDDFT) calculations are in good agreement with a simpler tight-binding approach that can be easily extended to much larger systems. These fundamental insights enable the development of novel plasmonic devices based upon chemically available molecules, which, unlike colloidal or lithographic nanostructures, are free from structural imperfections. We further show a strong interaction between plasmons in neighboring molecules, quantified in significant energy shifts and field enhancement, and enabling molecular-based plasmonic designs. Our findings suggest new paradigms for electro-optical modulation and switching, single-electron detection, and sensing using individual molecules.

Spontaneous emission in paired graphene plasmonic waveguide structures

The coupling between a single emitter and surface plasmons in paired graphene layers and in paired graphene ribbons are studied. For paired graphene layers, the coupling between surface plasmons in graphene layers is strong at low photon energy and small gap between layers, which results in strong enhancement of the emitter's emission. The excitation efficiency of surface plasmons by a single emitter can be increased to nearly 1 in paired graphene layers. With the increase of the photon energy, emitter's emission in paired layers is weakened and could be lower than that in graphene monolayer. For graphene paired ribbons, numerical simulations show similar properties of emission enhancement and high excitation efficiency of surface plasmons. The emission enhancement and the excitation efficiency of surface plasmons can be improved by narrowing the ribbon width.

Graphene sustained nonlinear modes in dielectric waveguides

We discuss the existence of nonlinear modes sustained by graphene layers in dielectric waveguides. Taking advantage of the almost two dimensional nature of graphene, we introduce the nonlinear effect as a parameter in the continuity equations. We then apply our modeling to a simple slab waveguide to enlighten how graphene can be used to induce huge nonlinear phase shifts at easily accessible power levels.

Bends and splitters in graphene nanoribbon waveguides

We investigate the performance of bends and splitters in graphene nanoribbon waveguides. Although the graphene waveguides are lossy themselves, we show that bends and splitters do not induce any additional loss provided that the nanoribbon width is sub-wavelength. We use transmission line theory to qualitatively interpret the behavior observed in our simulation. Our results pave a promising way to realize ultra-compact devices operating in the terahertz region.

A perfect absorber made of a graphene micro-ribbon metamaterial

Metamaterial-based perfect absorbers promise many applications. Perfect absorption is characterized by the complete suppression of transmission and reflection and complete dissipation of the incident energy by the absorptive meta-atoms. A certain absorption spectrum is usually assigned to a bulk medium and serves as a signature of the respective material. Here we show how to use graphene flakes as building blocks for perfect absorbers. Then, an absorbing meta-atom only consists of a molecular monolayer placed at an appropriate distance from a metallic ground plate. We show that the functionality of such device is intuitively and correctly explained by a Fabry-Perot model.

Plasmon-induced doping of graphene

A metallic nanoantenna, under resonant illumination, injects nonequilibrium hot electrons into a nearby graphene structure, effectively doping the material. A prominent change in carrier density was observed for a plasmonic antenna-patterned graphene sheet following laser excitation, shifting the Dirac point, as determined from the gate-controlled transport characteristic. The effect is due to hot electron generation resulting from the decay of the nanoantenna plasmon following resonant excitation. The effect is highly tunable, depending on the resonant frequency of the plasmonic antenna, as well as on the incident laser power. Hot electron-doped graphene represents a new type of hybrid material that shows great promise for optoelectronic device applications.

Broadband subwavelength imaging using a tunable graphene-lens

Graphene as a one-atom-thick planar sheet can support surface plasmons at infrared (IR) and terahertz (THz) frequencies, opening up exciting possibilities for the emerging research field of graphene plasmonics. Here, we theoretically report that a layered graphene-lens (GL) enables the enhancement of evanescent waves for near-field subdiffractive imaging. Compared to other resonant imaging devices like superlenses, the nonresonant operation of the GL provides the advantages of a broad intrinsic bandwidth and a low sensitivity to losses, while still maintaining a good subwavelength resolution of better than λ/10. Most importantly, thanks to the large tunability of the graphene, we show that our GL is a continuously frequency-tunable subwavelength-imaging device in the IR and THz regions, thus allowing for ultrabroadband spectral applications.

Excitation of plasmonic waves in graphene by guided-mode resonances

We propose an active plasmonic device based on graphene. Highly confined plasmonic waves in monolayer graphene are efficiently excited using an etched diffractive grating on silicon. The guided-wave resonance of the combined structure creates a sharp notch on the normal-incidence transmission spectra, as the incident optical wave couples to the graphene plasmonic wave. This structure can be used as a highly tunable optical filter or a broad-band modulator because the resonant wavelength can be quickly tuned over a wide wavelength range by a small change in the Fermi energy level of the graphene. In this paper, we analyze the performance of this device with finite-difference time-domain simulations. We compare the proposed structure with recently demonstrated graphene nanoribbons based on bound plasmonic oscillations.

Characteristics of electro-refractive modulating based on Graphene-Oxide-Silicon waveguide

Graphene has attracted a high level of research interest because of its outstanding electronic transport properties and optical properties. Based on the Kubo formalism and the Maxwell equations, it's demonstrated that the optical conductivity of graphene can be controlled through the applied voltage. And we find that the graphene-oxide-silicon (GOS) based waveguide can be made into either the electro-absorptive or electron-refractive modulators. Using graphene as the active medium, we present a new electro-refractive Mach-Zender interferometer based on the GOS structure. This new GOS-based electron-refractive modulation mechanism can enable novel architectures for on-chip optical communications.

A plethora of plasmonics from the laboratory for nanophotonics at Rice University

The study of the surface plasmons of noble metals has emerged as one of the most rapidly growing and dynamic topics in nanoscience. Key advances in the synthesis of noble metal nanoparticles and nanostructures have resulted in a broad variety of structures whose geometries can be controlled systematically at the nanoscale. Arising from these efforts is a new level of insight and understanding regarding the fundamental properties of localized plasmons supported by these structures, and, in particular, the properties of interacting plasmon systems. This additional insight has led to the design of plasmonic systems that support coherent phenomena, such as Fano resonances. A broad range of applications are emerging for these structures: single- nanoparticle and nanogap chemical sensors, low-loss plasmon waveguides, and active plasmonic devices and detectors. Applications in biomedicine that exploit the strong photothermal response of nanoparticle plasmons have developed and advanced into clinical trials. The Laboratory for Nanophotonics at Rice has been home to many of these advances. Here, we showcase a variety of functional plasmonic materials and nanodevices emerging from our individual and collaborative efforts.

A plasmonic Fano switch

Plasmonic clusters can support Fano resonances, where the line shape characteristics are controlled by cluster geometry. Here we show that clusters with a hemicircular central disk surrounded by a circular ring of closely spaced, coupled nanodisks yield Fano-like and non-Fano-like spectra for orthogonal incident polarization orientations. When this structure is incorporated into an uniquely broadband, liquid crystal device geometry, the entire Fano resonance spectrum can be switched on and off in a voltage-dependent manner. A reversible transition between the Fano-like and non-Fano-like spectra is induced by relatively low (∼6 V) applied voltages, resulting in a complete on/off switching of the transparency window.

OWL-based nanomasks for preparing graphene ribbons with sub-10 nm gaps

We report a simple and highly efficient method for creating graphene nanostructures with gaps that can be controlled on the sub-10 nm length scale by utilizing etch masks comprised of electrochemically synthesized multisegmented metal nanowires. This method involves depositing striped nanowires with Au and Ni segments on a graphene-coated substrate, chemically etching the Ni segments, and using a reactive ion etch to remove the graphene not protected by the remaining Au segments. Graphene nanoribbons with gaps as small as 6 nm are fabricated and characterized with atomic force microscopy, scanning electron microscopy, and Raman spectroscopy. The high level of control afforded by electrochemical synthesis of the nanowires allows us to specify the dimensions of the nanoribbon, as well as the number, location, and size of nanogaps within the nanoribbon. In addition, the generality of this technique is demonstrated by creating silicon nanostructures with nanogaps.

Recent advances in transformation optics

Within the past a few years, transformation optics has emerged as a new research area, since it provides a general methodology and design tool for manipulating electromagnetic waves in a prescribed manner. Using transformation optics, researchers have demonstrated a host of striking phenomena and devices; many of which were only thought possible in science fiction. In this paper, we review the most recent advances in transformation optics. We focus on the theory, design, fabrication and characterization of transformation devices such as the carpet cloak, "Janus" lens and plasmonic cloak at optical frequencies, which allow routing light at the nanoscale. We also provide an outlook of the challenges and future directions in this fascinating area of transformation optics.

Optical nano-imaging of gate-tunable graphene plasmons

The ability to manipulate optical fields and the energy flow of light is central to modern information and communication technologies, as well as quantum information processing schemes. However, because photons do not possess charge, a way of controlling them efficiently by electrical means has so far proved elusive. A promising way to achieve electric control of light could be through plasmon polaritons—coupled excitations of photons and charge carriers—in graphene. In this two-dimensional sheet of carbon atoms, it is expected that plasmon polaritons and their associated optical fields can readily be tuned electrically by varying the graphene carrier density. Although evidence of optical graphene plasmon resonances has recently been obtained spectroscopically, no experiments so far have directly resolved propagating plasmons in real space. Here we launch and detect propagating optical plasmons in tapered graphene nanostructures using near-field scattering microscopy with infrared excitation light. We provide real-space images of plasmon fields, and find that the extracted plasmon wavelength is very short—more than 40 times smaller than the wavelength of illumination. We exploit this strong optical field confinement to turn a graphene nanostructure into a tunable resonant plasmonic cavity with extremely small mode volume. The cavity resonance is controlled in situ by gating the graphene, and in particular, complete switching on and off of the plasmon modes is demonstrated, thus paving the way towards graphene-based optical transistors. This successful alliance between nanoelectronics and nano-optics enables the development of active subwavelength-scale optics and a plethora of nano-optoelectronic devices and functionalities, such as tunable metamaterials, nanoscale optical processing, and strongly enhanced light–matter interactions for quantum devices and biosensing applications.

Strong coupling of surface plasmon polaritons in monolayer graphene sheet arrays

Here we investigate theoretically and numerically the coupling between surface plasmon polaritons (SPPs) in monolayer graphene sheet arrays that have a period much smaller than the wavelength. We show that when the collective SPP is excited with an out-of-phase illumination, the beam tends to propagate toward the opposite direction of the Bloch momentum, reflecting a negative coupling between the constituent SPPs. In contrast, for in-phase illumination, the incident beam is split into two collective SPPs that are highly collimated and display low propagation loss. Moreover, the coupling between the individual SPPs results in a reduction of the modal wavelength of the SPP in comparison with that of a single graphene sheet.

Surface plasmons at the interface between graphene and Kerr-type nonlinear media

The properties of surface plasmons localized at the interface between graphene and Kerr-type nonlinear substrates are investigated analytically. Although the relative propagation distance remains the same, the dispersion of graphene plasmons may be affected much by the inevitable nonlinear effect of substrates. Specifically, the wavelength of graphene plasmons can be tuned by adjusting the nonlinear permittivity of substrates.

Graphene photonics, plasmonics, and broadband optoelectronic devices

Graphene has been hailed as a wonderful material in electronics, and recently, it is the rising star in photonics, as well. The wonderful optical properties of graphene afford multiple functions of signal emitting, transmitting, modulating, and detection to be realized in one material. In this paper, the latest progress in graphene photonics, plasmonics, and broadband optoelectronic devices is reviewed. Particular emphasis is placed on the ability to integrate graphene photonics onto the silicon platform to afford broadband operation in light routing and amplification, which involves components like polarizer, modulator, and photodetector. Other functions like saturable absorber and optical limiter are also reviewed.

Optical Excitations and Field Enhancement in Short Graphene Nanoribbons

The optical excitations of elongated graphene nanoflakes of finite length are investigated theoretically through quantum chemistry semiempirical approaches. The spectra and the resulting dipole fields are analyzed, accounting in full atomistic details for quantum confinement effects, which are crucial in the nanoscale regime. We find that the optical spectra of these nanostructures are dominated at low energy by excitations with strong intensity, comprised of characteristic coherent combinations of a few single-particle transitions with comparable weight. They give rise to stationary collective oscillations of the photoexcited carrier density extending throughout the flake and to a strong dipole and field enhancement. This behavior is robust with respect to width and length variations, thus ensuring tunability in a large frequency range. The implications for nanoantennas and other nanoplasmonic applications are discussed for realistic geometries.

Quantum finite-size effects in graphene plasmons

Graphene plasmons are emerging as an alternative solution to noble metal plasmons, adding the advantages of tunability via electrostatic doping and long lifetimes. These excitations have been so far described using classical electrodynamics, with the carbon layer represented by a local conductivity. However, the question remains, how accurately is such a classical description representing graphene? What is the minimum size for which nonlocal and quantum finite-size effects can be ignored in the plasmons of small graphene structures? Here, we provide a clear answer to these questions by performing first-principles calculations of the optical response of doped nanostructured graphene obtained from a tight-binding model for the electronic structure and the random-phase approximation for the dielectric response. The resulting plasmon energies are in good agreement with classical local electromagnetic theory down to ∼10 nm sizes, below which plasmons split into several resonances that emphasize the molecular character of the carbon structures and the quantum nature of their optical excitations. Additionally, finite-size effects produce substantial plasmon broadening compared to homogeneous graphene up to sizes well above 20 nm in nanodisks and 10 nm in nanoribbons. The atomic structure of edge terminations is shown to be critical, with zigzag edges contributing to plasmon broadening significantly more than armchair edges. This study demonstrates the ability of graphene nanostructures to host well-defined plasmons down to sizes below 10 nm, and it delineates a roadmap for understanding their main characteristics, including the role of finite size and nonlocality, thus providing a solid background for the emerging field of graphene nanoplasmonics.