Graphene Plasmons Research Articles

Efficient All-Optical Plasmonic Modulators with Atomically Thin Van Der Waals Heterostructures.

All-optical modulators are attracting significant attention due to their intrinsic perspective on high-speed, low-loss, and broadband performance, which are promising to replace their electrical counterparts for future information communication technology. However, high-power consumption and large footprint remain obstacles for the prevailing nonlinear optical methods due to the weak photon-photon interaction. Here, efficient all-optical mid-infrared plasmonic waveguide and free-space modulators in atomically thin graphene-MoS2 heterostructures based on the ultrafast and efficient doping of graphene with the photogenerated carrier in the monolayer MoS2 are reported. Plasmonic modulation of 44 cm-1 is demonstrated by an LED with light intensity down to 0.15 mW cm-2 , which is four orders of magnitude smaller than the prevailing graphene nonlinear all-optical modulators (≈103 mW cm-2 ). The ultrafast carrier transfer and recombination time of photogenerated carriers in the heterostructure may achieve ultrafast modulation of the graphene plasmon. The demonstration of the efficient all-optical mid-infrared plasmonic modulators, with chip-scale integrability and deep-sub wavelength light field confinement derived from the van der Waals heterostructures, may be an important step toward on-chip all-optical devices.

Active spatial control of terahertz plasmons in graphene

Graphene offers the possibility for actively controlling plasmon confinement and propagation by tailoring its spatial conductivity. However, implementation of this concept has been hampered because it is difficult to control the conductivity pattern without disturbing the electromagnetic environment of graphene plasmons. Here we demonstrate full electrical control of plasmon reflection/transmission in graphene at electronic boundaries induced by a transparent patterned zinc oxide gate, which is designed to minimize the electromagnetic coupling to graphene in the terahertz range. This approach enables plasmons to be confined to desired regions. Our approach might be applied to various types of plasmonic devices, paving the way for implementing a programmable plasmonic circuit.

Conversion of Solar Radiation into Vapor: New Possibilities Offered by Nanomaterials (Review)

The current state in converting solar radiation into steam using nanotechnologies and nanomaterials is considered. The main attention is paid to the use of novel nanostructured materials, including graphene components to generate steam, in particular, to the use of the latter as floating or volumetric solar absorbers. The absorbers of the above types have been classified, their physical and technological characteristics have been considered, and preferable variants for various applications have been established. It is stressed that the question on the efficiency of surface or volumetric absorbers has to be considered as applied to specific applications. The thermophysical processes that occur during the radiation absorption and the heating of the nanofluid have been described for different absorption patterns. Models that underlie the description of the thermophotonics and nanoplasmonics processes during the solar radiation absorption by nanocomponents have been formulated. The basic processes that occur under nanoplasmonic heating of nanocomponents by solar radiation and the heat-and-mass exchange between nanocomponents and the surrounding medium have been investigated. Special attention is paid to steam generation by plasmonic nanoparticles and graphene flakes, which are currently considered the basic elements for prospective efficient solar radiation conversion systems. The properties of the materials used to absorb solar radiation and their components are described. The materials that have already shown a high solar absorption coefficient are listed. The properties of nanofluids used for volumetric absorption of solar radiation and the subsequent localized heating are described. The significant role of novel steam-generation mechanisms based on plasmonic nanoparticles and graphene flakes is stressed. Mesoscopic and nanostructured solar radiation absorbers are presented that ensure the generation of steam used not only to produce energy but also to desalinate and sterilize water. The main unsolved problems of applying nanocomponents for the solar thermal industry are considered and new tasks are set, the solution of which will allow taking important steps towards using the above systems for solar energy conversion.

Single photon pulse induced transient entanglement force

We show that a single photon pulse incident on two interacting two-level atoms induces a transient entanglement force between them. After absorption of a multi-mode Fock state pulse, the time-dependent atomic interaction mediated by the vacuum fluctuations changes from the van der Waals interaction to the resonant dipole–dipole interaction (RDDI). We explicitly show that the RDDI force induced by the single photon pulse fundamentally arises from the two-body transient entanglement between the atoms. This single photon pulse induced entanglement force can be continuously tuned from being repulsive to attractive by varying the polarization of the pulse. We further demonstrate that the entanglement force can be enhanced by more than three orders of magnitude if the atomic interactions are mediated by graphene plasmons. These results demonstrate the potential of shaped single photon pulses as a powerful tool to manipulate this entanglement force and also provides a new approach to witness transient atom–atom entanglement.

Multiple Fano Resonances with Tunable Electromagnetic Properties in Graphene Plasmonic Metamolecules

Multiple Fano resonances (FRs) can be produced by destroying the symmetry of structure or adding additional nanoparticles without changing the spatial symmetry, which has been proved in noble metal structures. However, due to the disadvantages of low modulation depth, large damping rate, and broadband spectral responses, many resonance applications are limited. In this research paper, we propose a graphene plasmonic metamolecule (PMM) by adding an additional 12 nanodiscs around a graphene heptamer, where two Fano resonance modes with different wavelengths are observed in the extinction spectrum. The competition between the two FRs as well as the modulation depth of each FR is investigated by varying the materials and the geometrical parameters of the nanostructure. A simple trimer model, which emulates the radical distribution of the PMM, is employed to understand the electromagnetic field behaviors during the variation of the parameters. Our proposed graphene nanostructures might find significant applications in the fields of single molecule detection, chemical or biochemical sensing, and nanoantenna.

Wideband tunable perfect absorption of graphene plasmons via attenuated total reflection in Otto prism configuration

Abstract A strategy is proposed to achieve wideband tunable perfect plasmonic absorption in graphene nanoribbons by employing attenuated total refraction (ATR) in Otto prism configuration. In this configuration, the Otto prism with a deep-subwavelength dielectric spacer is used to generate tunneling evanescent waves to excite localized plasmons in graphene nanoribbons. The influence of the configuration parameters on the absorption spectra of graphene plasmons is studied systematically, and the key finding is that perfect absorption can be achieved by actively controlling the incident angle of light under ATR conditions, which provides an effective degree of freedom to tune the absorption properties of graphene plasmons. Based on this result, it is further demonstrated that by simultaneously tuning the incident angle and the graphene Fermi energy, the tunable absorption waveband can be significantly enlarged, which is about 3 times wider than the conventional cavity-enhanced configuration. Our proposed strategy to achieve wideband, tunable graphene plasmons could be useful in various infrared plasmonic devices.

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.

Electromagnetic induced transparency in graphene waveguide structure for Terahertz application

Electromagnetic Induced transparency (EIT) has been considered as a possible manner to implement plasmonic devices for slow light and sensing applications. In this paper, we proposed a novel design of graphene waveguide structure, which provides a tunable EIT response. Numerical results reveal that the plasmon induced transparency spectral response occurs due to the interference between quasi-dark mode and quasi-bright mode which exist in the loaded graphene resonator of the device. The EIT peak frequency can be tuned by changing the Fermi level of the graphene. An elaborate analysis of the relationship between the transparency frequency and graphene’s Fermi level, in turn, proved the two-dimensional nature of the graphene plasmon. Compared with previous designs, the proposed design can work in the THz frequency and provides a much sharper EIT window which is quite useful for sensing applications.

Graphene Plasmonic Crystal: Two-Dimensional Gate-Controlled Chemical Potential for Creation of Photonic Bandgap

A new design of graphene-based plasmonic waveguide is presented and its transmission properties are studied. The transmission channel is such designed that the chemical potential of graphene is periodically changed in two dimensions by application of voltage bias through a patterned substrate. Because of periodicity of structure, it shows a forbidden bandgap at terahertz (THz) frequencies similar to a conventional photonic crystal. The effect of different structure parameters on the rejection band frequency range is numerically studied and the switching function of the proposed waveguide is observed with rejection efficiency of more than 99%. The reported relation between center frequency of the rejection band and also FWHM with the related chemical potential of the 2D-GPC confirm the real-time tunability of the proposed structure employing an external bias voltage. According to the ultra-integrated size of the structure and its remarkable optical efficiency in THz range, such a realization can pave the way for further development in band rejection–based nanoplasmonic applications such as filtering and switching devices.

Strong Light–Matter Interactions Enabled by Polaritons in Atomically Thin Materials

AbstractPolaritons, resulting from the hybridization of light with polarization charges formed at the boundaries between media with positive and negative dielectric response functions, can focus light into regions much smaller than its associated free‐space wavelength. This property is paramount for a plethora of applications in nanophotonics, ranging from biological sensing to photocatalysis to nonlinear and quantum optics. In the two‐dimensional (2D) limit, represented by atomically thin and van der Waals (vdW) materials of single‐layers bound by weak vdW attraction, polaritons are characterized by extremely small wavelengths associated with extreme optical confinement, and furthermore can exhibit long lifetimes, electrical tunability, and extreme sensitivity to their dielectric environment, among many other desirable qualities in nano‐optical device applications. Here, the fundamentals of polaritons in atomically thin materials are summarized, emphasizing plasmon and exciton polaritons, their strong light–matter interactions, and nonlinear plasmonics. More specifically, this review opens with a pedagogical discussion of plasmons in extended and nanostructured graphene, providing a classical electrodynamical model in a nonretarded theoretical framework, and the ultraconfined acoustic plasmons supported by hybrid graphene–dielectric–metal structures. In addition, the basic principles are introduced and the recent developments on nonlinear graphene plasmonics and on strong coupling physics with atomically thin transition metal dichalcogenides are reviewed. Finally, potentially new, promising research directions in the burgeoning field of 2D nanophotonics are identified.

Complete Complex Amplitude Modulation with Electronically Tunable Graphene Plasmonic Metamolecules

Dynamic high-resolution wavefront modulation of light is a long-standing quest in photonics. Metasurfaces have shown potential for realizing light manipulation with subwavelength resolution through nanoscale optical elements, or metaatoms, to overcome the limitations of conventional spatial light modulators. State-of-the-art active metasurfaces operate via phase modulation of the metaatoms, and their inability to also independently control the scattered amplitude leads to an inferior reconstruction of the desired wavefronts. This fundamental problem posed severe performance limitations particularly for applications relying on subwavelength spatiotemporal complex field modulation, which includes dynamic holography, high-resolution imaging, optical tweezing, and optical information processing. Here, we present the "metamolecule" strategy, which incorporates two independent subwavelength scatterers composed of noble metal antennas coupled to gate-tunable graphene plasmonic nanoresonators. The two-parametric control of the metamolecule secures the complete control of both amplitude and phase of light, enabling 2π phase shift as well as large amplitude modulation including perfect absorption. We further develop a generalized graphical model to examine the underlying requirements for complete complex amplitude modulation, offering intuitive design guidelines to maximize the tunability in metasurfaces. To illustrate the reconfigurable capability of our designs, we demonstrate dynamic beam steering and holographic wavefront reconstruction in periodically arranged metamolecules.

Effects of Mid‐Infrared Graphene Plasmons on Photothermal Heating

Herein, the plasmonic heating of graphene‐based systems under the mid‐infrared laser irradiation is theoretically investigated, where periodic arrays of graphene plasmonic resonators are placed on dielectric thin films. Optical resonances are sensitive to structural parameters and the number of graphene layers. Under mid‐infrared laser irradiation, the steady‐state temperature gradients are calculated. It is found that graphene plasmons significantly enhance the confinement of electromagnetic fields in the system and lead to a large temperature increase compared with the case without graphene. The correlations between temperature change and the optical power, laser spot, and thermal conductivity of dielectric layer in these systems are discussed. The numerical results are in accordance with experiments.

Enhanced optical absorption of graphene by plasmon

The plasmons in graphene have the superior properties to metal surface plasmons, such as high field confinement, low Ohmic loss and long wave propagation, highly tunable via electrostatic. More importantly, the frequency of plasmons ranges from terahertz to infrared which indicates that graphene is an ideal candidate for terahertz plamsonics. On the other hand, the strong coupling between incident photons and plasmons in graphene can lead the optical absorption to be enhanced. However, it is difficult for light to couple directly with plasmons in graphene, for the momentum of incident photons cannot match the plasmons in graphene. A metal grating can be used to compensate for the momentum of photons so that it can match that of plasmons in graphene. In this work, we theoretically investigate the effect of plasmons on the terahertz optical absorption of graphene with grating based on finite difference time domain. A great enhancement of electric field component of light field can be obtained near the gold grating strip in the sheet of graphene. Thus, the photons, of which the momentum is compensated for by the grating, can strongly couple with plasmons in graphene. An obviously decrease of the transmission of the graphene structure can be seen at the resonant frequency. The transmission peak corresponds to the resonant frequency spliting into two peaks due to the fact that two plasmon polariton modes are formed by the coupling of photons and palsmons. So we also study the plasmon polariton modes made by coupling photon with palsmon based on the many-body self-consistent method. Two plasmon polariton modes are obtained and an obviously splitting at the resonant frequency can be seen due to the coupling between photons and plasmons. The work conduces to deepening the understanding of the photoelectric properties of graphene and the terahertz plasmonics based on graphene.

Near-field infrared microscopy of graphene on metal substrate

Graphene plasmons, collective oscillation modes of electrons in graphene, have recently attracted intense attention in both the fundamental researches and the applications because of their strong field confinement, low loss and excellent tunability. The dispersion of graphene plasmons can be significantly modified in the system of graphene on metal substrate, in which the screening of the long-range part of the electron-electron interactions by nearby metal can lead to many novel quantum effects, such as acoustic plasmons, quantum nonlocal effects and renormalization of band structure. Scattering-type scanning near-field optical microscopy (s-SNOM) which consists of a laser coupled to the tip of an atomic force microscopy (AFM), is an effective technique to directly probe plasmons in two-dimensional materials including graphene, and the graphene plasmons can be observed visually by real-space imaging. But so far the detailed s-SNOM studies of graphene/metal system have not been reported. One potential challenge is that the near-field response of highly conductive metal substrate may partially or entirely obscure that of graphene, making it difficult to further explore graphene by using s-SNOM. Here in this paper, we report the direct observation of near-field optical response of graphene in a graphene/metal system excited by a mid-infrared quantum cascade laser. From a close examination of the data of graphene/Cu compared with that of h-BN/Cu, we are able to identify experimental features due to the near-field response of graphene. Surprisingly, two completely different behaviors are observed in the s-SNOM data for different graphene samples on Cu substrates with similar surface step geometries. These results suggest that the near-field response of graphene/metal system is not completely dominated by the metal substrate, and that two completely different near-field response behaviors of graphene may be attributed to their intrinsic properties affected by metal substrates themselves rather than surface step geometries of metal substrate. In addition, following this approach it is possible to distinguish the near-field optical responses of graphene from that of graphene/metal system. Our work reveals the clear signatures of the near-field optical response of graphene on metal substrate, which provides the foundation for probing plasmons in these systems by using the s-SNOM and understanding many novel quantum phenomena therein.

Plasmons in graphene traversed by relativistic electronic beam with arbitrary trajectories

SynopsisIn this work we study the energy loss of a beam of relativistic electrons impinging on a graphene sheet under oblique incidence. Using several models for the electric conductivity of graphene, we cover a wide range of frequencies of the energy loss spectrum, from the terahertz to the ultraviolet. To describe this spectrum, we consider the contribution of electronic excitations as well as that of transition radiation.

Terahertz Frequency Comb in Graphene Field-Effect Transistors

Graphene Field-effect transistors (GFETs) are excellent candidates for all-electric, low-power radiation sources and detectors based on integrated circuit technology. In this work, we show that a hydrodynamic instability can be ex¬plored (the Dyakonov–Shur instability) to excite the graphene plasmons. The instability can be sustained with the help of a source-to-drain current and con¬trolled with the gate voltage. It is shown that the plasmons radiate a frequency comb in the Terahertz (THz) range. It is argued how this can pave the stage for a new generation of low power THz sources in integrated-circuit technology.

Design and Circuit Modeling of Graphene Plasmonic Nanoantennas

Nanonntennas are critical elements of nanoscale wireless communication technologies with potential to overcome some of the limitations of on-chip interconnects. In this paper, we model the optical response of graphene-based nanoantennas (GNAs) using an equivalent resistive-inductive-capacitive (RLC) circuit by incorporating plasmonic effects that are present in graphene at terahertz (THz) frequencies. The equivalent circuit model is used to estimate the resonance characteristics of the nanoantenna, thereby facilitating geometry and material optimization. Three GNA structures are considered, namely bowtie, circular, and rectangular dimers with vacuum and silicon dioxide dielectric environment. We characterize the radiation efficiency, the Purcell factor, the quality factor, and the resonant frequency of various antennas with respect to the physical properties of the surrounding dielectric media and the graphene sheet. The GNAs are designed for a resonant frequency in the range of 10 – 25 THz with field enhancements around 104 – 105 and radiation efficiency ~5 – 16%. The circuit model is applied to examine the trade-off between the radiation efficiency and the field enhancement in the antenna gap. While bowtie antennas have higher field enhancement due to charge accumulation in their pointed structure, they suffer from low radiation efficiency stemming from the large spreading resistance losses. The circuit model is validated against finite-difference time-domain (FDTD) simulations conducted in Lumerical, and excellent match between the model and numerical results is demonstrated in the THz regime. The circuit models can be readily used in a hierarchical circuit simulator to design and optimize optical nanoantennas for THz communication in high-performance computing systems.

Optical surface second harmonic generation from plasmonic graphene-coated nanoshells: influence of shape, size, dielectric core and embedding medium

We study optical surface second-harmonic generation (SHG) from the plasmonic nanoshells because they can improve and enhance nonlinear optical effects. We also investigate the SHG from the surface of spherical and cylindrical core-shell nanoparticles coated with a strong nonlinear material such as graphene due to its attractive plasmonic behaviour and unique properties of the surface plasmons in graphene. We demonstrate theoretically a giant and tunable second-order harmonic radiation which enhanced through the excitation of the surface plasmon resonance, can be observed at the surface of both spherical and cylindrical nanoshells because of symmetry-breaking at interface, which makes SHG as a valuable and powerful technique for applications in sensing and surface spectroscopy. We present the surface SHG strongly depends on the particle shape and size, the dielectric of embedding medium and core, the type of metal and graphene as a coating nonlinear material.

Highly resolved ultra-strong coupling between graphene plasmons and intersubband polaritons

The interaction between graphene surface plasmons and a semiconductor quantum well has been investigated by means of scattering matrix simulations. Due to the strong confinement factor of the graphene layer, a large Rabi splitting arises from the interaction with intersubband transitions. By varying the Fermi energy in the graphene and the doping in the quantum well, the resulting polariton states show features of strong and ultra-strong coupling. The system has been modeled with the coupled-mode theory to find the highest quality factor for the polariton resonance, reaching a “highly resolved” ultra-strong coupling regime.

Graphene plasmonically induced analogue of tunable electromagnetically induced transparency without structurally or spatially asymmetry

Electromagnetically induced transparency (EIT) arises from the coherent coupling and interference between a superradiant (bright) mode in one resonator and a subradiant (dark) mode in an adjacent resonator. Generally, the two adjacent resonators are structurally or spatially asymmetric. Here, by numerical simulation, we demonstrate that tunable EIT can be induced by graphene ribbon pairs without structurally or spatially asymmetry. The mechanism originates from the fact that the resonate frequencies of the bright mode and the dark mode supported by the symmetrical graphene ribbon pairs can be respectively tuned by electrical doping levels, and when they are tuned to be equal the graphene plasmon coupling and interference occurs. The EIT in symmetrical nanostructure which avoids deliberately breaking the element symmetry in shape as well as in size facilitates the design and fabrication of the structure. In addition, the work regarding to EIT in the structurally symmetric could provide a fresh contribution to a more comprehensive physical understanding of Fano resonance.