Graphene Terahertz Plasmon Oscillators

In this paper we propose and discuss coherent terahertz sources based on charge density wave (plasmon) amplification in two-dimensional graphene. The coupling of the plasmons to interband electron-hole transitions in population inverted graphene layers can lead to plasmon amplification through stimulated emission. Plasmon gain values in graphene can be very large due to the small group velocity of the plasmons and the strong confinement of the plasmon field in the vicinity of the graphene layer. We present a transmission line model for plasmon propagation in graphene that includes plasmon dissipation and plasmon interband gain due to stimulated emission. Using this model, we discuss design for terahertz plasmon oscillators and derive the threshold condition for oscillation taking into account internal losses and also losses due to external coupling. The threshold condition is shown to depend on the ratio of the external impedance and the characteristic impedance of the plasmon transmission line. The large gain values available at terahertz frequencies in graphene can lead to integrated oscillators that have dimensions in the 1-10 mum range.     

Negative terahertz conductivity of graphene when pumping by optical plasmons

The diffusion pumping of graphene by optical plasmons, propagating in metal, and separated from the graphene by a semiconductor layer has been investigated theoretically. It is shown that pumping of graphene with optical plasmons provides maximum negative terahertz conductivity of graphene at a lower (approximately by 25%) pumping power compared to a diffusion pumping of graphene with optical radiation.     

A graphene-based surface plasmon sensor

We present the application of graphene as a plasmon sensor. It was found that the electronic transport of chemical vapor deposition CVD-synthesized graphene is sensitive to surface plasmons generated by the illumination of metal nanoparticles. The observed change in electronic conduction can be up to seven times larger than the intrinsic photoresponse of graphene. A study of the mechanism revealed local field-assisted oxygen desorption induced by surface plasmons to be the cause of this intriguing behavior. A detailed investigation of the wavelength and spacing dependence of the plasmon-graphene coupling proves that graphene can be used as a sensitive, high resolution electronic plasmon detector. This finding shows the potential of devices exploiting the novel properties of graphene and surface plasmons.      

Substrate Phonon‐Mediated Plasmon Hybridization in Coplanar Graphene Nanostructures for Broadband Plasmonic Circuits

The mode hybridization between adjacent graphene nanoribbons determines the integration density of graphene‐based plasmonic devices. Here, plasmon hybridization in graphene nanostructures is demonstrated through the characterization of the coupling strength of plasmons in graphene nanoribbons as a function of charge density and inter‐ribbon spacing using Fourier transform infrared microscopy. In combination with numerical simulations, it is shown that the plasmon coupling is strongly mediated by the substrate phonons. For polar substrates, the plasmon coupling strength is limited by the plasmon–phonon interactions. In contrast, a nonpolar substrate affects neither the energy distribution of the original plasmon modes in graphene nanostructures nor their plasmon interactions, which increases exponentially as the inter‐ribbon spacing decreases. To further explore the potential of graphene broadband plasmonics on nonpolar substrates, a scheme is proposed that uses a metal–dielectric heterostructure to prevent the overlap of plasmons between neighboring graphene nanoribbons. The device structures retain the plasmon resonance frequency of the graphene ribbons and maximally isolate the plasmonic components from the surrounding electromagnetic environment, allowing modular design in integrated plasmonic circuits.     

Synergistic graphene/aluminum surface plasmon coupling for zinc oxide lasing improvement

Collective oscillations of free electrons generate plasmons on the surface of a material.A whispering-gallery microcavity effectively confines the light field on its surface based on the total reflection from its internal wall.When these two kinds of electromagnetic waves meet each other,the stimulated emissions from an individual ZnO microrod were enhanced more than 50-fold and the threshold was reduced after the whispering-gallery microcavity was coated with a monolayer of graphene and A1 nanoparticles.The improvement of the lasing performance was attributed to the synergistic energy coupling of the graphene/Al surface plasmons with ZnO excitons.The lasing characteristics and the coupling mechanism were investigated systematically.     

Electron-electron and electron-hole pairing in graphene structures

The superconducting pairing of electrons in doped graphene owing to in-plane and out-of-plane phonons is considered. It is shown that the structure of the order parameter in the valley space substantially affects conditions of the pairing. Electron-hole pairing in a graphene bilayer in the strong coupling regime is also considered. Taking into account retardation of the screened Coulomb pairing potential shows a significant competition between the electron-hole direct attraction and their repulsion owing to virtual plasmons and single-particle excitations.     

Electrothermal Control of Graphene Plasmon–Phonon Polaritons

Graphene plasmons are known to offer an unprecedented level of confinement and enhancement of electromagnetic field. They are hence amenable to interacting strongly with various other excitations (for example, phonons) in their surroundings and are an ideal platform to study the properties of hybrid optical modes. Conversely, the thermally induced motion of particles and quasiparticles can in turn interact with electronic degrees of freedom in graphene, including the collective plasmon modes via the Coulomb interaction, which opens up new pathways to manipulate and control the behavior of these modes. This study demonstrates tunable electrothermal control of coupling between graphene mid‐infrared (mid‐IR) plasmons and IR active optical phonons in silicon nitride. This study utilizes graphene nanoribbons functioning as both localized plasmonic resonators and local Joule heaters upon application of an external bias. In the latter role, they achieve up to ≈100 K of temperature variation within the device area. This study observes increased modal splitting of two plasmon–phonon polariton hybrid modes with temperature, which is a manifestation of increased plasmon–phonon coupling strength. Additionally, this study also reports on the existence of a thermally excited hybrid plasmon–phonon mode. This work can open the door for future optoelectronic devices such as electrically switchable graphene mid‐infrared plasmon sources.     

Plasmons in graphene: Recent progress and applications

Owing to its excellent electrical, mechanical, thermal and optical properties, graphene has attracted great interests since it was successfully exfoliated in 2004. Its two dimensional nature and superior properties meet the need of surface plasmons and greatly enrich the field of plasmonics. Recent progress and applications of graphene plasmonics will be reviewed, including the theoretical mechanisms, experimental observations, and meaningful applications. With relatively low loss, high confinement, flexible feature, and good tunability, graphene can be a promising plasmonic material alternative to the noble metals. Optics transformation, plasmonic metamaterials, light harvesting etc. are realized in graphene based devices, which are useful for applications in electronics, optics, energy storage, THz technology and so on. Moreover, the fine biocompatibility of graphene makes it a very well candidate for applications in biotechnology and medical science.     

Atomically localized plasmon enhancement in monolayer graphene

Plasmons in graphene can be tuned by using electrostatic gating or chemical doping, and the ability to confine plasmons in very small regions could have applications in optoelectronics, plasmonics and transformation optics. However, little is known about how atomic-scale defects influence the plasmonic properties of graphene. Moreover, the smallest localized plasmon resonance observed in any material to date has been limited to around 10 nm. Here, we show that surface plasmon resonances in graphene can be enhanced locally at the atomic scale. Using electron energy-loss spectrum imaging in an aberration-corrected scanning transmission electron microscope, we find that a single point defect can act as an atomic antenna in the petahertz (10(15) Hz) frequency range, leading to surface plasmon resonances at the subnanometre scale.     

Infrared spectroscopy of tunable Dirac terahertz magneto-plasmons in graphene

We present infrared spectroscopy study of plasmon excitations in graphene in high magnetic fields. The plasmon resonance in patterned graphene disks splits into edge and bulk plasmon modes in magnetic fields. Remarkably, the edge plasmons develop increasingly longer lifetimes in high fields due to the suppression of backscattering. Moreover, due to the linear band structure of graphene, the splitting of the edge and bulk plasmon modes develops a strong doping dependence, which differs from the behavior of conventional semiconductor two-dimensional electron gas (2DEG) systems. We also observe the appearance of a higher order mode indicating an anharmonic confinement potential even in these well-defined circular disks. Our work not only opens an avenue for the investigation of the properties of Dirac magnetoplasmons but also supports the great potential of graphene for tunable terahertz magneto-optical devices.     

Enhancement of mechanical properties of low carbon steel joints via graphene addition

Steel base metal laps or welding electrode surfaces were coated using graphene suspensions with various concentrations, and then the steel plates were welded using the shielded metal arc welding process. Microstructural observations showed that the addition of graphene to the weldment significantly refines the microstructure and promotes the formation of fine acicular ferrite. The results of mechanical testing indicated that with lower concentrations of graphene in the weldment, both the strength and ductility improve, but the hardness remains unchanged in comparison to the unreinforced weld metal. However, reinforcing with a higher concentration of graphene gives rise to the significant enhancement of the hardness and strength without deterioration of the ductility.     

Evidence of Plasmonic Coupling in Gallium Nanoparticles/Graphene/SiC

Graphene is emerging as a promising material for plasmonics applications due to its strong light-matter interactions, most of which are theoretically predicted but not yet experimentally realized. Therefore, the integration of plasmonic nanoparticles to create metal nanoparticle/graphene composites enables numerous phenomena important for a range of applications from photonics to catalysis. For these applications it is important to articulate the coupling of photon-based excitations such as the interaction between plasmons in each of the material components, as well as their charge-based interactions dependent upon the energy alignment at the metal/graphene interface. These coupled phenomena underpin an active application area in graphene-based composites due to nanoparticle-dependent surface-enhanced Raman scattering (SERS) of graphene phonon modes. This study reveals the coupling of a graphene/SiC support with Ga-nanoparticle-localized surface plasmon resonance, which is of particular interest due to its ability to be tuned across the UV into the near-IR region. This work is the first demonstration of the evolving plasmon resonance on graphene during the synthesis of surface-supported metal nanoparticles, thus providing evidence for the theoretically predicted screening revealed by a damped resonance with little energy shift. Therefore, the role of the graphene/substrate heterojunction in tailoring the plasmon resonance for nanoplasmonic applications is shown. Additionally, the coupled phenomena between the graphene-Ga plasmon properties, charge transfer, and SERS of graphene vibrational modes are explored.     

Graphene-enabled silver nanoantenna sensors

Silver is the ideal material for plasmonics because of its low loss at optical frequencies but is often replaced by a more lossy metal, gold. This is because of silver's tendency to tarnish and roughen, forming Ag(2)S on its surface, dramatically diminishing optical properties and rendering it unreliable for applications. By passivating the surface of silver nanostructures with monolayer graphene, atmospheric sulfur containing compounds are unable to penetrate the graphene to degrade the surface of the silver. Preventing this sulfidation eliminates the increased material damping and scattering losses originating from the unintentional Ag(2)S layer. Because it is atomically thin, graphene does not interfere with the ability of localized surface plasmons to interact with the environment in sensing applications. Furthermore, after 30 days graphene-passivated silver (Ag-Gr) nanoantennas exhibit a 2600% higher sensitivity over that of bare Ag nanoantennas and 2 orders of magnitude improvement in peak width endurance. By employing graphene in this manner, the excellent optical properties and large spectral range of silver can be functionally utilized in a variety of nanoscale plasmonic devices and applications.     

In‐Plane Electrical Connectivity and Near‐Field Concentration of Isolated Graphene Resonators Realized by Ion Beams

Graphene plasmons provide great opportunities in light–matter interactions benefiting from the extreme confinement and electrical tunability. Structured graphene cavities possess enhanced confinements in 3D and steerable plasmon resonances, potential in applications for sensing and emission control at the nanoscale. Besides graphene boundaries obtained by mask lithography, graphene defects engineered by ion beams have shown efficient plasmon reflections. In this paper, near‐field responses of structured graphene achieved by ion beam direct‐writing are investigated. Graphene nanoresonators are fabricated easily and precisely with a spatial resolution better than 30 nm. Breathing modes are observed in graphene disks. The amorphous carbons around weaken the response of edge modes in the resonators, but meanwhile render the isolated resonators in‐plane electrical connections, where near‐fields are proved gate‐tunable. The realization of gate‐tunable near‐fields of graphene 2D resonators opens up tunable near‐field couplings with matters. Moreover, graphene nonconcentric rings with engineered near‐field confinement distributions are demonstrated, where the quadrupole plasmon modes are excited. Near‐field mappings reveal concentrations at the scale of within certain zones which can be engineered. The realization of electrically tunable graphene nanoresonators by ion beam direct‐writing is promising for active manipulation of emission and sensing at the nanoscale.     

State-of-the-art graphene high-frequency electronics

High-performance graphene transistors for radio frequency applications have received much attention and significant progress has been achieved. However, devices based on large-area synthetic graphene, which have direct technological relevance, are still typically outperformed by those based on mechanically exfoliated graphene. Here, we report devices with intrinsic cutoff frequency above 300 GHz, based on both wafer-scale CVD grown graphene and epitaxial graphene on SiC, thus surpassing previous records on any graphene material. We also demonstrate devices with optimized architecture exhibiting voltage and power gains reaching 20 dB and a wafer-scale integrated graphene amplifier circuit with voltage amplification.     

Graphene enhances the specificity of the polymerase chain reaction

Graphene can inhibit non-specific DNA fragments, and the specificity of the polymerase chain reaction (PCR) can be retained even after eight rounds of repeated amplification in the presence of graphene in the form of reduced graphene oxide (RGO). In the figure, the numbers at the top give the number of rounds of PCR; lanes marked with C correspond to controls (no RGO), and the concentration of RGO in the other samples is 12 μg mL(-1) .     

Plasmonic properties of metallic nanoparticles: the effects of size quantization

We examine the size quantization of plasmons in metallic nanoparticles using time-dependent density functional theory. For small particles in the quantum limit, we identify "quantum core plasmons" and "classical surface plasmons", both of which are collective oscillations comprised of multiple single-particle transitions. As particle size increases, the response of the classical surface plasmons becomes much larger than that of the quantum core plasmons.     

Coupled Surface Plasmons at 3·391 μm

Abstract

Using 3·391 μm wavelength radiation to excite surface plasmons means that the conventional dimensional constraints imposed upon the dielectric thickness required for coupled surface plasmons is relaxed. Results are presented and analysed for coupled surface plasmons using silver layers spaced by a variable air gap of several micrometres thickness. Adjusting the air gap allows the determination of the dispersion curve for the coupled plasmons as a function of gap thickness down to a gap of about 5 μm.     

Effects of Electronic Quantum Interference, Photonic-Crystal Cavity, Spontaneous Emission, Longitudinal Field, Surface-Plasmon Modes, and Surface–Plasmon–Polariton for Optical Amplification

Some possibilities for coherent optical amplification of a normally incident and weak radiation field are reviewed based on various physical mechanisms, such as electronic quantum interference induced by a coupling laser field in a three-level system, field enhancement through the cavity confinement of a radiation field in a photonic crystal and field concentration seen in a transmitted near field through a metallic surface grating due to excitation of surface-plasmon-polariton modes. Numerical results are presented and discussed to demonstrate these interesting effects. The modification to the spontaneous emission inside a photonic crystal is also studied. The important role played by a longitudinal field resulting from the absorption by an induced three-dimensional plasma wave inside a doped semiconductor is analyzed using a nonlocal and nonadiabatic model. Moreover, the coupling between two-dimensional plasmons and surface plasmon modes in the nonretardation limit is explored.     

Tunable Low Loss 1D Surface Plasmons in InAs Nanowires

Due to the ability to manipulate photons at nanoscale, plasmonics has become one of the most important branches in nanophotonics. The prerequisites for the technological application of plasmons include high confining ability (λ₀/λ_p), low damping, and easy tunability. However, plasmons in typical plasmonic materials, i.e., noble metals, cannot satisfy these three requirements simultaneously and cause a disconnection to modern electronics. Here, the indium arsenide (InAs) nanowire is identified as a material that satisfies all the three prerequisites, providing a natural analogy with modern electronics. The dispersion relation of InAs plasmons is determined using the nanoinfrared imaging technique, and show that their associated wavelengths and damping ratio can be tuned by altering the nanowire diameter and dielectric environment. The InAs plasmons possess advantages such as high confining ability, low loss, and ease of fabrication. The observation of InAs plasmons could enable novel plasmonic circuits for future subwavelength applications.