Excitation Of Graphene Plasmons Research Articles

First-principles calculations of plasmon excitations in graphene, silicene, and germanene

Plasmon excitations in graphene, silicene, and germanene are studied using linear-response time-dependent density functional theory within the random phase approximation (RPA). In this work, we examine both the plasmon dispersion behavior and lifetime of extrinsic and intrinsic plasmons for these three materials. For extrinsic plasmons, we found that their properties are closely related to Landau damping. In the region without single-particle excitation (SPE), the plasmon dispersion shows a $\sqrt{q}$ behavior and the lifetime is infinite at the RPA level, while in the SPE region, the plasmon dispersion shows a quasilinear behavior and the lifetime is finite. Moreover, for intrinsic plasmons, unlike graphene, the plasmon dispersion behavior of silicene and germanene exhibits a two-peak structure, which can be attributed to the complex and hybridized band structure of these two materials.

Significant enhancement of third harmonic generation in graphene layers placed on a random grating supporting plasmonic Anderson localized states

We first theoretically investigate the third-harmonic generation (THG) in a metasurface in which trilayer graphene is deposited over a periodic structure of GaP-CaF2 unit cells. The results of simulation confirm that the output intensity of third-harmonic wave enhances by 229 times around the resonance frequency compared to a bare trilayer graphene surrounded by air. This is due to increase of nonlinear response because of the field enhancement in the graphene layer owing to the excitation of graphene plasmons. Second, to obtain more enhancement for THG, we propose and simulate a metasurface in which the trilayer graphene is deposited on a disordered GaP-CaF2 grating. For this random structure, significant THG occurs at multiple resonance frequencies induced by Anderson localization effect. Our simulation results demonstrate that the maximum output intensity of THG wave increases by 125 times compared to the periodic structure and 28,625 times compared to the bare trilayer graphene. The effects of disorder level are investigated on the output intensity of the THG signal. The number of peaks and the maximum value of THG wave increase with the disorder level. Finally, we study dependence of THG output intensity on the Fermi energy level of graphene. The emission peaks of the THG wave are blue-shifted as Fermi energy increases. Our theoretical prediction of significant enhancement of THG in the graphene based disordered structures needs some experimental evidences in the future to be employed in different applications such as imaging of biological tissues.

Relaxing Graphene Plasmon Excitation Constraints Through the Use of an Epsilon-Near-Zero Substrate

Graphene plasmons have attracted significant attention due to their tunability, potentially long propagation lengths and ultracompact wavelengths. However, the latter characteristic imposes challenges to light-plasmon coupling in practical applications, generally requiring sophisticated coupling setups, extremely high doping levels and/or graphene nanostructuring close to the resolution limit of current lithography techniques. Here, we propose and theoretically demonstrate a method for alleviating such a technological strain through the use of a practical substrate whose low and negative dielectric function naturally enlarges the graphene polariton wavelength to more manageable levels. We consider silicon carbide (SiC), as it exhibits a dielectric function whose real part is between \(-1\) and 0, while its imaginary part remains lower than 0.05, in the 951 to 970 cm\(^{-1}\) mid-infrared spectral range. Our calculations show hybridization with the substrate’s phonon polariton, resulting in a polariton wavelength that is an order of magnitude longer than obtained with a silicon dioxide substrate, while the propagation length increases by the same amount.

Graphene Plasmon Excitation with Ground-State Two-Level Quantum Emitters.

The transmission of a two-level quantum emitter in its ground state through a graphene nanosheet is investigated. The graphene plasmons (GPs) field distribution, especially the opposite orientations of the vertical electric field components on the two sides of the graphene nanosheet, produces a significant nonadiabatic process during the interaction between the emitter and the localized GPs. By taking into account the counterrotating terms, the excitation of the quantum emitter with simultaneous emission of a GP has a large probability. This happens for emitter speeds of about 10^{-4} times the speed of light. For accelerated emitters, the GPs exhibit thermal field photon distribution with a high temperature. As a consequence, this study provides a promising platform to observe the dynamical Casimir effect as well as a simulation of the Unruh effect.

Photonic and plasmonic transition radiation from graphene

We theoretically study the transition radiation in the framework of full Maxwell equations, when a swift electron crosses a monolayer graphene. Based on the Sommerfeld integration, we demonstrate in the frequency domain the spatial distribution of this free-electron radiation, which clearly shows the broadband excitation of both photons and graphene plasmons. Moreover, the radiation spectra for photons and graphene plasmons are analytically derived. We find that the excitation of photons and graphene plasmons favors different particle velocities. To be specific, a higher particle velocity gives rise to the excitation of photons with better directivity and higher intensity, while a lower particle velocity enables the efficient excitation of graphene plasmons in a broader frequency range. Our work indicates that the interaction between swift charged particles and various 2D materials or van der Waals heterostructures is promising for the design of terahertz on-chip radiation sources.

Dirac surface plasmons in photoexcited bismuth telluride nanowires: optical pump-terahertz probe spectroscopy

Collective excitation of Dirac plasmons in graphene and topological insulators has opened new possibilities of tunable plasmonic materials ranging from THz to mid-infrared regions. Using time resolved Optical Pump-Terahertz Probe (OPTP) spectroscopy, we demonstrate the presence of plasmonic oscillations in bismuth telluride nanowires (Bi2Te3 NWs) after photoexcitation using an 800 nm pump pulse. In the frequency domain, the differential conductivity (Δσ = σpump on - σpump off) spectrum shows a Lorentzian response where the resonance frequency (ωp), attributed to surface plasmon oscillations, shifts with photogenerated carrier density (n) as . This dependence establishes the absorption of THz radiation by the Dirac surface plasmon oscillations of the charge carriers in the Topological Surface States (TSS) of Bi2Te3 NWs. Moreover, we obtain a modulation depth, tunable by pump fluence, of ∼40% over the spectral range of 0.5 to 2.5 THz. In addition, the time evolution of Δσ(t) represents a long relaxation channel lasting for more than 50 ps. We model the decay dynamics of Δσ(t) using coupled second order rate equations, highlighting the contributions from surface recombination as well as from trap mediated relaxation channels of the photoinjected carriers.

Excitation of localized graphene plasmons by aperiodic self-assembled arrays of metallic antennas

Infrared (IR) and terahertz plasmons in two-dimensional (2D) materials are commonly excited by metallic or dielectric grating couplers with deep-submicron features fabricated by e-beam lithography. Mass reproduction of such gratings at macroscopic scales is a labor-consuming and expensive technology. Here, we show that localized plasmons in graphene can be generated on macroscopic scales with couplers based on randomly oriented particle-like nanorods (NRs) in close proximity to graphene layer. We monitor the excitation of graphene plasmons indirectly by tracking the changes in reflection/absorption spectra of methylene blue (MB) or polymethyl methacrylate(PMMA) molecules deposited on the structure. Hybridization of spectrally broad graphene plasmon and narrow molecular oscillators results in enhanced oscillator strengths and Fano scattering related lines asymmetry in reflection spectra.

Electric Control of Spin‐Orbit Coupling in Graphene‐Based Nanostructures with Broken Rotational Symmetry

AbstractSpin and orbital angular momenta of light are important degrees of freedom in nanophotonics which control light propagation, optical forces, and information encoding. Here, it is shown that graphene‐supported plasmonic nanostructures with broken rotational symmetry provide a surprising spin to orbital angular momentum conversion, which can be continuously controlled by changing the electrochemical potential of graphene. Upon resonant illumination by a circularly polarized plane wave, a polygonal array of indium‐tin‐oxide nanoparticles on a graphene sheet generates the scattered field carrying electrically‐tunable orbital angular momentum. This unique photonic spin‐orbit interaction occurs due to the strong coupling between graphene plasmon polaritons and localized surface plasmons of the nanoparticles and leads to the controlled directional excitation of graphene plasmons. The tuneable spin‐orbit conversion paves the way for high‐rate information encoding in optical communications, electric steering functionalities in optical tweezers, and nanorouting of higher‐dimensional entangled photon states.

Direct S-matrix calculation for diffractive structures and metasurfaces.

The paper presents a derivation of analytical components of S matrices for arbitrary planar diffractive structures and metasurfaces in the Fourier domain. The attained general formulas for S-matrix components can be applied within both formulations in the Cartesian and curvilinear metric. A numerical method based on these results can benefit from all previous improvements of the Fourier domain methods. In addition, we provide expressions for S-matrix calculation in the case of periodically corrugated layers of two-dimensional materials, which are valid for arbitrary corrugation depth-to-period ratios. As an example, the derived equations are used to simulate resonant grating excitation of graphene plasmons and the impact of a silica interlayer on corresponding reflection curves.

Quantum Control of Graphene Plasmon Excitation and Propagation at Heaviside Potential Steps.

Quantum mechanical effects of single particles can affect the collective plasmon behaviors substantially. In this work, the quantum control of plasmon excitation and propagation in graphene is demonstrated by adopting the variable quantum transmission of carriers at Heaviside potential steps as a tuning knob. First, the plasmon reflection is revealed to be tunable within a broad range by varying the ratio γ between the carrier energy and potential height, which originates from the quantum mechanical effect of carrier propagation at potential steps. Moreover, the plasmon excitation by free-space photos can be regulated from fully suppressed to fully launched in graphene potential wells also through adjusting γ, which defines the degrees of the carrier confinement in the potential wells. These discovered quantum plasmon effects offer a unified quantum-mechanical solution toward ultimate control of both plasmon launching and propagating, which are indispensable processes in building plasmon circuitry.

Nonlinear Light Mixing by Graphene Plasmons.

Graphene is known to possess strong optical nonlinearity which turned out to be suitable for creation of efficient saturable absorbers in mode locked fiber lasers. Nonlinear response of graphene can be further enhanced by the presence of graphene plasmons. Here, we report a novel nonlinear effect observed in nanostructured graphene which comes about due to excitation of graphene plasmons. We experimentally detect and theoretically explain enhanced mixing of near-infrared and mid-infrared light in arrays of graphene nanoribbons. Strong compression of light by graphene plasmons implies that the described effect of light mixing is nonlocal in nature and orders of magnitude larger than the conventional local graphene nonlinearity. Both second and third order nonlinear effects were observed in our experiments with the recalculated third-order nonlinearity coefficient reaching values of 4.5 × 10-6 esu. The suggested effect could be used in variety of applications including nonlinear light modulators, light multiplexers, light logic, and sensing devices.

First-principles calculations and model analysis of plasmon excitations in graphene and graphene/hBN heterostructure

Plasmon excitations in free-standing graphene and graphene/hexagonal boron nitride (hBN) heterostructure are studied using linear-response time-dependent density functional theory within the random phase approximation. Within a single theoretical framework, we examine both the plasmon dispersion behavior and lifetime (linewidth) of Dirac and $\ensuremath{\pi}$ plasmons on an equal footing. Particular attention is paid to the influence of the hBN substrate and the anisotropic effect. Furthermore, a model-based analysis indicates that the correct dispersion behavior of $\ensuremath{\pi}$ plasmons should be ${\ensuremath{\omega}}_{\ensuremath{\pi}}(q)=\sqrt{{E}_{g}^{2}+\ensuremath{\beta}q\phantom{{}^{l}}}$ for small $q$'s, where ${E}_{g}$ is the band gap at the $M$ point in the Brillouin zone, and $\ensuremath{\beta}$ is a fitting parameter. This model is radically different from previous proposals, but in good agreement with our calculated results from first principles.

Plasmon Generation through Electron Tunneling in Graphene

The short wavelength of graphene plasmons relative to the light wavelength makes them attractive for applications in optoelectronics and sensing. However, this property limits their coupling to external light and our ability to create and detect them. More efficient ways of generating plasmons are therefore desirable. Here we demonstrate through realistic theoretical simulations that graphene plasmons can be efficiently excited via electron tunneling in a sandwich structure formed by two graphene monolayers separated by a few atomic layers of hexagonal boron nitride. We predict plasmon generation rates of ∼1012−1014/s over an area of the squared plasmon wavelength for realistic values of the spacing and bias voltage, while the yield (plasmons per tunneled electron) has unity order. Our results support electrical excitation of graphene plasmons in tunneling devices as a viable mechanism for the development of optics-free ultrathin plasmonic devices.

A new transfer matrix for investigation of surface plasmon modes in multilayer structures containing anisotropic graphene layers

In this paper, we derive a new transfer matrix for optical calculations of anisotropic graphene layers in the presence of an external magnetic field. We employ this method to calculate the reflectance of a configuration which can be used for the optical excitation of surface plasmon modes in a magnetostatically biased single graphene layer surrounded by two dielectric media under illumination of a transverse magnetic (TM) polarized wave. Due to the external magnetic field, both transverse electric (TE) and magnetic polarizations exist in the reflected wave. There is a dip in the reflectance spectrum of the reflected TM polarized wave which corresponds to the excitation of graphene plasmons in the terahertz region. The frequency at which reflectance becomes minimum increases with increasing the external magnetic field. However, the reflectance of reflected TE polarized wave becomes maximum when graphene plasmons are excited. We use the proposed transfer matrix method to obtain the dispersion relation of hybrid TE-TM polarized surface waves supported by the air-graphene-SiO2 structure at different external magnetic fields. Our results exactly coincide with the previous plasmon dispersion relation. Next, we consider a configuration for the excitation of surface plasmons in a multilayered structure containing three magnetostatically graphene layers. The reflectance spectrum of TM reflected waves for this structure is calculated at different external magnetic fields and for each value of magnetic field three dips appear in the reflectance which shift to higher frequencies with increase of the magnetic field. Finally, the plasmon dispersion curve corresponding to three graphene layers separated with different dielectric layers is calculated at different magnetic fields. For each value of magnetic field, there are three branches in the dispersion curve corresponding to three plasmon modes. At the same frequency, the wave number of each plasmon mode decreases with increasing the magnetic field leading to its easier optical excitation from air.

Intensity dependences of the nonlinear optical excitation of plasmons in graphene.

Recently, we demonstrated an all-optical coupling scheme for plasmons, which takes advantage of the intrinsic nonlinear optical response of graphene. Frequency mixing using free-space, visible light pulses generates surface plasmons in a planar graphene sample, where the phase matching condition can define both the wavevector and energy of surface waves and intraband transitions. Here, we also show that the plasmon generation process is strongly intensity-dependent, with resonance features washed out for absorbed pulse fluences greater than 0.1 J m-2 This implies a subtle interplay between the nonlinear generation process and sample heating. We discuss these effects in terms of a non-equilibrium charge distribution using a two-temperature model.This article is part of the themed issue 'New horizons for nanophotonics'.

Unidirectional excitation of graphene plasmons in Au-graphene composite structures by a linearly polarized light beam.

Controllable manipulation of propagating surface plasmon polaritons is critical in plasmonics and important for nanophotonic applications. Here, we demonstrate theoretically that graphene plasmons (GPs) can be unidirectionally excited in an Au-graphene composite structure by a linearly polarized optical wave at the wavelength of 10.2 µm. The unidirectional ratio can reach as large as 900 with the incidence angle at 37.7° off normal, which is obtained by the angular spectrum of GPs. Moreover, the physical mechanism behind the unidirectional excitation is revealed to be the interference between anti-symmetric and symmetric amplitude distributions of GPs, which are induced by the gold rod antenna under the normal and grazing illuminations, respectively.

Excitation of plasmon wakes in two-dimensional electron systems by moving external charged particles

ABSTRACTWe use the dielectric response formalism to explore the wake effect due to excitation of the sheet plasmons in graphene and in a two-dimensional (2D) electron gas with a parabolic energy band, caused by an externally moving charged particle. Using the random phase approximation to obtain the relevant polarisation functions, we have found similarities as well as some differences in the V-shaped wake patterns that result from different structures of the single-particle excitation spectra in those two electron systems. While both systems exhibit a velocity threshold for the charged particle to be able to excite a wake, graphene shows some irregularities in the wake oscillations behind that particle, whereas the 2D electron gas exhibits a larger opening angle of its V-shaped wake.

Publisher's Note: Modeling the excitation of graphene plasmons in periodic grids of graphene ribbons: An analytical approach [Phys. Rev. B 94 , 195421 (2016)

Publisher's Note: Modeling the excitation of graphene plasmons in periodic grids of graphene ribbons: An analytical approach [Phys. Rev. B <b>94</b> , 195421 (2016)

Modeling the excitation of graphene plasmons in periodic grids of graphene ribbons: An analytical approach

We study electromagnetic scattering and subsequent plasmonic excitations in periodic grids of graphene ribbons. To address this problem, we develop an analytical method to describe the plasmon-assisted absorption of electromagnetic radiation by a periodic structure of graphene ribbons forming a diffraction grating for THz and mid-IR light. The major advantage of this method lies in its ability to accurately describe the excitation of graphene surface plasmons (GSPs) in one-dimensional (1D) graphene gratings without the use of both time-consuming, and computationally-demanding full-wave numerical simulations. We thus provide analytical expressions for the reflectance, transmittance and plasmon-enhanced absorbance spectra, which can be readily evaluated in any personal laptop with little-to-none programming. We also introduce a semi-analytical method to benchmark our previous results and further compare the theoretical data with spectra taken from experiments, to which we observe a very good agreement. These theoretical tools may therefore be applied to design new experiments and cutting-edge nanophotonic devices based on graphene plasmonics.

Acoustic terahertz graphene plasmons revealed by photocurrent nanoscopy.

Terahertz (THz) fields are widely used for sensing, communication and quality control. In future applications, they could be efficiently confined, enhanced and manipulated well below the classical diffraction limit through the excitation of graphene plasmons (GPs). These possibilities emerge from the strongly reduced GP wavelength, λp, compared with the photon wavelength, λ0, which can be controlled by modulating the carrier density of graphene via electrical gating. Recently, GPs in a graphene/insulator/metal configuration have been predicted to exhibit a linear dispersion (thus called acoustic plasmons) and a further reduced wavelength, implying an improved field confinement, analogous to plasmons in two-dimensional electron gases (2DEGs) near conductive substrates. Although infrared GPs have been visualized by scattering-type scanning near-field optical microscopy (s-SNOM), the real-space imaging of strongly confined THz plasmons in graphene and 2DEGs has been elusive so far-only GPs with nearly free-space wavelengths have been observed. Here we demonstrate real-space imaging of acoustic THz plasmons in a graphene photodetector with split-gate architecture. To that end, we introduce nanoscale-resolved THz photocurrent near-field microscopy, where near-field excited GPs are detected thermoelectrically rather than optically. This on-chip detection simplifies GP imaging as sophisticated s-SNOM detection schemes can be avoided. The photocurrent images reveal strongly reduced GP wavelengths (λp ≈ λ0/66), a linear dispersion resulting from the coupling of GPs with the metal gate below the graphene, and that plasmon damping at positive carrier densities is dominated by Coulomb impurity scattering.