Graphene Plasmons Research Articles

Graphene coated micro-channel fiber sensor based on localized surface plasmon resonance

In this paper, a novel localized surface plasmon resonance (LSPR)-based micro-channel photonic crystal fiber (PCF) sensor is proposed. The LSPR-PCF sensor is designed with the three micro-channels, and the metal nanostraps and graphene are coated onto the PCF as the sensing elements. To realize high refractive index (RI) sensitivity, the influence of different plasmon materials, structure parameters, and graphene layers on the sensing performance of the LSPR-PCF sensor is investigated. The maximum wavelength sensitivity and amplitude sensitivity of the optimized LSPR-PCF sensor are up to 45800 nm/RIU and 1818RIU−1, respectively, when the RI changes from 1.35 to 1.42. Furthermore, the proposed LSPR-PCF sensor achieves the maximum figure of merit of 594.8, alone with a resolution of 2.18×10−6RIU. Because of its simple structure and excellent sensing performance, the proposed LSPR-PCF sensor has potential applications in biochemistry and environmental science.

Analog Quantum Valley–Hall and Quantum Spin Hall Plasmons in Graphene Metasurfaces with Low Point Group Symmetry

AbstractIn recent years, topological physics of classical waves in artificial crystals has become an emerging field of research. While Dirac cones and valley‐related physics are conventionally studied in these systems with C6v and C3v point‐group symmetries, here analog quantum valley Hall and quantum spin Hall plasmons in graphene metasurfaces with lower point‐group symmetries are explored. First, it is shown that a single‐layer graphene sheet with rectangle holes respecting to the C2v point group symmetry can host a mirror (σv) symmetry‐protected Dirac cone along the X–M edge of the Brillouin zone. Then we demonstrate that introducing further circular holes to the graphene sheet can break the mirror symmetry (i.e., reducing C2v symmetry to C1v) and thus gap out the Dirac cone, which allows us to explore the valley and layer‐pseudospin related topological plasmons in these graphene metasurfaces with low point group symmetry. Valley‐locking unidirectional propagations along the domain–wall interface of a single‐layer graphene metasurface and layer‐pseudospin converter in a double‐layer graphene metasurface are explicitly demonstrated for graphene plasmons in the THz range. This work provides a new design principle for exploring Dirac cones, valley, and pseudospin related physics using much lower point‐group symmetries.

Observation of hydrodynamic plasmons and energy waves in graphene.

Thermally excited electrons and holes form a quantum-critical Dirac fluid in ultraclean graphene and their electrodynamic responses are described by a universal hydrodynamic theory. The hydrodynamic Dirac fluid can host intriguing collective excitations distinctively different from those in a Fermi liquid1-4. Here we report the observation of the hydrodynamic plasmon and energy wave in ultraclean graphene. We use the on-chip terahertz (THz) spectroscopy technique to measure the THz absorption spectra of a graphene microribbon as well as the propagation of the energy wave in graphene close to charge neutrality. We observe a prominent high-frequency hydrodynamic bipolar-plasmon resonance and a weaker low-frequency energy-wave resonance of the Dirac fluid in ultraclean graphene. The hydrodynamic bipolar plasmon is characterized by the antiphase oscillation of massless electrons and holes in graphene. The hydrodynamic energy wave is an electron-hole sound mode with both charge carriers oscillating in phase and moving together. The spatial-temporal imaging technique shows that the energy wave propagates at a characteristic speed of [Formula: see text] near the charge neutrality2-4. Our observations open new opportunities to explore collective hydrodynamic excitations in graphene systems.

Effect of Uniform Strain on Graphene Surface Plasmon Excitations

This paper reports a study of the reflectance of the optical sensors based on graphene under uniform strain. Assuming the graphene layer is surrounded by two different semi-infinite dielectric media, the generalized Fresnel coefficients are derived as a function of usual quantities (e.g., dielectric constants, incident angles, and strain) and anisotropic optical conductivity. The strain not only changes the electronic band structure but also can be employed to tune the electronic collective excitations (plasmons) and thus the optical reflectance of graphene monolayers. One of the most common techniques for plasmon excitation is the Kretschmann configuration. It is based on the observation of a sharp minimum in the reflection coefficient versus the angle (or wavelength) curve. Because strain induces anisotropy in graphene optical conductivity the strain-dependent orientation plays an important role to manipulate the variation of graphene plasmon energy, which may be useful to synchronize graphene properties in plasmonic devices to enhance light-matter interactions.

Plasmon resonances of graphene-assisted core-bishell nanoparticles

We study the Localized Surface Plasmon Resonance (LSPR) in graphene-assisted core-bishell nanoparticles which consist of a graphene layer (outer shell) wrapped around a metal shell and either a dielectric or a metal core. Small nanoparticles with a size much smaller than the wavelength of incident light are assumed, and the quasi-static approximation is applied to develop analytic equations to describe the absorption, scattering, and extinction efficiencies . The proposed nanostructures exhibit two LSPRs; one is in the visible range and corresponds to a plasmon mode of the core-inner shell composite, while the second lies in the near infrared (NIR) and is induced by the graphene plasmons excited at the outer shell. Interestingly, the LSPR of graphene has an ultra-narrow bandwidth and can be tuned in the NIR by altering the physical parameters of graphene, such as the Fermi energy and the number of graphene layers. Therefore, the LSPR peak of graphene is promising for medical applications. In addition, the LSPR of graphene can be tuned to the visible range near the position of the first LSPR, resulting in two narrow linewidth peaks. These resonance peaks could be beneficial for highly sensitive LSPR-based sensors.

Nanoscale infrared imaging and spectroscopy of hot-electron plasmons in graphene

We report a nanoscale infrared (IR) imaging and spectroscopy study of hot-electron plasmons in graphene, which are excited by the sharp metallic probe of the scattering-type scanning near-field optical microscope (s-SNOM) illuminated with a mid-IR femtosecond pulsed laser. We found the average electron temperature $({T}_{e})$ can reach as high as $\ensuremath{\sim}1400\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ within the pulse duration, which can be controlled by tuning the laser power. With s-SNOM, we monitored both the plasmon interference fringes and the hybrid plasmon-phonon resonances of graphene. When graphene is heavily doped, a higher ${T}_{e}$ leads to a smaller plasmon wavelength and a weaker plasmon-phonon resonance intensity. At the charge neutrality point, on the other hand, the plasmon-phonon resonance intensity is enhanced when ${T}_{e}$ increases. With quantitative modeling and theoretical analysis, we concluded that the observed plasmonic responses of hot electrons are governed by the temperature dependencies of chemical potential, electron scattering, and thermal carrier generation. The competition of these factors leads to distinct ${T}_{e}$ dependence of graphene plasmons at various doping levels.

Extreme enhancement of optical force via the acoustic graphene plasmon mode.

We have investigated the effect of enhanced optical force via the acoustic graphene plasmon (AGP) cavities with the ultra-small mode volumes. The AGP mode can generate stronger field confinement and higher momentum, which could provide giant optical force, and has no polarization preference for the optical source. We have demonstrated that the trapping potential and force applied on polystyrene nanoparticle in the AGP cavities are as high as -13.6 × 102 kBT/mW and 2.5 nN/mW, respectively. The effect of radius of rounded corners and gap distance of AGP cavities on the optical force has been studied. Compared with an ideal nanocube, nanocube with rounded corners is more in line with the actual situation of the device. These results show that the larger radius of nanocube rounded corners, the smaller trapping potential and force provided by AGP cavities. Our results pave a new idea for the investigation of optical field and optical force via acoustic plasmon mode.

Nonlocal effects investigation via the coupling between localized and acoustic plasmons.

A scheme to investigate nonlocal effects in metal using the coupling between localized graphene plasmons (GPs) and acoustic plasmons (APs) is proposed. Because of the extremely strong field confinement property, the APs on a configuration consisting of monolayer graphene and a metal film have different dispersions when the nonlocal response is considered or not. A graphene nanoribbon array can efficiently couple incident light to the localized GPs on the ribbons and subsequently the APs. The strong coupling between the two kinds of plasmon, equivalent to electric field dipole interaction, is highly related to the acoustic plasmonic dispersion and induces different absorption spectra, depending on the dispersion. Using a very simple model, nonlocal effects can be extracted from the spectra. The investigation provides a promising platform to manipulate nanophotonics and study nonlocal effects.

Topological super-modes engineering with acoustic graphene plasmons.

Acoustic graphene plasmons (AGPs) in a graphene-dielectric-metal structure possess extreme field localization and low loss, which have promising applications in strong photon-matter interaction and integrated photonic devices. Here, we propose two kinds of one-dimensional crystals supporting propagating AGPs with different topological properties, which is confirmed by the Zak phase calculations and the electric field symmetry analysis. Moreover, by combining these two plasmonic crystals to form a superlattice system, the super-modes exist because of the coupling between isolated topological interface states. A flat-like dispersion of super-modes is observed by designing the superlattice. These results should find applications in optical sensing and integrating photonic devices with plasmonic crystals.

Entanglement mediated by DC current induced nonreciprocal graphene plasmonics.

We investigate entanglement mediated by DC current induced nonreciprocal graphene plasmon polaritons. Nonreciprocal systems are ideal for the enhancement, control, and preservation of entanglement due to the potential for unidirectional beam-like wave propagation, i.e., efficiently transporting photons from one emitter to another. Using a quantum master equation and three-dimensional Green's function analysis, we investigate a system consisting of two two-level emitters dominantly interacting via electric current induced nonreciprocal plasmonic modes of a graphene waveguide. We use concurrence as a measure of entanglement. We show that nonreciprocal graphene plasmon polaritons are a promising candidate to generate and mediate concurrence, where it is shown that there is good enhancement and control of entanglement over vacuum, which is beneficial for the broad applications of entanglement as a quantum resource. We believe our findings contribute to the development of quantum devices, enabling efficient and tunable entanglement between two-level systems, which is a central goal in quantum technologies.

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.

Miniaturized infrared spectrometer based on the tunable graphene plasmonic filter.

Miniaturization of a conventional spectrometer is challenging because of the tradeoffs of size, cost, signal-to-noise ratio, and spectral resolution, etc. Here, a new type of miniaturized infrared spectrometer based on the integration of tunable graphene plasmonic filters and infrared detectors is proposed. The transmittance spectrum of a graphene plasmonic filter can be tuned by varying the Fermi energy of the graphene, allowing light incident on the graphene plasmonic filter to be dynamically modulated in a way that encodes its spectral information in the receiving infrared detector. The incident spectrum can then be reconstructed by using decoding algorithms such as ridge regression and neural networks. The factors that influence spectrometer performance are investigated in detail. It is found that the graphene carrier mobility and the signal-to-noise ratio are two key parameters in determining the resolution and precision of the spectrum reconstruction. The mechanism behind our observations can be well understood in the framework of the Wiener deconvolution theory. Moreover, a hybrid decoding (or recovery) algorithm that combines ridge regression and a neural network is proposed that demonstrates a better spectral recovery performance than either the ridge regression or a deep neural network alone, being able to achieve a sub-hundred nanometer spectral resolution across the 8∼14 µm wavelength range. The size of the proposed spectrometer is comparable to a microchip and has the potential to be integrated within portable devices for infrared spectral imaging applications.

Hybrid quantum dot-graphene layers with improved optical properties in the terahertz spectrum region

Recently, it has been shown that the graphene plasmonic (GP) effect can enhance the optical properties of quantum dots (QDs) through the Purcell's effect. This can happen when the electromagnetic field is concentrated tightly in a region where the ground state (S-state) and the first excited state (P-state) of electron are present, and the S-to-P transition frequency falls onto the frequency of plasmonic resonance of graphene layers. Here, we use a hybrid composition of graphene layers, consisting of a disk and four L-shaped layers, and a QD aimed to enhance the linear and third-order optical susceptibilities of QD in the terahertz (THz)/far infrared (FIR) spectrum region. Unlike the previous works, where much more graphene layers were used, in this work, we use fewer layers beneath the wetting layer and with a noticeably simpler scheme, we achieve comparable results in transition dipole moments (TDMs) and optical properties. By changing the Fermi's energy of graphene, we could reach 90-fold enhancement for TDM and 8 × 10 3 - and 6 × 10 7 -fold enhancements, respectively, for linear and third-order susceptibilities. Our scheme can noticeably reduce the difficulties in fabricating hybrid graphene-QD systems. • Graphene flakes in nanodisk and L-shaped geometry have been placed in conic-shaped InAs/GaAS quantum dots (QDs). • The emission of QDs has been tuned with graphene plasmonic (GP). • With use of GP effect, S-to-P transition dipole moment (TDM) of QD has been enhanced 120 times. • Linear susceptibility of QD has been multiplied by a factor of 8 × 10 3 . • Third-order susceptibility of QD has been multiplied by a factor of 6 × 10 7 . • GP has been used as a passive method to enhance significantly the optical properties of QDs.

Study of magnetoplasmons in graphene rings with two-dimensional finite element method

Graphene plasmons are important collective excitations in graphene, which play a key role in determining the optical properties of graphene. They have quite lots of unique features in comparison with classical plasmons in noble metals. Of them, the active tunability is the most attractive, which is realized by external gating (equivalently electric field). As is well known, graphene also has strong magnetic response (e.g. room temperature quantum Hall effect), so magnetic field can act as another degree of freedom for actively tuning graphene plasmons, with the new quasi particles being so-called graphene magneto-plasmons. Because of the two-dimensional nature of graphene, the numerical studies (or full wave simulations) of graphene magneto-plasmons are usually carried out through a three-dimensional approximation, e.g. treating two-dimensional graphene as a very thin three-dimensional film. Actually, this treatment takes quite some time and requires high memory consumption. Herein, starting from Coulomb law and charge conservation law, we propose an alternative numerical method, namely, two-dimensional finite element method, to solve this problem. All the calculations are now performed in two-dimensional graphene plane, and the usual three-dimensional approximation is not required. To characterize the excitations of graphene magneto-plasmons, the eigenvalue loss spectrum is introduced. Based on this method, graphene magneto-plasmons in graphene rings of four kinds are investigated. The strongest magneto-optic effect is observed in circular ring, which is consistent with its highest rotational symmetry. In all the rings, the lowest dipolar graphene magneto-plasmon always supports symmetric mode splitting, which can be further modified by the interaction between inner edge and outer edge of ring. As the hole size is very small, the edge current confined to the outer edge dominates, and that confined to the inner edge can be ignored; while increasing the hole size, the interaction between these two edges increases, which results in the reduction of the symmetric mode splitting; when the hole size is larger than a critical value, the symmetric mode splitting will disappear.

Graphene plasmonics for ultrasensitive imaging-based molecular fingerprint detection

A compact imaging-based metasurface nanophotonic sensor consisting of graphene nanoribbon arrays is proposed. It provides a chemically specific technique to resolve absorption fingerprints without the need for spectrometry and frequency scanning.

In English

The behavior of the diagonal components of the dielectric tensor and the behavior of the absorption cross-section in the different frequency ranges for the composite cylindrical nanostructures “metallic core – graphene shell” have been studied. In order to obtain the calculation formulas one uses the relations for the longitudinal and transverse components of the dielectric tensors for metallic core and graphene shell, which are determined by Drude model and Cubo model correspondingly. The consideration is carried out in the frameworks of “equivalent” elongated spheroid approach, according to which the defining dimensional parameter is effective aspect ratio, calculated from the condition of the equality of the corresponding axial inertia moments for two-layer cylinder and the “equivalent” elongated spheroid. The numerical results have been obtained for the nanocylinders with the cores of different metals, different radius and with the different number of graphene layers. The variation of amplitude and the variation of the location of extremes of the real and imaginary parts of the transverse component of the dielectric tensor under the increase in radius of the metallic core and the thickness of the graphene shell have been analyzed. It has been shown that the variation of the radius of the core has the significantly greater influence on the properties of the polarizability resonances and absorption cross-section than the variation of the number of graphene layers. The reasons of the presence of two maxima of the absorption cross-section for the metal-graphene cylinders which differ in both amplitude and width and located in infrared, violet and near ultraviolet parts of the spectrum and their relation with the surface plasmonic resonances in the metallic core and with the terahertz plasmons of graphene have been found. The factors which have an effect on amplitude and on the shift of the maxima of the absorption cross-section have been found. The reasons of the different width of maxima, which are located in the different spectral intervals, have been determined.

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.

THz Surface Plasmons in Wide and Freestanding Graphene Nanoribbon Arrays

Graphene is a thin-film carbon material that has immense potential as a key ingredient in new nanoelectronic and nanophotonic devices due to its unique characteristics. In particular, plasmons in graphene appear as a practical tool for the manipulation of light with potential applications from cancer treatment to solar cells. A motivating tunability of graphene properties has been observed in graphene nanoribbons (GNRs) due to their geometrically controllable bandgaps that, in turn, influence the plasmonic properties. The formidable effort made over recent years in developing GNR-based technologies is, however, weakened by a lack of predictive approaches that draw upon available semi-analytical electromagnetic models. An example of such a framework is used here, focusing on experimentally realized GNRs from 155 to 480 nm wide and organized as two-dimensional (2D) GNR arrays. The results show that the plasmon frequency behavior is highly affected by the experimental setup or geometrical factors. In particular, the bandgap of the analyzed systems is of the order of a few meV with a density of states opening around zero energy (Fermi level) in contrast to what is observed in graphene. From the plasmonic part, it is observed in all 2D GNR arrays that the frequency–momentum trend follows a q-like plasmon dispersion whose plasmon frequency can be increased substantially by increasing the ribbon width or charge density concentration. Forbidden plasmon regions are observed for high values of plasmon excitation angle or electron relaxation rate. From a sensing point of view, the important finding is the fact that 2D GNR arrays of 155 nm wide with high values of electron relaxation rate have plasmon responses similar to those observed for α−thrombin in water. Our predictions are projected to be of fast support for detecting plasmons in more complex designs of ribbon nanodevices with potential applications in molecular sensing of aqueous molecules.

Hybrid Image Graphene Polaritons with Extremely Confined Mode and Field Enhancement in the Terahertz Regime

AbstractGuiding terahertz waves below the diffraction limit of light typically excite graphene plasmons (GPs). However, the mode confinements of conventional GPs using graphene–dielectric interfaces are limited. To further squeeze the optical mode sizes, a graphene–dielectric–metal (GDM) configuration supporting an image graphene plasmon (IGP) has been reported recently through the plasmon interactions in a graphene sheet and its mirror image inside a metal with out‐of‐phase charge oscillations. This study proposes a waveguide structure consisting of a cylindrical dielectric wire over a GDM to achieve an ultraconfined mode area by coupling a dielectric waveguide and an IGP mode. Furthermore, this work examines the dependence of waveguide dimensions and material properties on the mode characteristics of the proposed hybrid IGP (HIGP). This HIGP achieves plasmon wavelengths 20 times shorter and normalized propagation lengths twice longer than GPs, accompanying a mode size of 3 × 10−8A0, where A0 is the diffraction‐limited mode area. Moreover, the simple structure makes the HIGP easy to be verified experimentally. This work demonstrates that the HIGP delivering strong light–matter interactions at extremely compact dimensions can be exploited for building highly integrated, enhanced nonlinear, high‐resolution sensing and imaging THz devices.

Ultrabroad reflective polarization converter in terahertz based on circular-end graphene rectangles

Polarization is one of the basic properties of electromagnetic waves. Control and manipulation of polarization by devices based on metasurfaces have versatile applications in modern technologies. We propose a broadband reflective cross-polarization converter in the THz region based on an array of circular-end graphene rectangular patches. Efficient polarization conversion was accomplished through dipole graphene plasmon resonances (DGPRs), either along the long or the short axis of the graphene patches. As the width of the rectangles increased, a new longitudinal dipole mode appeared due to the circular ends of the patches. It grew strong and connected the original DGPRs, constructing an ultrabroad conversion band with efficiency over 94% from approximately 2.5 to 5.0 THz. The relative bandwidth was nearly 66%, surpassing most broadband converters reported. Such broad conversion held for a wide incident angle range as DGPRs were robust to the direction of irradiation. Due to the adjustable optical properties of graphene, the broad conversion band can be tuned. These findings are inspiring for creating broadband, compact and dynamic optoelectrical devices in the THz range.