Graphene Plasmons Research Articles

Broadband high-efficiency near-infrared graphene phase modulators enabled by metal-nanoribbon integrated hybrid plasmonic waveguides.

The phase modulator is a key component in optical communications for its phase modulation functions. In this paper, we numerically demonstrate a variety of ultra-compact high-efficiency graphene phase modulators (GPMs) based on metal-nanoribbon integrated hybrid plasmonic waveguides in the near-infrared region. Benefiting from the good in-plane mode polarization matching and strong hybridsurface plasmon polariton and graphene interaction, the 20μm-length GPM can achieve excellent phase modulation performance with a good phase and amplitude decoupling effect, a low insertion loss around 0.3dB/μm, a high modulation efficiency with V π L π of 118.67Vμm at 1.55μm, which is 1-3 orders improvement compared to the state-of-the-art graphene modulators. Furthermore, it has a wide modulation bandwidth of 67.96GHz, a low energy consumption of 157.49fJ/bit, and a wide operating wavelength ranging from 1.3 to 1.8μm. By reducing the overlap width ofthe graphene-Al2O3-graphene capacitor, the modulation bandwidth and energy consumption of the modulator canbe further improved to 370.36GHz and 30.22 fJ/bit, respectively. These compact and energy-efficient GPMs mayhold a key to various high-speed telecommunications, interconnects, and other graphene-based integrated photonics applications.

FEM analysis of a λ3/125 high sensitivity graphene plasmonic biosensor for low hemoglobin concentration detection.

A highly sensitive plasmonic refractive index biosensor for hemoglobin protein detection in blood is presented in the near-infrared region. The proposed Au split-ring resonator structure with an extra arm is used to increase electric field enhancement intensity in the vicinity of the nanostructure, which excites localized surface plasmon resonances in the metal-dielectric interface and leads to unity absorption. The footprint of the proposed structure is λ3/125 (λ denoting center wavelength). Through the results from the finite element method (FEM), by variation of the spacer material, and inserting a graphene layer between the spacer and the gold nanostructure, maximum sensitivities of 1804.1 nm/RIU and 2448.45 nm/RIU are achieved, respectively.

Controlling two-photon emission from superluminal and accelerating index perturbations

Sources of photons with controllable quantum properties such as entanglement and squeezing are desired for applications in quantum information, metrology, and sensing. However, fine-grained control over these properties is hard to achieve, especially for two-photon sources. Here, we propose a new mechanism for generating entangled and squeezed photon pairs using superluminal and/or accelerating modulations of the refractive index in a medium. By leveraging time-changing dielectric media, where quantum vacuum fluctuations of the electromagnetic field can be converted into photon pairs, we show that energy- and momentum-conservation in multi-mode systems give rise to frequency and angle correlations of photon pairs which are controlled by the trajectory of the index modulation. These radiation effects are two-photon analogues of Cherenkov and synchrotron radiation by moving charged particles such as free electrons. We find the particularly intriguing result that synchrotron-like radiation into photon pairs exhibits frequency correlations which can enable the realization of a heralded single photon frequency comb. We conclude with a general discussion of experimental viability, showing how solitons, ultrashort pulses, and nonlinear waveguides may enable pathways to realize this two-photon emission mechanism. For completeness, we discuss in the Supplementary Information how these effects, sensitive to the local density of photonic states, can be strongly enhanced using photonic nanostructures. As an example, we show that index modulations propagating near the surface of graphene produce entangled pairs of graphene plasmons with high efficiency, leading to additional experimental opportunities.

Efficient mid-infrared wavelength converter based on plasmon-enhanced nonlinear response in graphene nanoribbons

Graphene plasmons with enhanced localized electric field have been used for boosting the light–matter interaction in linear optical nano-devices. Meanwhile, graphene is an excellent nonlinear material for several third-order nonlinear processes. We present a theoretical investigation of the mechanism of plasmon-enhanced third-order nonlinearity susceptibility of graphene nanoribbons. It is demonstrated that the third-order nonlinearity susceptibility of graphene nanoribbons with excited graphene surface plasmon polaritons can be an order of magnitude larger than the intrinsic susceptibility of a continuous graphene sheet. Combining these properties with the relaxed phase matching condition due to the ultrathin graphene, we propose a novel plasmon-enhanced mid-infrared (MIR) wavelength converter with arrays of graphene nanoribbons. The wavelength of signal light is in the MIR range, which can excite the tunable surface plasmons polaritons in arrays of graphene nanoribbons. The efficiency of the converter from MIR to near-infrared wavelength can be remarkably improved by 60 times compared with a graphene sheet without graphene plasmons. This work provides a novel idea for the efficient application of graphene in nonlinear optical nano-devices. The proposed MIR wavelength converter is compact, tunable and has promising potential in graphene-based MIR detectors with high detection efficiency.

Dynamically Reconfigurable Bipolar Optical Gradient Force Induced by Mid‐Infrared Graphene Plasmonic Tweezers for Sorting Dispersive Nanoscale Objects

AbstractExisting techniques for optical trapping and manipulation of microscopic objects, such as optical tweezers and plasmonic tweezers, are mostly based on visible and near‐infrared light sources. As it is in general more difficult to confine light to a specific length scale at a longer wavelength, these optical trapping and manipulation techniques have not been extended to the mid‐infrared spectral region or beyond. Here, it is shown that by taking advantage of the fact that many materials have large permittivity dispersions in the mid‐infrared region, optical trapping and manipulation using mid‐infrared excitation can achieve additional functionalities and benefits compared to the existing techniques in the visible and near‐infrared regions. In particular, it is demonstrated that by exploiting the exceedingly high field confinement and large frequency tunability of mid‐infrared graphene plasmonics, high‐performance and versatile mid‐infrared plasmonic tweezers can be realized to selectively trap or repel nanoscale objects of different materials in a dynamically reconfigurable way. This new technique can be utilized for sorting, filtering, and fractionating nanoscale objects in a mixture.

Quantum dot-like plasmonic modes in twisted bilayer graphene supercells

We derive a material-realistic real-space many-body Hamiltonian for twisted bilayer graphene from first principles, including both single-particle hopping terms for p z electrons and their long-range Coulomb interaction. By disentangling low- and high-energy subspaces of the electronic dispersion, we are able to utilize state-of-the-art constrained random phase approximation calculations to reliably describe the non-local background screening from the high-energy s, p x , and p y electron states which we find to be independent of the bilayer stacking and thus of the twisting angle. The twist-dependent low-energy screening from p z states is subsequently added to obtain a full screening model. We use this modeling scheme to study plasmons in electron-doped twisted bilayer graphene supercells. We find that the finite system size yields discretized plasmonic levels, which are controlled by the system size, doping level, and twisting angle. This tunability together with atomic-like charge distributions of some of the excitations renders these plasmonic excitations remarkably similar to the electronic states in electronic quantum dots. To emphasize this analogy in the following we refer to these supercells as plasmonic quantum dots. Based on a careful comparison to pristine AB-stacked bilayer graphene plasmons, we show that two kinds of plasmonic excitations arise, which differ in their layer polarization. Depending on this layer polarization the resulting plasmonic quantum dot states are either significantly or barely dependent on the twisting angle. Due to their tunability and their coupling to light, these plasmonic quantum dots form a versatile and promising platform for tailored light-matter interactions.

Tunable coupling of terahertz Dirac plasmons and phonons in transition-metal dichalcogenide-based van der Waals heterostructures

Dirac plasmons in graphene hybridize with phonons of transition metal dichalcogenides (TMDs) when the materials are combined in so-called van der Waals heterostructures (vdWh), thus forming surface plasmon-phonon polaritons (SPPPs). The extend to which these modes are coupled depends on the TMD composition and structure, but also on the plasmons’ properties. By performing realistic simulations that account for the contribution of each layer of the vdWh separately, we calculate how the strength of plasmon-phonon coupling depends on the number and composition of TMD layers, on the graphene Fermi energy and the specific phonon mode. From this, we present a semiclassical theory that is capable of capturing all relevant characteristics of the SPPPs. We find that it is possible to realize both strong and ultra-strong coupling regimes by tuning graphene’s Fermi energy and changing TMD layer number.

Tunable terahertz metasurface platform based on CVD graphene plasmonics.

Graphene plasmonics provides a powerful means to extend the reach of metasurface technology to the terahertz spectral region, with the distinct advantage of active tunability. Here we introduce a comprehensive design platform for the development of THz metasurfaces capable of complex wavefront manipulation functionalities, based on ribbon-shaped graphene plasmonic resonators combined with metallic antennas on a vertical cavity. Importantly, this approach is compatible with the electrical characteristics of graphene grown by chemical vapor deposition (CVD), which can provide the required mm-scale dimensions unlike higher-mobility exfoliated samples. We present a single device structure that can be electrically reconfigured to enable multiple functionalities with practical performance metrics, including tunable beam steering and focusing with variable numerical aperture. These capabilities are promising for a significant impact in a wide range of THz technologies for sensing, imaging, and future wireless communications.

Large-area nanoengineering of graphene corrugations for visible-frequency graphene plasmons.

Quantum confinement of the charge carriers of graphene is an effective way to engineer its properties. This is commonly realized through physical edges that are associated with the deterioration of mobility and strong suppression of plasmon resonances. Here, we demonstrate a simple, large-area, edge-free nanostructuring technique, based on amplifying random nanoscale structural corrugations to a level where they efficiently confine charge carriers, without inducing significant inter-valley scattering. This soft confinement allows the low-loss lateral ultra-confinement of graphene plasmons, scaling up their resonance frequency from the native terahertz to the commercially relevant visible range. Visible graphene plasmons localized into nanocorrugations mediate much stronger light-matter interactions (Raman enhancement) than previously achieved with graphene, enabling the detection of specific molecules from femtomolar solutions or ambient air. Moreover, nanocorrugated graphene sheets also support propagating visible plasmon modes, as revealed by scanning near-field optical microscopy observation of their interference patterns.

Tailoring Dirac Plasmons via Anisotropic Dielectric Environment by Design

Dirac plasmons in a two-dimensional (2D) crystal are strongly affected by the dielectric properties of the environment, due to interaction of their electric field lines with the surrounding medium. Using graphene as a 2D reservoir of free carriers, one can engineer a material configuration that provides an anisotropic environment to the plasmons. In this work, we discuss the physical properties of Dirac plasmons in graphene surrounded by an arbitrary anisotropic dielectric and exemplify how $h$-$\mathrm{BN}$--based heterostructures can be designed to bear the required anisotropic characteristics. We calculate how dielectric anisotropy impacts the spatial propagation of the plasmons and find that an anisotropy-induced plasmon mode emerges, together with a damping pathway, that stem from the out-of-plane off-diagonal elements in the dielectric tensor. Furthermore, we find that one can create hyperbolic plasmons by inheriting the dielectric hyperbolicity of the designed material environment. Strong control over plasmon propagation patterns can be realized in a similar manner. Finally, we show that in this way one can also control the polarization of the light-matter excitations that constitute the plasmon. Taken together, our results promote the design of the dielectric environment as an effective path to tailor the plasmonic response of graphene on the nanoscopic level.

In-plane plasmon coupling in topological insulator Bi2Se3 thin films

The surface states of the 3D topological insulator (TI), Bi2Se3, are known to host two-dimensional Dirac plasmon polaritons (DPPs) in the terahertz spectral range. In TI thin films, the DPPs excited on the top and bottom surfaces couple, leading to an acoustic mode and an optical plasmon mode. Vertical coupling in these materials is, therefore, reasonably well-understood, but in-plane coupling among localized TI DPPs has yet to be investigated. In this paper, we demonstrate in-plane DPP coupling in TI stripe arrays and show that they exhibit dipole–dipole type coupling. The coupling becomes negligible when the lattice constant is greater than approximately 2.8 times the stripe width, which is comparable to results for in-plane coupling of localized plasmons excited on metallic nanoparticles or graphene plasmon polaritons. This understanding could be leveraged for the creation of TI-based metasurfaces.

Imaging-based optical barcoding for relative humidity sensing based on meta-tip.

In a wide range of applications such as healthcare treatment, environmental monitoring, food processing and storage, and semiconductor chip manufacturing, relative humidity (RH) sensing is required. However, traditional fiber-optic humidity sensors face the challenges of miniaturization and indirectly obtaining humidity values. Here, we propose and demonstrate an optical barcode technique by cooperating with RH meta-tip, which can predict the humidity values directly. Such RH meta-tip is composed of fiber-optic sensor based on surface plasmon resonance (SPR) effect and graphene oxide film as humidity sensitizer. While SPR sensor is composed of multimode fiber (MMF) integrated with metallic metasurface. Dynamic time warping (DTW) algorithm is used to obtain the warp path distance (WPD) sequence between the measured reflection spectrum and the spectra of the precalibrated database. The distance sequence is transformed into a pseudo-color barcode, and the humidity value is corresponded to the lowest distance, which can be read by human eyes. The RH measurement depends on the collective changes of the reflection spectrum rather than tracking a single specific resonance peak/dip. This work can open up new doors to the development of a humidity sensor with direct RH recognition by human eyes.

On the role of the energy loss function in the image force on a charge moving over supported graphene

We use a dielectric response theory to describe electrodynamic forces on a charged particle moving parallel to a supported two-dimensional layer. Using a Kramers–Kronig relation, we show that the image force on the particle can be expressed in terms of the energy loss function of the target materials. This enables us to analyze the stopping and the image forces on the particle on equal footing in the frequency–momentum domain encompassing all the energy loss channels in the target. Using the example of a graphene layer on a silicon carbide substrate, we show that both the image and stopping forces can be decomposed into contributions coming from two modes arising from hybridization of the sheet plasmon in doped graphene and a transverse optical phonon in the substrate.

Enhanced Molecular Infrared Spectroscopy Employing Bilayer Graphene Acoustic Plasmon Resonator.

Graphene plasmon resonators with the ability to support plasmonic resonances in the infrared region make them a promising platform for plasmon-enhanced spectroscopy techniques. Here we propose a resonant graphene plasmonic system for infrared spectroscopy sensing that consists of continuous graphene and graphene ribbons separated by a nanometric gap. Such a bilayer graphene resonator can support acoustic graphene plasmons (AGPs) that provide ultraconfined electromagnetic fields and strong field enhancement inside the nano-gap. This allows us to selectively enhance the infrared absorption of protein molecules and precisely resolve the molecular structural information by sweeping graphene Fermi energy. Compared to the conventional graphene plasmonic sensors, the proposed bilayer AGP sensor provides better sensitivity and improvement of molecular vibrational fingerprints of nanoscale analyte samples. Our work provides a novel avenue for enhanced infrared spectroscopy sensing with ultrasmall volumes of molecules.

Broadband focusing of acoustic plasmons in graphene with an applied current

Non-reciprocal plasmons in current-driven, isotropic, and homogenous graphene with proximal metallic gates is theoretically explored. Nearby metallic gates screen the Coulomb interactions, leading to linearly dispersive acoustic plasmons residing close to its particle-hole continuum counterpart. We show that the applied bias leads to spectral broadband focused plasmons whose resonance linewidth is dependent on the angular direction relative to the current flow due to Landau damping. We predict that forward focused non-reciprocal plasmons are possible with accessible experimental parameters and setup.

Influence of Impurity Scattering on Surface Plasmons in Graphene in the Lindhard Approximation

We study the influence of impurity scattering on transverse magnetic (TM) and transverse electric (TE) surface plasmons (SPs) in graphene using the Lindhard approximation. We show how the behaviour and domains of TM SPs are affected by the impurity strength γ and determine the critical value γc below which no SPs exist. The quality factor of TM SPs, for single-band and two-band transitions, is proportional to the square of αλSP/γ, with α being the fine-structure constant and λSP being the plasmon wavelength. In addition, we show that impurity scattering suppresses TE SPs.

Plasmonic drag photocurrent in graphene at extreme nonlocality

It is commonly assumed that photocurrent in two-dimensional systems with centrosymmetric lattice is generated at structural inhomogenities, such as p-n junctions. Here, we study an alternative mechanism of photocurrent generation associated with inhomogenity of the driving electromagnetic field, termed as 'plasmonic drag'. It is associated with direct momentum transfer from field to conduction electrons, and can be characterized by a non-local non-linear conductivity $\sigma^{(2)}({\bf q},\omega)$. By constructing a classical kinetic model of non-linear conductivity with full account of non-locality, we show that it is resonantly enhanced for wave phase velocity coinciding with electron Fermi velocity. The enhancement is interpreted as phase locking between electrons and the wave. We discuss a possible experiment where non-uniform field is created by a propagating graphene plasmon, and find an upper limit of the current responsivity vs plasmon velocity. This limit is set by a competition between resonantly growing $\sigma^{(2)}({\bf q},\omega)$ and diverging kinetic energy of electrons as the wave velocity approaches Fermi velocity.

Accessing the Exceptional Points in a Graphene Plasmon–Vibrational Mode Coupled System

The coupling of plasmons and vibrational modes has been routinely observed in graphene and other 2D systems in both near-field and far-field spectroscopy. However, the relation between coupling strength and modal losses, and exceptional point physics has not been discussed. Here we apply a non-Hermitian framework to a model system of molecular layers on graphene and show that the transition point between strong and weak coupling regimes coincides with the exceptional point of non-Hermitian physics. We further show that the exceptional point can be conveniently located by changing the incident angle of the light and the graphene Fermi energy. Finally, we show that enhanced spectral sensitivity is obtained from small changes in molecular film thickness when the system is tuned to the exceptional point.

Coupling the graphene plasmonic with terahertz emission of truncated conic-shaped InAs/GaAs quantum dots: A passive approach to enhance the intersubband optical properties

Graphene plasmonic (GP) concentrates the electromagnetic waves in small zones beyond the optical diffraction limit. This effect provides strong interactions of radiation with atomic and molecular systems in the vicinity. Here, in this work, we use GP effect to enhance the intersubband optical response of truncated conic-shaped InAs/GaAs quantum dots (QDs) attached to wetting layer (WL). Monolayer graphene flakes with triangular geometries arranged in various configurations are located in the QDs to intensify the electric field and consequently to couple GP with terahertz radiation of QDs. The results show 120-fold enhancement in S-to-P transition dipole moment (TDM) and 1.3 × 10 4 and 6.5 × 10 7 times enhancements, respectively, in the linear and third-order optical susceptibility of QDs. The optimal QDs are those with height of 5 nm and base length of 10 nm. The results of this work are promising to improve the performance of QD-based lasers, detectors, light emitting diodes (LEDs), etc. • Graphene flakes in triangular geometry have been placed in trancated 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 1.3 × 10 4 . • Third-order susceptibility of QD has been multiplied by a factor of 6.5 × 10 7 .

Tunable phonon-plasmon hybridization in α-MoO3-graphene based van der Waals heterostructures.

The plasmon-phonon hybridization behavior between anisotropic phonon polaritons (APhP) of orthorhombic phase Molybdenum Trioxide (α - MoO3) and the plasmon-polaritons of Graphene layer - forming a van der Waals (vdW) heterostructure is investigated theoretically in this paper. It is found that in-plane APhP shows strong interaction with graphene plasmons lying in their close vicinity, leading to large Rabi splitting. Anisotropic behavior of biaxial MoO3 shows the polarization-dependent response with strong anti-crossing behavior at 0.55 eV and 0.3 eV of graphene's Fermi potential for [100] and [001] crystalline directions, respectively. Numerical results reveal unusual electric field confinement for the two arms of enhanced hybrid modes: the first being confined in the graphene layer representing plasmonic-like behavior. The second shows volume confined zigzag pattern in hyperbolic MoO3. It is also found that the various plasmon-phonon hybridized modes could be wavelength tuned, simply by varying the Fermi potential of the graphene layer. The coupling response of the hybrid structure is studied analytically using the coupled oscillator model. Furthermore, we also infer upon the coupling strength and frequency splitting between the two layers with respect to their structural parameters and interlayer spacing. Our work will provide an insight into the active tunable property of hybrid van der Waals (vdW) structure for their potential application in sensors, detectors, directional spontaneous emission, as well as for the tunable control of the propagating polaritons in fields of flat dispersion where strong localization of photons can be achieved, popularly known as the flatband optics.