Graphene plasmon waveguiding and hybridization in individual and paired nanoribbons

1 × 2 Graphene Surface Plasmon Waveguide Beam Splitter Based on Self-Imaging

Based on the principle of self-imaging, a 1 × 2 graphene waveguide beam splitter is proposed in this work, which can split the graphene surface plasmons excited by far-infrared light. The multimode interference process in the graphene waveguide is analyzed by guided-mode propagation analysis (MPA), and then the imaging position is calculated. The simulation results show that the incident beam can be obviously divided into two parts by the self-imaging of the graphene surface plasmon. In addition, the influences of the excited light wavelength, Fermi level, dielectric environment on the transmission efficiency are studied, which provide a reference for the research of graphene waveguide related devices.

A Metamaterials-Based Absorber Used for Switch Applications with Dynamically Variable Bandwidth in Terahertz Regime

A broadband absorber based on metamaterials of graphene and vanadium dioxide (VO2) is proposed and investigated in the terahertz (THz) regime, which can be used for switch applications with a dynamically variable bandwidth by electrically and thermally controlling the Fermi energy level of graphene and the conductivity of VO2, respectively. The proposed absorber turns 'on' from 1.5 to 5.4 THz, with the modulation depth reaching 97.1% and the absorptance exceeding 90% when the Fermi energy levels of graphene are set as 0.7 eV, and VO2 is in the metallic phase. On the contrary, the absorptance is close to zero and the absorber turns 'off' with the Fermi energy level setting at 0 eV and VO2 in the insulating phase. Furthermore, other four broadband absorption modes can be achieved utilizing the active materials graphene and VO2. The proposed terahertz absorber may benefit the areas of broadband switch, cloaking objects, THz communications and other applications.

Hybridization of graphene-gold plasmons for active control of mid-infrared radiation

Many applications in environmental and biological sensing, standoff detection, and astronomy rely on devices that operate in the mid-infrared range, where active devices can play a critical role in advancing discovery and innovation. Nanostructured graphene has been proposed for active miniaturized mid-infrared devices via excitation of tunable surface plasmons, but typically present low efficiencies due to weak coupling with free-space radiation and plasmon damping. Here we present a strategy to enhance the light-graphene coupling efficiency, in which graphene plasmons couple with gold localized plasmons, creating novel hybridized plasmonic modes. We demonstrate a metasurface in which hybrid plasmons are excited with transmission modulation rates of 17% under moderate doping (0.35 eV) and in ambient conditions. We also evaluate the metasurface as a mid-infrared modulator, measuring switching speeds of up to 16 kHz. Finally, we propose a scheme in which we can excite strongly coupled gold-graphene gap plasmons in the thermal radiation range, with applications to nonlinear optics, slow light, and sensing.

Tunable ultra-wideband graphene-based filter with a staggered structure

We present a tunable ultra-wideband band-stop filter utilizing graphene with a straightforward staggered structure. The transmission spectrum has been meticulously analyzed using the effective-index-based transfer matrix method (EIB-TMM). The results demonstrate that the filtering properties can be precisely tailored by manipulating the Fermi energy level of graphene. Importantly, we have successfully achieved a remarkable ultra-wideband stopband by optimizing the staggered parameters. Our exploration of redefining the staggered structure through adjustments to three critical parameters has revealed a crucial role in expanding bandwidth. This investigation deepens our understanding of how nonperiodic structures can effectively broaden bandwidth and holds great promise for the prospective design of ultra-wideband band-stop devices.

Reconstruction of the Near-Field Electric Field by SNOM Measurement

Scanning near-field optical microscope (SNOM) with nanoscale spatial resolution has been a powerful tool in studying the plasmonic properties of nano materials/structures. However, the quantification of the SNOM measurement remains a major challenge in the field due to the lack of reliable methodologies. We employed the point-dipole model to describe the tip-surface interaction upon laser illumination and theoretically derived the quantitative relationship between the measured results and the actual near-field electric field strength. Thus, we can experimentally reconstruct the near-field electric field through this theoretically calculated relationship. We also developed an experimental technique together with FEM simulation to get the above relationship experimentally and reconstruct the near-field electric field from the measurement by SNOM.

Multifunctional THz Graphene Antenna with 360∘ Continuous ϕ-Steering and θ-Control of Beam

A novel graphene antenna composed of a graphene dipole and four auxiliary graphene sheets oriented at 90∘ to each other is proposed and analyzed. The sheets play the role of reflectors. A detailed group-theoretical analysis of symmetry properties of the discussed antennas has been completed. Through electric field control of the chemical potentials of the graphene elements, the antenna can provide a quasi-omnidirectional diagram, a one- or two-directional beam regime, dynamic control of the beam width and, due to the vertical orientation of the dipole with respect to the base substrate, a 360∘ beam steering in the azimuth plane. An additional graphene layer on the base permits control of the radiation pattern in the θ-direction. Radiation patterns in different working states of the antenna are considered using symmetry arguments. We discuss the antenna parameters such as input reflection coefficient, total efficiency, front-to-back ratio, and gain. An equivalent circuit of the antenna is suggested. The proposed antenna operates at frequencies between 1.75 THz and 2.03 THz. Depending on the active regime defined by the chemical potentials set on the antenna graphene elements, the maximum gain varies from 0.86 to 1.63.

Polaritons in Van der Waals Heterostructures

2D monolayers supporting a wide variety of highly confined plasmons, phonon polaritons, and exciton polaritons can be vertically stacked in van der Waals heterostructures (vdWHs) with controlled constituent layers, stacking sequence, and even twist angles. vdWHs combine advantages of 2D material polaritons, rich optical structure design, and atomic scale integration, which have greatly extended the performance and functions of polaritons, such as wide frequency range, long lifetime, ultrafast all-optical modulation, and photonic crystals for nanoscale light. Here, the state of the art of 2D material polaritons in vdWHs from the perspective of design principles and potential applications is reviewed. Some fundamental properties of polaritons in vdWHs are initially discussed, followed by recent discoveries of plasmons, phonon polaritons, exciton polaritons, and their hybrid modes in vdWHs. The review concludes with a perspective discussion on potential applications of these polaritons such as nanophotonic integrated circuits, which will benefit from the intersection between nanophotonics and materials science.

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 k_BT/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.

Utilization of smartphones for the evaluation of Gr/Ni nanostructures magnetically controlled based on optical fibers surface plasmons

In the suggested optical fiber-based magnetoplasmonic system, we investigated the magnetic properties of graphene/nickel nanostructures. The plasmonic mode changes under the magnetic field observed in the intensity diagrams over time. To be accessible, cheap, and portable, we used a smartphone as a detector and processor. Considering the ambient noise and the light source, it was reported that the intensity of the changes improved up to 5 times. Further, the clad corrosion experiment carried out by pure dimethyl ketone in an intensity modulation by a smartphone camera and 10 seconds suggested removing fluorine polymer clad.

High-speed tunable optical absorber based on a coupled photonic crystal slab and monolayer graphene structure

Reconfigurable metasurfaces have been pursued intensively in recent years for the ability to modulate the light after fabrication. However, the optical performances of these devices are limited by the efficiency, actuation response speed and mechanical control for reconfigurability. In this paper, we propose a fast tunable optical absorber based on the critical coupling of resonance mode to absorptive medium and the plasma dispersion effect of free carriers in semiconductor. The tunable absorber structure includes a single-layer or bi-layer silicon photonic crystal slab (PCS) to induce a high-Q optical resonance, a monolayer graphene as the absorption material, and bottom reflector to remove transmission. By modulating the refractive index of PCS via the plasma dispersion of the free carrier, the critical coupling condition is shifted in spectrum, and the device acquires tuning capability between perfect absorption and total reflection of the incident monochromatic light beam. Simulation results show that, with silicon index change of 0.015, the tunable absorption of light can achieve the reflection/absorption switching, and full range of reflection phase control is feasible in the over coupling region. The proposed reconfigurable structure has potential applications in remote sensing, free-space communications, LiDAR, and imaging.

Perfect optical absorption in a single array of folded graphene ribbons

Due to its one atom thickness, optical absorption (OA) in graphene is a fundamental and challenging issue. Practically, the patterned graphene-dielectric-metal structure is commonly used to achieve perfect OA (POA). In this work, we propose a novel scenario to solve this issue, in which POA is obtained by using free-standing folded graphene ribbons (FGRs). We show several local resonances, e.g. a dipole state (Mode-I) and a bound state in continuum (BIC, Mode-II), will cause very efficient OA. At normal incidence, by choosing appropriate folding angle θ, 50% absorptance by the two states is easily achieved; at oblique incidence, the two states will result in roughly 98% absorptance as incidence angle φ≈40^∘. It is also interesting to see that the system has asymmetric OA spectra, e.g. POA of the former (latter) state existing in reverse (forward) incidence, respectively. Besides the angles θ and φ, POA here can also be actively tuned by electrostatic gating. As increasing Fermi level, POA of Mode-I will undergo a gradual blueshift, while that of Mode-II will experience a rapid blueshift and then be divided into three branches, due to Fano coupling to two guided modes. In reality, the achieved POA is well maintained even the dielectric substrates are used to support FGRs. Our work offers a remarkable scenario to achieve POA, and thus enhance light-matter interaction in graphene, which can build an alternative platform to study novel optical effects in general two-dimensional (2D) materials. The folding, mechanical operation in out-of-plane direction, may emerge as a new degree of freedom for optoelectronic device applications based on 2D materials.

Tunable high-quality-factor absorption in a graphene monolayer based on quasi-bound states in the continuum

A tunable graphene absorber, composed of a graphene monolayer and a substrate spaced by a subwavelength dielectric grating, is proposed and investigated. Strong light absorption in the graphene monolayer is achieved due to the formation of embedded optical quasi-bound states in the continuum in the subwavelength dielectric grating. The physical origin of the absorption with high quality factor is examined by investigating the electromagnetic field distributions. Interestingly, we found that the proposed absorber possesses high spatial directivity and performs similar to an antenna, which can also be utilized as a thermal emitter. Besides, the spectral position of the absorption peak can not only be adjusted by changing the geometrical parameters of dielectric grating, but it is also tunable by a small change in the Fermi level of the graphene sheet. This novel scheme to tune the absorption of graphene may find potential applications for the realization of ultrasensitive biosensors, photodetectors, and narrow-band filters.

Dispersion Theory of Surface Plasmon Polaritons on Bilayer Graphene Metasurfaces

Surface plasmon polaritons (SPPs) on the graphene metasurfaces (GSPs) are crucial to develop a series of novel functional devices that can merge the well-established plasmonics and novel nanomaterials. Dispersion theory on GSPs is an important aspect, which can provide a basic understanding of propagating waves and further guidance for potential applications based on graphene metamaterials. In this paper, the dispersion theory and its modal characteristics of GSPs on double-layer graphene metasurfaces consisting of the same upper and lower graphene micro-ribbon arrays deposited on the dielectric medium are presented. In order to obtain its dispersion expressions of GSP mode on the structure, an analytical approach is provided by directly solving the Maxwell’s equations in each region and then applying periodical conductivity boundary onto the double interfaces. The obtained dispersion expressions show that GSPs split into two newly symmetric and antisymmetric modes compared to that on the single graphene metasurface. Further, the resultant dispersion relation and its propagating properties as a function of some important physical parameters, such as spacer, ribbon width, and substrate, are treated and investigated in the Terahertz band, signifying great potentials in constructing various novel graphene-based plasmonic devices, such as deeply sub-wavelength waveguides, lenses, sensors, emitters, etc.

Simulating a graphene-based acousto-plasmonic biosensor to eliminate the interference of surrounding medium

The presence of species other than the target biomolecules in the fluidic analyte used in the refractive index biosensor based on the surface plasmon resonances (SPRs) can lead to measurement ambiguity. Using graphene-based acousto-plasmonic biosensors, we propose two methods to eliminate any possible ambiguity in interpreting the measured results. First, we take advantage of the dynamic tunability of graphene SPRs in the acousto-plasmonic biosensor with a surface acoustic wave (SAW) induced uniform grating, performing measurements at different applied voltages. Second, a single measurement employing a similar biosensor but with SAW-induced dual-segment gratings. The numerical results show the capability of both methods in decoupling the effect of the target analyte from the other species in the fluid, enabling interpreting the measurement results with no ambiguity. We also report the results of our numerical investigation on the effect of measuring parameters like the target layer effective refractive index and thickness, and the fluid effective refractive index, in addition to the controlling parameters of the proposed acousto-plasmonic biosensor, including graphene Fermi energy and electrical signaling on the sensing characteristics. Both types of proposed biosensors show promising features for developing the next generation lab-on-a-chip biosensors with minimal cross-sensitivities to non-target biomolecules.

Simulation of scanning near-field optical microscopy spectra of 1D plasmonic graphene junctions

We present numerical simulations of scattering-type scanning near-field optical microscopy (s-SNOM) of 1D plasmonic graphene junctions. A comprehensive analysis of simulated s-SNOM spectra is performed for three types of junctions. We find conditions when the conventional interpretation of the plasmon reflection coefficients from s-SNOM measurements does not apply. Our approach can be used for other conducting 2D materials to provide a comprehensive understanding of the s-SNOM techniques for probing the local transport properties of 2D materials.

Slow Light Effect and Tunable Channel in Graphene Grating Plasmonic Waveguide

A graphene plasmon waveguide composed of silicon grating substrate and a silica separator is proposed to generate the slow-light effect. A bias voltage is applied to tune the optical conductivity of graphene. The tunability of the slow-light working channel can be achieved due to the adjustable bias voltage. With an increase in the bias voltage, the working channel exhibited obvious linear blue-shift. The linear correlation coefficient between the working channel and the bias voltage was up to 0.9974. The average value of the normalized delay bandwidth product (NDBP) with different bias voltages was 3.61. In addition, we also studied the tunable group velocity at a specific working channel. Due to the tunability of this miniaturized waveguide structure, it can be used in a variety of applications including optical storage devices, optical buffers and optical switches.

Oxygen Generation Using Catalytic Nano/Micromotors

Gaseous oxygen plays a vital role in driving the metabolism of living organisms and has multiple agricultural, medical, and technological applications. Different methods have been discovered to produce oxygen, including plants, oxygen concentrators and catalytic reactions. However, many such approaches are relatively expensive, involve challenges, complexities in post-production processes or generate undesired reaction products. Catalytic oxygen generation using hydrogen peroxide is one of the simplest and cleanest methods to produce oxygen in the required quantities. Chemically powered micro/nanomotors, capable of self-propulsion in liquid media, offer convenient and economic platforms for on-the-fly generation of gaseous oxygen on demand. Micromotors have opened up opportunities for controlled oxygen generation and transport under complex conditions, critical medical diagnostics and therapy. Mobile oxygen micro-carriers help better understand the energy transduction efficiencies of micro/nanoscopic active matter by careful selection of catalytic materials, fuel compositions and concentrations, catalyst surface curvatures and catalytic particle size, which opens avenues for controllable oxygen release on the level of a single catalytic microreactor. This review discusses various micro/nanomotor systems capable of functioning as mobile oxygen generators while highlighting their features, efficiencies and application potentials in different fields.

Numerical Investigation of Graphene and STO Based Tunable Terahertz Absorber with Switchable Bifunctionality of Broadband and Narrowband Absorption

A graphene metamaterial and strontium titanate (STO)-based terahertz absorber with tunable and switchable bifunctionality has been numerically investigated in this work. Through electrically tuning the Fermi energy level of the cross-shaped graphene, the bandwidth of the proposed absorber varies continuously from 0.12 THz to 0.38 THz with the absorptance exceeding 90%, which indicates the functionality of broadband absorption. When the Fermi energy level of the cross-shaped graphene is 0 eV, the proposed absorber exhibits the other functionality of narrowband absorption owing to the thermal control of the relative permittivity of STO, and the rate of change of the center frequency is 50% ranging from 0.56 THz to 0.84 THz. The peak intensity of the narrowband absorption approximates to nearly 100% through adjusting the Fermi energy level of the graphene strips. The calculated results indicate that it is not sensitive to the polarization for wide incidence angles. The proposed absorber can realize tunable bifunctionality of broadband absorption with a tunable bandwidth and narrowband absorption with a tunable center frequency, which provides an alternative design opinion of the tunable terahertz devices with high performance for high-density integrated systems.

Plasmonic metal nanostructures with extremely small features: new effects, fabrication and applications

Surface plasmons in metals promise many fascinating properties and applications in optics, sensing, photonics and nonlinear fields. Plasmonic nanostructures with extremely small features especially demonstrate amazing new effects as the feature sizes scale down to the sub-nanometer scale, such as quantum size effects, quantum tunneling, spill-out of electrons and nonlocal states etc. The unusual physical, optical and photo-electronic properties observed in metallic structures with extreme feature sizes enable their unique applications in electromagnetic field focusing, spectra enhancing, imaging, quantum photonics, etc. In this review, we focus on the new effects, fabrication and applications of plasmonic metal nanostructures with extremely small features. For simplicity and consistency, we will focus our topic on the plasmonic metal nanostructures with feature sizes of sub-nanometers. Subsequently, we discussed four main and typical plasmonic metal nanostructures with extremely small features, including: (1) ultra-sharp plasmonic metal nanotips; (2) ultra-thin plasmonic metal films; (3) ultra-small plasmonic metal particles and (4) ultra-small plasmonic metal nanogaps. Additionally, the corresponding fascinating new effects (quantum nonlinear, non-locality, quantum size effect and quantum tunneling), applications (spectral enhancement, high-order harmonic wave generation, sensing and terahertz wave detection) and reliable fabrication methods will also be discussed. We end the discussion with a brief summary and outlook of the main challenges and possible breakthroughs in the field. We hope our discussion can inspire the broader design, fabrication and application of plasmonic metal nanostructures with extremely small feature sizes in the future.

Visualization of Plasmonic Couplings Using Ultrafast Electron Microscopy

Hybrids of graphene and metal plasmonic nanostructures are promising building blocks for applications in optoelectronics, surface-enhanced scattering, biosensing, and quantum information. An understanding of the coupling mechanism in these hybrid systems is of vital importance to its applications. Previous efforts in this field mainly focused on spectroscopic studies of strong coupling within the hybrids with no spatial resolution. Here we report direct imaging of the local plasmonic coupling between single Au nanocapsules and graphene step edges at the nanometer scale by photon-induced near-field electron microscopy in an ultrafast electron microscope for the first time. The proximity of a step in the graphene to the nanocapsule causes asymmetric surface charge density at the ends of the nanocapsules. Computational electromagnetic simulations confirm the experimental observations. The results reported here indicate that this hybrid system could be used to manipulate the localized electromagnetic field on the nanoscale, enabling promising future plasmonic devices.

Multi-Beam Steering for 6G Communications Based on Graphene Metasurfaces

As communication technology is entering the 6G era, a great demand for high-performance devices operating in the terahertz (THz) band has emerged. As an important part of 6G technology, indoor communication requires multi-beam steering and tracking to serve multi-users. In this paper, we have designed a graphene metasurface that can realize multi-beam steering for directional radiations. The designed metasurface consists of graphene ribbons, dielectric spacer, and metal substrate. By designing the graphene ribbons and controlling the applied voltage on them, we have obtained single-, double-, and triple-beam steering. In addition, we have also numerically calculated the far-field distributions of the steered multi-beam with a diffraction distance of 2 m. Our design has potential applications in future indoor directional 6G communications.

A Tunable “Ancient Coin”-Type Perfect Absorber with High Refractive Index Sensitivity and Good Angular Polarization Tolerance

In this paper, we design and present a graphene-based “ancient coin”-type dual-band perfect metamaterial absorber, which is composed of a silver layer, silicon dioxide layer, and a top “ancient coin” graphene layer. The absorption performance of the absorber is affected by the hollowed-out square in the center of the graphene layer and geometric parameters of the remaining nano disk. The optical properties of graphene can be changed by adjusting the voltage, to control the absorption performance of the absorber dynamically. In addition, the centrally symmetric pattern structure greatly eliminates the polarization angle dependence of our proposed absorber, and it exhibits good angular polarization tolerance. Furthermore, the proposed “ancient coin”-type absorber shows great application potential as a sensor or detector in biopharmaceutical, optical imaging, and other fields due to its strong tunability and high refractive index sensitivity.

High-performance tunable resonant electro-optical modulator based on suspended graphene waveguides

The exceptional tunable waveguiding characteristics of graphene surface plasmons have remained unrivaled since it has inspired many electro-optical (EO) devices in terahertz (THz) and mid-infrared (MIR) photonic circuits. We propose and numerically investigate a low-loss, highly extinctive resonant EO modulator based on a suspended graphene plasmonic waveguide. Unlike other resonance-based modulators, the input power has negligible interaction with lossy resonance cavity in on-state, remarkably reducing the losses. Achieving the insertion loss (IL) of 1.3 dB and the extinction ratio (ER) of 22 dB within a footprint less than 3 µm² substantiates the superiority of the proposed structure. The charge transport simulations are first conducted to calculate the steady-state charge distribution. The three-dimensional finite-difference time-domain (3D-FDTD) method is utilized to monitor the guided wave propagation and modulation properties. We show that the transmission spectrum is highly dependent upon geometric parameters of the structure, and the modulator can be effectively tuned to operate at the desired wavelength by applying a suitable gate voltage. Simulation results show the modulation bandwidth of 71 GHz corresponding to the total capacitance of 4.8 fF within the active area. The novel EO modulator structure has shown great potentiality and flexibility to find other applications in MIR and THz integrated circuits like controllable notch filters and switches.

Graphene Nanoribbon Gap Waveguides for Dispersionless and Low-Loss Propagation with Deep-Subwavelength Confinement

Surface plasmon polaritons (SPPs) have been attracting considerable attention owing to their unique capabilities of manipulating light. However, the intractable dispersion and high loss are two major obstacles for attaining high-performance plasmonic devices. Here, a graphene nanoribbon gap waveguide (GNRGW) is proposed for guiding dispersionless gap SPPs (GSPPs) with deep-subwavelength confinement and low loss. An analytical model is developed to analyze the GSPPs, in which a reflection phase shift is employed to successfully deal with the influence caused by the boundaries of the graphene nanoribbon (GNR). It is demonstrated that a pulse with a 4 μm bandwidth and a 10 nm mode width can propagate in the linear passive system without waveform distortion, which is very robust against the shape change of the GNR. The decrease in the pulse amplitude is only 10% for a propagation distance of 1 μm. Furthermore, an array consisting of several GNRGWs is employed as a multichannel optical switch. When the separation is larger than 40 nm, each channel can be controlled independently by tuning the chemical potential of the corresponding GNR. The proposed GNRGW may raise great interest in studying dispersionless and low-loss nanophotonic devices, with potential applications in the distortionless transmission of nanoscale signals, electro-optic nanocircuits, and high-density on-chip communications.

Tunable reflective dual-band line-to-circular polarization convertor with opposite handedness based on graphene metasurfaces

In this letter, we propose a dual-band tunable reflective linear-to-circular (LTC) polarization converter, which is composed of a graphene sheet etched with an I-shaped carved-hollow array. In the mid-infrared region, two LTC bands with opposite handedness are simultaneously realized due to the excitation of the three graphene surface plasmon (GSP) modes. The band of line-to-right-circular-polarization (LTRCP) ranges from 9.87 to 11.03THz with ellipticity χ <-0.95, and from 9.69 to 11.36 THz with an axial ratio of less than 3 dB; the band of line-to-left-circular-polarization (LTLCP) ranges from 13.16 to 14.43THz with χ >0.95, and from 12.79 to 14.61 THz with an axial ratio of less than 3 dB. The tunable responses of the reflective polarizer with Fermi energy (Ef) and electron scattering time (τ) are discussed, and especially the perfect LTLCP can be changed to LTRCP with increasing Ef. Also, the influences of geometric parameters, incident angle, and polarization angle on the performances of the dual-band LTC are also investigated, and it is found that our polarizer converter shows angle insensitivity. All simulation results are conducted by the finite element method. Our design enriches the research of tunable LTC polarizers and has potential applications in integrated terahertz systems.

Nanoimaging of Low-Loss Plasmonic Waveguide Modes in a Graphene Nanoribbon

Graphene nanoribbons are predicted to support low-loss and tunable plasmonic waveguide modes with an ultrasmall mode area. Experimental observation of the plasmonic waveguide modes in graphene nanoribbons, however, is challenging because conventional wet lithography has difficulty creating a clean graphene nanoribbon with a low edge roughness. Here, we use a dry lithography method to fabricate ultraclean and low-roughness graphene nanoribbons, which are then encapsulated in hexagonal boron nitride (hBN). We demonstrate low-loss plasmon propagation with a quality factor up to 35 in the ultraclean nanoribbon waveguide using cryogenic infrared nanoscopy. In addition, we observe both the fundamental and the higher-order plasmonic waveguide modes for the first time. All the plasmon waveguide modes can be tuned through electrostatic gating. The observed tunable plasmon waveguide modes in ultraclean graphene nanoribbons agree well with the finite-difference time-domain (FDTD) simulation results. They are promising for reconfigurable photonic circuits and devices at a subwavelength scale.

Efficient excitation of hybrid modes on a double-layer graphene with metallic slit grating

The excitation of double-layer hybrid plasmonic modes is investigated by the finite element method. The hybrid modes, verified as the standing even order of both symmetric and anti-symmetric modes, are effectively generated. There are several advances in comparison with using the Si grating: the metallic grating not only compensates phase mismatch, but also acts as a magnetic polariton. The dependences of each hybrid mode on the geometric parameters are analyzed respectively. Interestingly, a second spectra splitting occurs at each hybrid resonant mode with an obliquely incident light. At last, the excitation efficiency can be further enhanced to 90% using the Salisbury screen. The proposed hybrid system can be utilized to design various double-layer graphene-based plasmonic devices, including tunable optical switches, thermal emitters, multiband absorbers, sensors, etc.

Coupling of waveguide mode and graphene plasmons

Photonic waveguides with graphene layers have been recently studied for their potential as fast and low-power electro-optic modulators with small footprints. We show that in the optical wavelength range of 1.55 μm, surface plasmons supported by the graphene layer with the chemical potential exceeding ~0.5 eV can couple with the waveguide mode and affect its propagation. This effect might be possibly utilized in technical applications as a very low-power amplitude modulation, temperature sensing, etc.

Anomalous redshift of graphene absorption induced by plasmon-cavity competition

Anomalous redshift of the absorption peak of graphene in the cavity system is numerically and experimentally demonstrated. It is observed that the absorption peak exhibits a redshift as the Fermi level of graphene increases, which is contrary to the ordinary trend of graphene plasmons. The influencing factors, including the electron mobility of graphene, the cavity length, and the ribbon width, are comprehensively analyzed. Such anomalous redshift can be explained by the competition between the graphene plasmon mode and the optical cavity mode. The study herein could be beneficial for the design of graphene-based plasmonic devices.

Charge accumulation resulting in metallization of II-VI semiconductor (ZnX X = O, S, Se) films neighboring polar liquid crystal molecules and their surface plasmonic response in the visible region

The surfaces of some IIB-VI semiconductors (ZnX, X = O, S, Se) are metallized by neighboring highly polar and atomically vertically aligned (VA) liquid crystal (LC) molecules. Owing to polar catastrophe, the charge carriers swarm in an extremely thin layer and the density can achieve 4.86 × 1028 m-3 close to the LC layer, which can be regarded as a 2-dimensional electron gas (2DEG). Using density functional theory (DFT), it was found that the dielectric functions of the modified layer become negative in the visible region. This indicates the semiconductor/LC platform is an ideal active plasmonic candidate, apart from the lossy metal constituents. Experimentally, after mediation with phase gratings written in the LC system, surface plasmon polaritons (SPPs) can be excited at the semiconductor surface and localized charges are gathered in an adjacent LC layer. With the help of the enhanced static electric field from the metallic surface, significantly more 2D diffraction orders in many rows and columns and a huge energy transfer between the laser beams and SPPs was observed, which is consistent with the metallization results and the bidirectional coupling between the SPPs and incident lights. The generalization of the II-VI semiconductors means the system has great promise for use in practical applications owing to the ultra-low loss. The novel insights regarding this combination with liquid crystals will be beneficial for real-time holographic displays and the study of tunable epsilon near zero points.

Investigation of a new graphene strain sensor based on surface plasmon resonance

The high confinement of surface plasmon polaritons in graphene nanostructures at infrared frequencies can enhance the light-matter interactions, which open up intriguing possibilities for the sensing. Strain sensors have attracted much attention due to their unique electromechanical properties. In this paper, a surface plasmon resonance based graphene strain sensor is presented. The considered sensing platform consists of arrays of graphene ribbons placed on a flexible substrate which enables efficient coupling of an electromagnetic field into localized surface plasmons. When the strain stretching is applied to the configuration, the localized surface plasmon resonance frequency sensitively shift. The strain is then detected by measuring the frequency shifts of the localized plasmon resonances. This provides a new optical method for graphene strain sensing. Our results show that the tensile direction is the key parameter for strain sensing. Besides, the sensitivity and the figure of merit were calculated to evaluate the performance of the proposed sensor. The calculated figure of merit can be up to two orders of magnitude, which could be potentially useful from a practical point of view.

Continuously tunable metasurfaces controlled by single electrode uniform bias-voltage based on nonuniform periodic rectangular graphene arrays

Metasurfaces, the two-dimensional artificial metamaterials, have attracted intensive attention due to their abnormal ability to manipulate the electromagnetic wave. Although there have been considerable efforts to design and fabricate beam steering devices, continuously tunable devices with a uniform bias-voltage have not been achieved. Finding new ways to realize more convenient and simpler wavefront modulation of light still requires research efforts. In this article, a series of novel reflective metasurfaces are proposed to continuously modulate the wavefront of terahertz light by uniformly adjusting the bias-voltage. By introducing the innovation of nonuniform periodic structures, we realize the gradient distribution of the reflected light phase-changing-rate which is the velocity of phase changing with Fermi energy. Based on strict phase distribution design scheme, a beam scanner and a variable-focus reflective metalens are both demonstrated successfully. Furthermore, dynamic and continuous control of either the beam azimuth of beam scanner or the focal length of metalens can be achieved by uniformly tuning the Fermi energy of graphene. Our work provides a potentially efficient method for the development and simplification of the adjustable wavefront controlling devices.

Graphene Plasmonics: Fully Atomistic Approach for Realistic Structures

We demonstrate that the plasmonic properties of realistic graphene and graphene-based materials can effectively and accurately be modeled by a novel, fully atomistic, yet classical, approach, named ωFQ. Such a model is able to reproduce all plasmonic features of these materials and their dependence on shape, dimension, and fundamental physical parameters (Fermi energy, relaxation time, and two-dimensional electron density). Remarkably, ωFQ is able to accurately reproduce experimental data for realistic structures of hundreds of nanometers (∼370k atoms), which cannot be afforded by any ab initio method. Also, the atomistic nature of ωFQ permits the investigation of complex shapes, which can hardly be dealt with by exploiting widespread continuum approaches.

Fast electrical modulation of strong near-field interactions between erbium emitters and graphene

Combining the quantum optical properties of single-photon emitters with the strong near-field interactions available in nanophotonic and plasmonic systems is a powerful way of creating quantum manipulation and metrological functionalities. The ability to actively and dynamically modulate emitter-environment interactions is of particular interest in this regard. While thermal, mechanical and optical modulation have been demonstrated, electrical modulation has remained an outstanding challenge. Here we realize fast, all-electrical modulation of the near-field interactions between a nanolayer of erbium emitters and graphene, by in-situ tuning the Fermi energy of graphene. We demonstrate strong interactions with a  >1000-fold increased decay rate for  ~25% of the emitters, and electrically modulate these interactions with frequencies up to 300 kHz - orders of magnitude faster than the emitter's radiative decay (~100 Hz). This constitutes an enabling platform for integrated quantum technologies, opening routes to quantum entanglement generation by collective plasmon emission or photon emission with controlled waveform.

Tuning Anderson localization of edge-mode graphene plasmons in randomly gated nanoribbons

Edge-mode graphene plasmons (EGPs) supported by graphene nanoribbons are highly confined, and they can show versatile tunability under electrostatic bias. In order to efficiently enhance and actively control the near-field intensity in integrated plasmonic devices, we theoretically study Anderson localization of EGPs in a graphene nanoribbon with an underlying electrode array in this work. By randomly arranging the electrodes in the array, positional disorder is introduced in the graphene nanoribbon system. Consequently, the Anderson localization of EGPs occurs with an exponentially decreased electric field, reduced propagation length, and rapid disappearance of the cross-correlation coefficient. Physically, inhomogeneous gating effectively creates a disordered distribution of Fermi levels in the graphene nanoribbon, which provides adequate fluctuation of the effective refractive index and results in strong localization of the EGPs at mid-infrared regime. By changing electrode array arrangements, the EGPs can be trapped at distinct locations in the nanoribbon. Further considering that the Fermi-level disorder can be introduced by randomly modulating the electrostatic bias, we apply different gate voltages at different electrodes in the array. Electrically tunable Anderson localization of EGPs are eventually realized in those randomly gated nanoribbons. Moreover, by combining both the positional and Fermi-level disorders in the system, the Anderson localization becomes more actively controlled in this electrically gated graphene nanoribbons. It is shown that the local field can be selectively trapped at single distinct location, or even several locations along the graphene nanoribbon. This investigation extends the Anderson localization to the EGPs in the mid-infrared range and enriches the graphene-based active plasmonic devices.

Excitation of graphene surface plasmons polaritons by guided-mode resonances with high efficiency

An Otto-like configuration for the excitation of graphene surface plasmon polaritons (GSPPs) is proposed. The configuration is composed of a metallic grating-dielectric-waveguide structure and a monolayer graphene with a subwavelength vacuum gap between them. The evanescent field located at the bottom surface of the dielectric waveguide corresponding to grating-coupled guided-mode resonances (GMRs) is utilized to efficiently excite the highly confined GSPPs. The finite difference time domain method is used to investigate the behaviors of the GMR-GSPP hybrid modes. The dispersion relations of GMRs and GSPPs are calculated and the numerical results further identify the excitation of GMR-GSPP hybrid modes. By changing the gap between the graphene layer and the bottom of the dielectric waveguide and the Fermi energy of graphene, the resonant frequencies of GMR-GSPP hybrid modes can be continuously tuned. When the optimized excitation condition is satisfied, the maximum energy enhancement factor in the gap can reach about 500 at the resonant frequencies. The proposed structure can be used to realize highly sensitive, compatible with planar fabrication technology, and electrically (mechanically) tunable sensors.

Enhanced LSPR performance of graphene nanoribbons-silver nanoparticles hybrid as a colorimetric sensor for sequential detection of dopamine and glutathione

In the present study, a novel plasmonic sensing platform was proposed for sequential colorimetric detection of dopamine (DA) and glutathione (GSH) in human serum sample by taking advantage of plasmon hybridization in graphene nanoribbons/sliver nanoparticles (GNR/Ag NPs) hybrid. DA was detected based on etching strategy and morphology transition of label-free Ag NPs hybridized with GNR. As a result of the etching process, hexagonal Ag NPs were changed to smaller corner-truncated nanoparticles and a blue shift was observed in its plasmonic band, accompanied by the color change from green to red. Sequentially, GSH induced aggregation of Ag NPs which resulted in a decrease in absorption intensity of Ag NPs plasmonic band and a color change from red to gray. By employing GNR/Ag NPs hybrid as a sensitive colorimetric sensor, DA and GSH were successfully detected in low concentrations of 0.04 μM and 0.23 μM, respectively. The same experiment was carried out in the absence of GNR and the detection limits were obtained 0.46 and 1.2 μM for DA and GSH, respectively. These results confirmed the effective role of GNR on the sensitivity improvement of GNR/Ag NPs hybrid. The proposed simple and sensitive sensing approach offered a beneficial and promising platform for sequential detection of DA and GSH in the biological samples.

Spectrometer-Free Graphene Plasmonics Based Refractive Index Sensor

We propose a spectrometer-free refractive index sensor based on a graphene plasmonic structure. The spectrometer-free feature of the device is realized thanks to the dynamic tunability of graphene's chemical potential, through electrostatic biasing. The proposed sensor exhibits a 1566 nm/RIU sensitivity, a 250.6 RIU-1 figure of merit in the optical mode of operation and a 713.2 meV/RIU sensitivity, a 246.8 RIU-1 figure of merit in the electrical mode of operation. This performance outlines the optimized operation of this spectrometer-free sensor that simplifies its design and can bring terahertz sensing one step closer to its practical realization, with promising applications in biosensing and/or gas sensing.

Electrical Phase Control Based on Graphene Surface Plasmon Polaritons in Mid-infrared

Phase modulation of light is the core of many optoelectronic applications, such as electro-optic switch, sensors and modulators. Graphene Surface plasmon polaritons (SPPs) exhibit unique properties in phase modulation including dynamic tunability, a small driving voltage and small device size. In this paper, the novel phase modulation capability of graphene SPPs in mid-infrared are confirmed through theory and simulation. The results show that graphene SPPs can realize continuous tuning of the phase shift at multiple wavelengths in mid-infrared, covering the phase range from 0° to 360°. Based on these results, a sandwich waveguide structure of dielectric–graphene–dielectric with a device length of 800 nm is proposed, which shows up to 381° phase modulation range at an operating wavelength of 6.55 µm, given a 1 V driving voltage. In addition, the structure size is much shorter than the wavelength in mid-infrared and can realize sub-wavelength operation. This work paves the way to develop graphene-based tunable devices for mid-infrared wave-front control.

A Triple-Band Hybridization Coherent Perfect Absorber Based on Graphene Metamaterial

In this paper, a triple-band hybridization coherent perfect absorber based on graphene metamaterial is proposed, which consists of graphene concentric nanorings with different sizes and a metallic mirror separated by SiO2 layer. Based on the finite-difference time-domain (FDTD) solution, triple-band coherent perfect absorption is achieved at frequencies from 0.6 THz to 1.8 THz, which results from the surface plasmon resonance hybridization. The wavelength of the absorption peak can be rapidly changed by varying the Fermi level of graphene. Most importantly, the wavelength of the absorption peak can be independently tuned by varying the Fermi level of the single graphene nanoring. Moreover, the triple hybridization perfect absorber is angle-insensitive because of the perfect symmetry structure of the graphene nanorings. Therefore, our results may widely inspire optoelectronic and micro-nano applications, such as cloaking, tunable sensor, etc.

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

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

Giant optical activity in plasmonic chiral structure via double-layer graphene moiré stacking in mid-infrared region

The plasmonic metamaterials and metasurfaces play a critical role in manipulating lights in the mid-infrared spectral region. Here, we first propose a novel plasmonic chiral structure with the giant optical activity in the mid-infrared spectral region. The chiral structure consists of the moiré patterns, which are formed by stacking double-layer graphene nanoribbons with a relative in-plane rotation angle. It is demonstrated that the graphene-based plasmonic structure with moiré patterns exhibits the strong circular dichroism. The giant chiroptical response can be precisely controlled by changing the rotation angle and Fermi level of graphene. Furthermore, a dielectric interlayer is inserted between two layers of graphene to obtain the stronger circular dichroism. Impressively, the strongest circular dichroism can reach 5.94 deg at 13.6 µm when the thickness of dielectric interlayer is 20 nm. The proposed structure with graphene-based moiré patterns can be superior to conventional graphene chiral metamaterials due to some advantage of rotation-dependent chirality, flexible tunability and cost-effective fabrication. It will advance many essential mid-infrared applications, such as chiral sensors, thermal imaging and chiroptical detectors.

Dual-Mode On-to-Off Modulation of Plasmon-Induced Transparency and Coupling Effect in Patterned Graphene-Based Terahertz Metasurface

The plasmon-induced transparency (PIT), which is destructive interference between the superradiation mode and the subradiation mode, is studied in patterned graphene-based terahertz metasurface composed of graphene ribbons and graphene strips. As the results of finite-difference time-domain (FDTD) simulation and coupled-mode theory (CMT) fitting, the PIT can be dynamically modulated by the dual-mode. The left (right) transmission dip is mainly tailored by the gate voltage applied to graphene ribbons (stripes), respectively, meaning a dual-mode on-to-off modulator is realized. Surprisingly, an absorbance of 50% and slow-light property of 0.7 ps are also achieved, demonstrating the proposed PIT metasurface has important applications in absorption and slow-light. In addition, coupling effects between the graphene ribbons and the graphene strips in PIT metasurface with different structural parameters also are studied in detail. Thus, the proposed structure provides a new basis for the dual-mode on-to-off multi-function modulators.

Mode Conversion of the Edge Modes in the Graphene Double-Ribbon Bend

In this paper, a new kind of graphene double-ribbon bend structure, which can support two edge graphene surface plasmons (EGSPs) modes, is proposed. In this double-ribbon bend, one edge mode can be partly converted into another one. We attribute the mode conversion mechanism to the interference between the two edge plasmonic modes. Based on the finite element method (FEM), we calculate the transmission and loss of EGSPs propagating along this graphene double-ribbon bend in the mid-infrared range under different parameters.

Multifunctional graphene metasurface to generate and steer vortex waves

Graphene, an innovated 2D material with atomic thickness, is a very promising candidate and has drawn great attentions in various applications. Graphene metasurface enables dynamic control of various wavefronts, achieving distinguished functionalities. The flexibility of graphene metasurface makes it possible to implement multifunctional devices with ease. In this work, a novel design of multifunctional graphene metasurface, which can combine the functionalities of generating and steering vortex waves, has been proposed. The multifunctional graphene metasurface consists of a large array of graphene reflective unit cells. Each unit cell is controlled independently by its size and external static gate voltage. By scrutinizing the reflective property of the graphene cell, the graphene metasurface is designed to realize multi-functionalities. Simulation results show that vortex wave can be generated and steered. This work can establish a methodology to design multifunctional graphene metasurfaces, and the tunability of graphene opens the gate to the design and fabrication of reconfigurable graphene devices.

Tunable plasmonic force switch based on graphene nano-ring resonator for nanomanipulation

Using a plasmonic graphene ring resonator of resonant frequency 10.38 THz coupled to a plasmonic graphene waveguide, we design a lab-on-a-chip optophoresis system that can function as an efficient plasmonic force switch. Finite difference time domain numerical simulations reveal that an appropriate choice of chemical potentials of the waveguide and ring resonator keeps the proposed structure in on-resonance condition, enabling the system to selectively trap a nanoparticle. Moreover, a change of 250 meV in the ring chemical potential (i.e., equivalent to 2.029 V change in the corresponding applied bias) switches the structure to a nearly perfect off-resonance condition, releasing the trapped particle. The equivalent plasmonic switch ON/OFF ratio at the waveguide output is -15.519 dB. The designed system has the capability of trapping, sorting, controlling, and separating PS nanoparticles of diameters ≥30 nm with a THz source intensity of 14.78 mW/µm² and ≥22 nm with 29.33 mW/µm².

Numerical simulations of tunable ultrashort power splitters based on slotted multimode interference couplers

This paper presents an ultracompact tunable device for power splitting and switching by tuning the Fermi energy level of monolayer patternless graphene underneath a slotted multimode interference (MMI) coupler operating in the mid-infrared, λ = 9-11 μm. By introducing a high-index silicon slot in the central region of the MMI structure, which can significantly shorten the beat length, the proposed device has an approximately 4.5-fold reduction in device length and a two-fold improvement in power transmission compared with conventional MMI couplers without slotting. The device has a footprint of only 0.30 × 0.65 μm² (<λ/10), making it the smallest power splitter and switcher. Over the bandwidth of 2 μm, the power transmission of the proposed device is nearly uniform. Extending the operating bandwidth is limited only by the practically achievable Fermi energy of graphene. For the fabrication tolerance, the numerical results show that the relative power variations are lower than 5%, even though the dimension variations are greater than 15%. With its advantages of tunability, compact footprint, and broadband operation, the proposed device is suitable for highly dense photonic integrated circuits.

Metamaterial Lensing Devices

In recent years, the development of metamaterials and metasurfaces has drawn great attention, enabling many important practical applications. Focusing and lensing components are of extreme importance because of their significant potential practical applications in biological imaging, display, and nanolithography fabrication. Metafocusing devices using ultrathin structures (also known as metasurfaces) with superlensing performance are key building blocks for developing integrated optical components with ultrasmall dimensions. In this article, we review the metamaterial superlensing devices working in transmission mode from the perfect lens to two-dimensional metasurfaces and present their working principles. Then we summarize important practical applications of metasurfaces, such as plasmonic lithography, holography, and imaging. Different typical designs and their focusing performance are also discussed in detail.

Dielectric-loaded black phosphorus surface plasmon polariton waveguides

By using the effective-index method (EIM) and the finite element methods (FEM), a surface plasmon polariton (SPP) waveguide structured by a dielectric ridge placed on monolayer black phosphorus is proposed and analyzed in the infrared spectral region. It is found that the strong anisotropic dispersion of the black phosphorus (BP) gives rise to direction-dependent waveguide modes in a dielectric-loaded black phosphorus waveguide (DLBPW). The effective mode index, propagation loss and cutoff wavelength of higher order modes are investigated along the armchair (AC) and the zigzag (ZZ) directions of the black phosphorus. Moreover, the propagation characteristics of single-mode are investigated for different widths of the dielectric ridge and different polarization directions of the black phosphorus. Via tuning the carrier density, the electromagnetic field propagation features can be effectively modified. Also, the coupling effect by adding more dielectric bridges can tune the propagation properties. These results will lead to great applications in black phosphorus-based optical integrated devices.