Graphene Plasmonic Structures Research Articles

Analytical calculation of beam profile and orbital angular momentum spectrum of Laguerre Gaussian beams reflected from a graphene plasmonic structure.

In this paper, Laguerre Gaussian (LG) beams with different topological charges are used for excitation of surface plasmon polaritons (SPPs) through a graphene layer inserted in the Otto-configuration. By utilizing the angular spectrum representation (ASR) and Lorenz-gauge vector potential, an explicit analytical expression is derived for the electromagnetic fields of the reflected beam. At the optimal excitation state of graphene SPPs, the reflected beam exhibits a distinctive field profile characterized by two identical crescent-shaped lobes separated by a vertical strip with null intensity. Furthermore, in the absence of external magnetic field, the orbital angular momentum (OAM) spectrum of the reflected beam at the optimal excitation of SPPs reveals the annihilation of central OAM mode and the generation of two equal OAM sidebands, regardless of the incident OAM topological charge. Furthermore, the phase distributions of electric field of the reflected beam confirm the existence of OAM sidebands in the vicinity of optimal SPPs excitation. As the system is taken away from the optimal excitation of SPPs by introduction of an external magnetic field or increasing the chemical potential or increasing the incident angle, both central and sideband modes appear in the OAM spectrum of the reflected beam. In this case, when the topological charge of the incident wave increases, the weight of central OAM mode decreases while the weight of sidebands increases. In contrast, in the presence of external magnetic field, at the optimal excitation of SPPs, both central OAM and sidebands modes exist in the reflected beam such that the weight of central modes increases with the external magnetic field. This effect is also confirmed by plotting the phase distributions of the reflected beam at different external magnetic fields and for different incident topological charges. Therefore, the manipulation of graphene plasmons characteristics leads to the control of OAM sideband generation.

Sensing and slow light applications of graphene plasmonic terahertz structure

In this work, Ansys FDTD is used to design and simulate a terahertz metamaterial structure based on periodic continuous pattern graphene monolayer, and the high-quality PIT phenomena are obtained by continuously adjusting structural parameters. To validate the designed structure, the simulated transmission curve (reflection curve) obtained is compared with the theoretical transmission curve (reflection curve) derived from coupled-mode theory. It is observed that these two results exhibit a remarkably high degree of overlap. The resonant frequency and Fermi energy reveals a perfect linear correlation between them with the resonant frequency increasing proportionally with Fermi energy increasing. Dynamic tuning of PIT can be realized by adjusting the Fermi energy of graphene. For a more in-depth study of its sensing characteristics, the structure is placed in different environments. As the refractive index of the detection medium increases, the resonant frequency gradually decreases, demonstrating a redshift phenomenon. By manipulating the resonant frequency of the PIT sensor, the selective detection of specific target can berealized. After analyzing the sensitivity and FOM values of the structure, it is found that the maximum sensitivity is 1.457 THz/RIU. At a resonant frequency of 6.8174 THz, FOM reaches 30.5652. In summary, the sensor structure designed in this work has dual frequency sensing characteristics and can be used for dual frequency detection. Moreover, compared with other sensor structures, it demonstrates superior sensing performance. Additionally, in studying the slow light effect of the structure, it is found that as the Fermi energy increases, the group index and phase shift at the transparency window continue to increase. At the Fermi energy of 1.2 eV, the group index reaches a high value of 584. This is because in the PIT phenomenon, transparent peaks are formed due to multimodal coupling. This coupling will significantly improve the dispersion characteristics near the transparent peak, resulting in a large group index near the transparent peak. Furthermore, with the increase of carrier mobility, the group index and phase shift of the structure also gradually increase. At a carrier mobility of 0.75 m²/(V·s), the group refractive index is 456, and reaches 1010 at 2.0 m²/(V·s). In this study, the slow-light performance of graphene structure can be optimized through jointly adjusting the Fermi energy and carrier mobility. This research provides theoretical support and methods for designing advanced graphene-based sensors and devices for slow-light applications.

Ambipolar charge-transfer graphene plasmonic cavities.

Plasmon polaritons in van der Waals materials hold promise for various photonics applications1-4. The deterministic imprinting of spatial patterns of high carrier density in plasmonic cavities and nanoscale circuitry can enable the realization of advanced nonlinear nanophotonic5 and strong light-matter interaction platforms6. Here we demonstrate an oxidation-activated charge transfer strategy to program ambipolar low-loss graphene plasmonic structures. By covering graphene with transition-metal dichalcogenides and subsequently oxidizing the transition-metal dichalcogenides into transition-metal oxides, we activate charge transfer rooted in the dissimilar work functions between transition-metal oxides and graphene. Nano-infrared imaging reveals ambipolar low-loss plasmon polaritons at the transition-metal-oxide/graphene interfaces. Further, by inserting dielectric van der Waals spacers, we can precisely control the electron and hole densities induced by oxidation-activated charge transfer and achieve plasmons with a near-intrinsic quality factor. Using this strategy, we imprint plasmonic cavities with laterally abrupt doping profiles with nanoscale precision and demonstrate plasmonic whispering-gallery resonators based on suspended graphene encapsulated in transition-metal oxides.

Quantum effects on the propagation of surface magnetoplasmon polaritons in a graphene-plasmonic structure

We investigate the properties of surface magnetoplasmon polaritons (SMPPs) in a graphene-plasmonic structure which is constructed as a graphene film sandwiched with two semi-infinite dielectrics under a perpendicular configuration. By solving Maxwell equations and quantum magneto-hydrodynamic equations with considering the quantum statistical and quantum diffraction effects, we deduce the dispersion relation of graphene SMPPs (GSMPPs) in detail. We show how the graphene electron density, the external magnetic field, and the dielectric constant, affect the features of the dispersion of GSMPPs in both classical and quantum cases. We find that the quantum effects (QEs) significantly alter the properties of GSMPPs, which are entirely different from those in a classical model. We find that the propagation speed of classical GSMPPs has small increases while the propagation speed of quantum GSMPPs has fast and sharp increases along with the increases in graphene electron density. We further find that the propagation speed decreases gradually by increasing the applied magnetic field in both classical and quantum GSMPPs. Moreover, we also find that the propagation speed of classical GSMPPs has fast decreases tending to zero at large wavenumber while the propagation speed of quantum GSMPPs has slow decreases tending to infinity with increasing the dielectric constant. Our findings elucidate that QEs play a crucial role in the properties of GSMPPs and their response to different parameters.

Dual Tunable Electromagnetically Induced Transparency Based on a Grating-Assisted Double-Layer Graphene Hybrid Structure at Terahertz Frequencies.

We propose a graphene plasmonic structure by applying two graphene layers mingled with a thin gold layer in a silicon grating. By utilizing the finite-difference time-domain (FDTD) method, we investigate the optical response of the system, and observe that the design achieves dual tunable electromagnetically induced transparency (EIT)-like effect at terahertz frequencies. The EIT-like effect arises from the destructive interference between the grapheme-layer bright modes and the gold-layer dark mode. The EIT-like phenomenon can be adjusted by the Fermi level, which is related to the applied voltage. The results show that the group delay of the present structure reaches 0.62 ps in the terahertz band, the group refractive index exceeds 1200, the maximum delay-bandwidth product is 0.972, and the EIT-like peak frequency transmittance is up to 0.89. This indicates that the device has good slow light performance. The proposed structure might enable promising applications in slow-light devices.

Electrical activating of the “nonradiative” terahertz plasmon modes in a periodic grating-gate graphene structure with asymmetrical gating

Electrical tuning the radiative damping of “nonradiative” (weak) terahertz plasmon mode in a periodic dual-grating-gate graphene structure is studied theoretically. Tuning the dc voltage value applied to one of the gate subgratings can change the radiative damping of the weak plasmon mode from zero to the dissipative damping of the plasmons in high-quality graphene allowing for maximum energy conversion from the incident wave to “nonradiative” plasmon mode. Radiative damping increases due to the increase of the dipole moment of this mode by virtue of its asymmetrical gating. Using the weak plasmon modes is important for effective excitation of the plasmon resonances in high-quality graphene as well as for realizing the concept of low-threshold terahertz lasers based on graphene plasmonic structures. Data availabilityData underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.

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.

An all-optical modulator based on a graphene–plasmonic slot waveguide at 1550 nm

An all-optical modulator with high modulation efficiency based on a graphene-plasmonic slot waveguide structure is proposed. The modulation efficiency is enhanced by the strong interaction between the light and the graphene–plasmonic structure. A modulation efficiency of 0.21 dB μm−1 is obtained with the signal light at a wavelength of 1550 nm. A graphene–plasmonic slot waveguide device of length of 10 μm is fabricated in the experiment. Benefiting from the strong modulation, it is a promising candidate as an on-chip optical modulator with high modulation efficiency and micrometer-scale footprint.

Design of a high-efficient and ultra-compact full-adder based on graphene-plasmonic structure

Full-adders are basic devices used widely in many computational circuits. In this paper, by utilizing the tuning capability of graphene-dielectric-metal waveguides, an efficient electro-optical full-adder with compact size is presented. In our structure, surface plasmon polaritons are stimulated at the graphene-dielectric interface by a TM mode incident wave with a wavelength of 13.8 μm. Addition operation is performed on electrical signals as inputs and results are obtained optically at output ports. The extinction ratios for Sum and Carry bits are calculated as 25.95 dB and 22.92 dB, respectively. The footprint of the structure is 1.3 μm2 which is considerably smaller than other reported full-adder structures. This ultra-compact footprint denotes high potentiality of the structure for using in integrated photonic circuits.

Giant effect of terahertz-radiation rectification in periodic graphene plasmonic structures

The rectification of terahertz radiation due to plasmonic nonlinearities in a periodic graphene structure with a dual-grating gate and an asymmetric unit cell is considered. The influence of unit-cell asymmetry on the plasmonic rectification of the terahertz radiation near higher plasmonic resonances is considered for the cases of electron and hole conduction in parts of the graphene unit cell. The current responsivity of the rectification of terahertz radiation due to the plasmonic effects of differential charge-carrier drag and electron–hole ratchet is calculated.

Dual tunable plasmon-induced transparency based on silicon-air grating coupled graphene structure in terahertz metamaterial.

A graphene plasmonic structure consists of three graphene layers mingled with a silicon-air grating is proposed. We theoretically predict and numerically simulate the plasmon-induced transparency effect in this system at terahertz wavelengths, and a dual plasmon-induced transparency peaks can be successfully tuned by virtually shifting the desired Fermi energy on graphene layers. We investigate the surface plasmon dispersion relation by means of analytic calculations, and we can achieve the numerical solution of propagation constant got by the dispersion relation. A suitable theoretical model is established to study spectral features in the plasmonic graphene system, and the theoretical results agree well with the simulations. The proposed model and findings may provide guidance for fundamental research of highly tunable optoelectronic devices.

Interaction between graphene-coated nanowires revisited with transformation optics.

The interaction between graphene-coated nanostructures provides interesting optical properties not found in isolated graphene plasmonic structures. However, full-analytical solutions, which can provide deep physical insights underlying the hybrid graphene plasmonic systems, are difficult to achieve. In this Letter, we deploy the theory of transformation optics to study the plasmonic interactions between two dielectric-core-graphene-shell nanowires. The scattering and absorption spectra as well as the field distributions are derived analytically. We find that the interaction between two graphene-coated nanowires results in polarization-independent multi-frequency Fano dips, which show a broadband red shift of bonding modes and a blue shift of anti-bonding modes when the nanowires approach each other. The analytical tool presented here offers a rigorous study of graphene plasmonic compound and can be extended to treat more complicated cases.

Electrically Tunable Absorption Enhancement with Spectral and Polarization Selectivity through Graphene Plasmonic Light Trapping

In this paper, anisotropic graphene plasmonic structures are explored for light trapping and absorption enhancement in surrounding media. It is shown that electrically tunable and versatile spectral and polarization selectivity can be realized. Particularly, it is possible to control absorption of the incident light’s polarization component at a specific wavelength by varying the Fermi energy with suitable geometric designs. It may find applications for new types of infrared and THz photodetectors and will promote the research of other novel polarization devices.

Far-field nanoscale infrared spectroscopy of vibrational fingerprints of molecules with graphene plasmons.

Infrared spectroscopy, especially for molecular vibrations in the fingerprint region between 600 and 1,500 cm−1, is a powerful characterization method for bulk materials. However, molecular fingerprinting at the nanoscale level still remains a significant challenge, due to weak light–matter interaction between micron-wavelengthed infrared light and nano-sized molecules. Here we demonstrate molecular fingerprinting at the nanoscale level using our specially designed graphene plasmonic structure on CaF2 nanofilm. This structure not only avoids the plasmon–phonon hybridization, but also provides in situ electrically-tunable graphene plasmon covering the entire molecular fingerprint region, which was previously unattainable. In addition, undisturbed and highly confined graphene plasmon offers simultaneous detection of in-plane and out-of-plane vibrational modes with ultrahigh detection sensitivity down to the sub-monolayer level, significantly pushing the current detection limit of far-field mid-infrared spectroscopies. Our results provide a platform, fulfilling the long-awaited expectation of high sensitivity and selectivity far-field fingerprint detection of nano-scale molecules for numerous applications.

Highly tunable hybrid metamaterials employing split-ring resonators strongly coupled to graphene surface plasmons.

Metamaterials and plasmonics are powerful tools for unconventional manipulation and harnessing of light. Metamaterials can be engineered to possess intriguing properties lacking in natural materials, such as negative refractive index. Plasmonics offers capabilities of confining light in subwavelength dimensions and enhancing light–matter interactions. Recently, the technological potential of graphene-based plasmonics has been recognized as the latter features large tunability, higher field-confinement and lower loss compared with metal-based plasmonics. Here, we introduce hybrid structures comprising graphene plasmonic resonators coupled to conventional split-ring resonators, thus demonstrating a type of highly tunable metamaterial, where the interaction between the two resonances reaches the strong-coupling regime. Such hybrid metamaterials are employed as high-speed THz modulators, exhibiting ∼60% transmission modulation and operating speed in excess of 40 MHz. This device concept also provides a platform for exploring cavity-enhanced light–matter interactions and optical processes in graphene plasmonic structures for applications including sensing, photo-detection and nonlinear frequency generation.

Active graphene plasmonic grating for terahertz beam scanning device

We have proposed an electrically controllable terahertz (THz) beam scanning device based on asymmetric graphene plasmonic grating. It is proved that the dispersion relation of surface plasmon waves can be efficiently manipulated on the periodic metal–dielectric–graphene plasmonic structure by an external bias voltage, and thus the derivation angle of the THz beam can be actively tuned. The numerical results show that the radiation angle of THz waves through this device can be tuned from −18° to 18° with the bias changing from 53.5mV into 368.8mV at the operating frequency of 0.5THz. This electrically active beam scanning device has important application in the THz imaging, radar and communication systems.

Plasmon-plasmon hybridization and bandwidth enhancement in nanostructured graphene.

Graphene plasmonic structures with long-range layering periodicity are presented. Resonance energy scaling with the number of graphene layers involved in plasmonic excitation allows these structures to support multiple plasmonic modes that couple and hybridize due to their physical proximity. Hybridized states exhibit bandwidth enhancements of 100-200% compared to unhybridized modes, and resonance energies deviate from what is usually observed in coupled plasmonic systems. Origins of this behavior are discussed, and experimental observations are computationally modeled. This work is a precursor and template for the study of plasmonic hybridization in other two-dimensional material systems with layering periodicity.

Graphene optical-to-thermal converter

Infrared plasmons in doped graphene nanostructures produce large optical absorption that can be used for narrow-band thermal light emission at tunable frequencies that strongly depend on the doping charge. By virtue of Kirchhoff's law, thermal light emission is proportional to the absorption, thus resulting in narrow emission lines associated with the electrically controlled plasmons of heated graphene. Here, we show that realistic designs of graphene plasmonic structures can release over 90% of the emission through individual infrared lines with 1% bandwidth. We examine anisotropic graphene structures in which efficient heating can be produced upon optical pumping tuned to a plasmonic absorption resonance situated in the blue region relative to the thermal emission. An incoherent thermal light converter is thus achieved. Our results open a radically different approach for designing tunable nanoscale infrared light sources.

Coupling of plasmon modes in graphene microstructures

A variety of different graphene plasmonic structures and devices have been proposed and demonstrated experimentally. Plasmon modes in graphene microstructures interact strongly via the depolarization fields. An accurate quantitative description of the coupling between plasmon modes is required for designing and understanding complex plasmonic devices. Drawing inspiration from microphotonics, we present a coupled-mode theory for graphene plasmonics, in which the plasmon eigenmodes of a coupled system are expressed in terms of the plasmon eigenmodes of its uncoupled sub-systems. The coupled-mode theory enables accurate computation of the coupling between the plasmon modes and of the resulting dynamics. We compare theory with experiments performed on the plasmon modes in coupled arrays of graphene strips. In experiments, we tune the coupling by changing the spacing between the graphene strips in the array. Our results show that the coupling parameters obtained from the coupled-mode theory and the plasmon frequency changes resulting from this coupling agree very well with experiments. The work presented here provides a framework for designing and understanding coupled graphene plasmonic structures.

Graphene-based plasmonic photodetector for photonic integrated circuits

We developed a planar-type graphene-based plasmonic photodetector (PD) for the development of all-graphene photonic-integrated-circuits (PICs). By configuring the graphene plasmonic waveguide and PD structure all-in-one, the proposed graphene PD detects horizontally incident light. The photocurrent profile with opposite polarity is the maximum at graphene-electrode interfaces due to a Schottky-like barrier effect at the interface. The photocurrent amplitude increases with an increase of the graphene-metal interface length. Obtaining time constants of less than 39.7 ms for the time response, we concluded that the proposed graphene PD could be exploited further for application in all graphene-based PICs.