Graphene Plasmon Resonances Research Articles

Effects of Mid‐Infrared Graphene Plasmons on Photothermal Heating

Herein, the plasmonic heating of graphene‐based systems under the mid‐infrared laser irradiation is theoretically investigated, where periodic arrays of graphene plasmonic resonators are placed on dielectric thin films. Optical resonances are sensitive to structural parameters and the number of graphene layers. Under mid‐infrared laser irradiation, the steady‐state temperature gradients are calculated. It is found that graphene plasmons significantly enhance the confinement of electromagnetic fields in the system and lead to a large temperature increase compared with the case without graphene. The correlations between temperature change and the optical power, laser spot, and thermal conductivity of dielectric layer in these systems are discussed. The numerical results are in accordance with experiments.

Multilayer Graphene Terahertz Plasmonic Structures for Enhanced Frequency Tuning Range

Graphene plasmonics has recently found a variety of applications in terahertz photonic devices. High spatial confinement and large frequency tunability are two key advantages of graphene plasmonics. Nevertheless, the frequency tuning range of plasmonic devices employing single-layer graphene is ultimately limited by its carrier density tuning range. Here, we demonstrate that the frequency tuning range of graphene-based plasmonic devices can be further extended by employing multilayer graphene structures. Both our experimental investigations and theoretical calculations show that the frequency tuning range of gate-controlled graphene plasmonic resonators can be significantly enhanced by employing two or three layers of stacked graphene, which is a result of the carrier distributions in multiple layers leading to higher total optical conductivity. However, contrary to the previous prediction, stacking even more graphene layers yields little additional benefit, as the interlayer charge screening effect leads to insignificant gate-induced carrier density in additional graphene layers. Our findings provide new insights for designing and optimizing graphene-based plasmonic structures for various photonic device applications, such as modulators, sensors, and detectors.

Hybrid metal-graphene plasmonic sensor for multi-spectral sensing in both near- and mid-infrared ranges.

This paper proposes a hybrid metal-graphene plasmonic sensor which can simultaneously perform multi-spectral sensing in near- and mid-IR ranges. The proposed sensor consists of an array of asymmetric gold nano-antennas integrated with an unpatterned graphene sheet. The gold antennas support sharp Fano-resonances for near-IR sensing while the excitation of graphene plasmonic resonances extend the sensing spectra to the mid-IR range. Such a broadband spectral range goes far beyond previously demonstrated multi-spectral plasmonic sensors. The sensitivity and figure of merit (FOM) as well as their dependence on the thickness of the sensing layer and Fermi energy of graphene are studied systematically. This new type of sensor combines the advantages of conventional metallic plasmonic sensors and graphene plasmonic sensors and may open a new door for high-performance, multi-functional plasmonic sensing.

Graphene Antidot Terahertz Plasmonic Metasurfaces Employing Self-Aligned Metal Cores for Sensing Applications

Surface plasmons in graphene have great potential for molecular sensing applications thanks to their exceedingly high sensitivity to environmental changes. Here, we demonstrate a type of hybrid graphene–metal metasurface that supports strong graphene plasmonic resonances in the terahertz range. Each unit cell of such a hybrid metasurface consists of a graphene antidot enclosing a metal disk realized using a self-aligned photolithography process. This hybrid design combines the advantages of both graphene- and metal-based photonic structures, leading to ∼3 times stronger tunable plasmonic resonances and an order of magnitude larger near-field intensity enhancement with respect to those of bare graphene antidot metasurfaces.

Edge Doping Effect to the Surface Plasmon Resonances in Graphene Nanoribbons

In this work, we investigated the edge doping effect to graphene plasmon resonances in graphene nanoribbons (GNRs), which is known to affect the electronic doping of GNRs but has not been systematically studied. We found the Fermi levels, which reflect that the sheet charge carrier densities, extracted from the graphene plasmon resonance frequencies, vary across GNRs of different widths on the same sample. Using Raman spectroscopy, we confirmed that the variation of the sheet charge carrier density is caused by edge doping, which has a stronger effect on narrower GNRs. To further understand the edge doping effect, electron beam irradiation (EBI) is applied to modify the charge state of the edge. Using EBI, we successfully demonstrated the tuning of the graphene plasmon resonances due to the change of the edge doping states. These findings demonstrated the importance of the edge doping effect in determination of the surface plasmon frequency in GNRs.

Numerical Study of Angle-Insensitive and Tunable Dual-Band THz Absorber Using Periodic Cross-Shaped Graphene Arrays.

A dual-band terahertz (THz) absorber using the periodic cross-shaped graphene arrays is presented. It is shown that the dual-band light absorption enhancement of graphene results from the edge graphene plasmon (EGP) resonance, and the locations of the two absorption peaks can be precisely estimated by using the Fabry-Pérot (F-P) cavity model. Slight residual reflection remains at the two absorption peaks because the input impedance of the cross-arm cannot be perfectly matched with the free space impedance. In addition, the locations of the two absorption bands can be simultaneously tuned by changing the Fermi level of graphene, and they can be independently tuned by changing the width or the length of the cross-arm of graphene. Excellent angle-insensitivity dual-band absorption enhancement of graphene can be maintained for both the transverse electric (TE) and transverse magnetic (TM) polarizations.

Excitation and tuning of a dual graphene plasmonic wave based on a trapezoidal grating structure

The independent excitation and tuning of a dual-band graphene plasmonic wave are realized in a hybrid structure that consists of two graphene monolayers placed above and below the trapezoidal grating. Because of the transparency of graphene in the mid-infrared range, the incident light can travel through the first graphene layer to be diffracted by the grating structure and couple its energy to both graphene layers. Numerical simulations are performed using the finite difference time domain method. Results show that the plasmon resonances corresponding to the two graphene monolayers are excited at 9.8 and 10.9μm, which agrees well with the theoretical analysis. Because of the fast and efficient electrical tunability of graphene, the resonance wavelengths can be tuned individually by changing the chemical potential of the corresponding graphene. Furthermore, the effects of geometric parameters and the refractive index of the surrounding media are studied. The results show that the structure can achieve an optimal sensing coefficient at 0.4 and 0.5eV for the top and bottom graphene plasmon resonances, respectively. The proposed structure provides an alternative option to engineer the proposed structure for sensing with high detection accuracy at mid-infrared wavelengths.

Graphene Plasmonics: Approaching the Intrinsic Lifetime and Modulating a Graphene Plasmonic Resonance at a Few Hundred GHz (Advanced Optical Materials 11/2019)

Graphene Plasmonics: Approaching the Intrinsic Lifetime and Modulating a Graphene Plasmonic Resonance at a Few Hundred GHz (Advanced Optical Materials 11/2019)

Approaching the Intrinsic Lifetime and Modulating a Graphene Plasmonic Resonance at a Few Hundred GHz

AbstractGraphene‐based metamaterials exhibit large tunability, low insertion loss, and high field confinement compared with metal‐based structures. Graphene‐based metamaterials may find numerous applications in optical modulators, optoelectronic devices, and chemical sensors in the few‐hundred‐GHz and low‐THz regime, constituting a new frontier as electronics and wireless communication continuously push further to higher frequencies. However, the main challenge is overcoming loss in graphene‐based resonant applications. Here, a low‐loss graphene plasmonic resonant modulator is demonstrated that operates at a few hundred GHz and 1 THz. The modulation depth is in excess of 35% with two layers of graphene. Numerical simulations show good agreement with the experimental results and reveal 100–200 fs lifetimes, which are approaching the intrinsic lifetime of the graphene plasmon due to the suppression of loss from phonon and edge scattering. The above values are the closest to electronic frequencies and represent the longest plasmon lifetime and largest modulation depth in graphene plasmon resonance devices reported so far.

Surface-enhanced Raman scattering in graphene deposited on AlxGa1−xN/GaN axial heterostructure nanowires

The surface-enhanced Raman scattering in graphene deposited on AlxGa1−xN/GaN axial heterostructure nanowires was investigated. It was found that the intensity of graphene Raman spectra is not correlated with aluminium content. Analysis of graphene Raman band parameters, Kelvin probe force microscopy, and electroreflectance showed a screening of polarisation charges. Theoretical calculations demonstrated that plasmon resonance in graphene is far beyond the Raman spectral range. This excludes the presence of an electromagnetic mechanism of surface-enhanced Raman scattering, hence suggesting a chemical mechanism of enhancement.

Design, Theory, and Circuit Model of Wideband, Tunable and Polarization-Insensitive Terahertz Absorber Based on Graphene

We present a wideband tunable absorber which is insensitive to polarization in terahertz region. This absorber is composed of dielectric and graphene sheet layers with crossover elliptical dielectric cylinders on top of the structure. Graphene plasmon resonances are used to achieve wideband absorption with relative bandwidth of 68% for the absorption above 90% in THz regime. Thanks to the symmetric design, the structure is polarization-insensitive. Furthermore, the absorber has a good performance for wide angle of the incident waves. Also, the equivalent circuit model of the absorber is derived and the results are in good agreement with the simulation ones. The presented method for broadening the absorption spectrum and the technique of equivalent circuit model derivation can be used for related structures which have many applications in similar devices.

Scattering From Graphene-Based Multilayered Spherical Structures

In this paper, modified Mie-Lorentz coefficients for graphene-based multilayered spherical structures are extracted. To this end, electromagnetic fields in each region of the solution domain are expanded in terms of vector spherical harmonics. By properly choosing the geometrical dimensions of the spheres, graphene coatings are considered as thin shells with the surface conductivities calculated by the Kubo formulas. Later, graphene surface currents are incorporated in the boundary conditions on tangential magnetic fields. The aforementioned procedure is conducted at the interface of two adjacent layers to relate the Mie coefficients of the successive layers recurrently. The validity of the extracted equations is verified by calculating the extinction cross section of various graphene-based multilayered spherical structures and comparing them with the results of finite element method. Possibility of tuning the graphene plasmonic resonances in multiple layers by varying graphene carrier doping density is discussed, as well. The potential application of our analysis is scattering shaping of graphene-based multilayered spherical configurations for cloaking, super-scattering, medical treatment, and tagging.

Dynamically Tunable and High-Contrast Graphene-Based Terahertz Electro-Optic Modulator**Supported by Scientific Research Plan Projects of Shaanxi Education Department (203011730)

We propose and discuss terahertz (THz) electro-optic modulator induced by periodically patterned graphene microcavity. Due to the joint effect of graphene plasmon resonances and Fabry-Perot (F-P) oscillations, plasmon-induced transparency (PIT) effect is achieved and the operation frequency can be dynamically tuned by graphene Fermi energies and structural parameters. The results reveal that modulation depth (MD) larger than 80% is obtained across a wide frequency range from 4.2 THz to 9.4 THz, and the largest MD and Q factor reaches 95% and 15.8, respectively. In addition, operation frequency range and MD can also be tuned by optimizing the structure parameters. This investigation promises the development of high-performance widely tunable THz modulator in chip integrated photonic circuits.

Tunable Fano Resonance Based Mode Interference in Waveguide-Cavity-Graphene Hybrid Structure

In this paper, a graphene strip is introduced into a metal-insulator-metal (MIM)-integrated square cavity hybrid structure; the transmission spectra are theoretically investigated by the finite different time domain (FDTD) methods. An asymmetric Fano resonance dip that has high figure of merit (FOM) value appears in the transmission band. According to the multimode interference coupled mode theory (MICMT) analytical method, the Fano resonance originates from the coherent coupling between TM10 cavity magnetic mode and graphene plasmonic resonance electric mode. The center wavelength, full width at half maximum (FWHM), and FOM value of the Fano resonance can be tuned dynamically by altering the Fermi level of the graphene. Through breaking the symmetry of the hybrid structure or introducing double graphene strips with different Fermi level into hybrid structure, double Fano resonance are realized. This study can provide some theoretical basis and design reference for designing ultrahigh sensitivity plasmonic sensor.

Enhancement and control of the Goos—Hänchen shift by nonlinear surface plasmon resonance in graphene**Project supported by the National Natural Science Foundation of China (Grant No. 61505111).

The Goos–Hänchen (GH) shift of graphene in the terahertz frequency range is investigated, and an extremely high GH shift is obtained owing to the excitation of surface plasmon resonance in graphene in the modified Otto configuration. It is shown that the GH shift can be positive or negative, and can be enhanced by introducing a nonlinearity in the substrate. Large and bistable GH shifts are demonstrated to be due to the hysteretic behavior of the reflectance phase. The bistable GH shift can be manipulated by changing the thickness of the air gap and the Fermi level or relaxation time of graphene.

Mie resonance induced broadband near-perfect absorption in nonstructured graphene loaded with periodical dielectric wires.

In general, there is a fundamental trade-off between the operational bandwidth and the attainable absorption. So, obtaining broadband wave absorption of a low reference standard such as 90% is not very difficult. However, when trying to obtain higher absorption such as 99%, the bandwidth will drop dramatically. Here, we demonstrate that broadband near-perfect absorption of over 99% absorption with a 60% relative bandwidth can be obtained utilizing single-layered and nonstructured graphene loaded with periodical dielectric wires. The absorption mechanism originates from the coupling of Mie resonances in dielectric wires excited by the incident wave to the graphene plasmon resonances, which introduces two absorption contributions: direct near-field absorption in the graphene and radiative emission into the graphene.

Broadband terahertz absorber based on multi-band continuous plasmon resonances in geometrically gradient dielectric-loaded graphene plasmon structure

We propose a broadband terahertz absorber consisting of nonstructured graphene loaded with arrays of elliptic dielectric cylinders. The relative bandwidth for the absorption above 90% reaches about 65%. The working mechanism of broad bandwidth mainly comes from two aspects. One is that the nonstructured graphene loaded with elliptic dielectric cylinders provides multiple discrete graphene plasmon resonances with large relative frequency interval. The other is that, for each discrete resonance, there exists a set of continuous plasmon resonances because the width of the dielectric structure varies continuously and gradiently. The broadband terahertz absorber we demonstrate here, based on geometrically gradient dielectric structures and nonstructured graphene, avoids the graphene processing, which shows great potential applications in related devices.

Dynamical Tuning of Graphene Plasmonic Resonances by Ultraviolet Illuminations

AbstractOne of the unique features of plasmonic resonances in graphene structures is the enabled tunability through external means. Up to now, electrical gating is extensively adopted to tune graphene plasmonic resonances. This method is, however, limited unfortunately only to graphene structures with electrical connections. Here, a simple and efficient method to tune graphene plasmonic resonances by ultraviolet illuminations in a dynamical and reversible way is demonstrated. It is purely an optical means and can be applied to all kinds of graphene structures without introducing electrodes, ion–gel, or conducting layers required in electrical gating, which is of great significance especially for isolated graphene structures. The optical tuning method could shed new light on graphene‐based tunable devices.

Tunable wavelength demultiplexer using modified graphene plasmonic split ring resonators for terahertz communication

This paper presents graphene modified ring resonator based wavelength demultiplexer (WDM) for THz device applications that is, a surface plasmon polaritons (SPPs) demultiplexer consisting of two nanostrip waveguides at input as well as output coupled to each other by a split ring resonator (SRR), which is modified in shape as compared to a simple ring-shaped resonator. A systematic analysis of the transmission spectra for the graphene based SRR poses clear insight on the demultiplexing phenomenon of the proposed nanodevice. The results show resonance peaks in the transmission spectrum, having a linear relationship with the chemical potential of graphene. The influence of structural parameters have also been analyzed. The tuning capability of graphene based tunable WDM, lays its foundation in the applications of optical switches, modulators, etc.

Mid-infrared sensing of molecular vibrational modes with tunable graphene plasmons.

We study the tunable plasmons based on a graphene integrated gold grating structure to sense the vibrational modes of nanometric molecules. The greatly enhanced light-matter interaction and the broadband tunability of the localized graphene plasmonic resonance enable accurate label-free identification of the molecular vibrational modes at subwavelength scale. Our results may accelerate the further development of novel cost-effective biosensors with superior molecular chemical fingerprint sensitivity in an active graphene plasmonic device.