Chemical Potential Of Graphene Layer Research Articles

Implementing the single/multiport tunable terahertz circularly polarized dielectric resonator antenna

A tunable terahertz circularly-polarized dielectric resonator antenna is implemented and numerically studied. An aperture coupled terahertz rectangular dielectric resonator antenna operating with the fundamental magnetic dipole is designed. The modal field distribution is perturbed using the material-perturbation technique and the electric field vectors are aligned in the horizontal plane with their orthogonal components having quadrature phase difference between them. Consequently, the circular polarization is obtained in the operating passband. The proposed antenna provides the 10-dB impedance bandwidth of 12.94% (4.27–4.86 THz) and 3 dB axial ratio bandwidth of 3.77% (4.43–4.60 THz). Moreover, tunability is achieved by modulation in the chemical potential of graphene layer coated at the top of the dielectric resonator by applying an electrostatic bias voltage. In addition, antenna provides the gain of 6.45 dBic along with 98% radiation efficiency. The proposed antenna can also be used for the implementation of multi-port wireless THz communication systems with the advantage of tunable CP response with impeccably high radiation performance parameters. A two-port multi input, multi output (MIMO) antenna is implemented with the usage of the proposed metal strip coated DR. This antenna provides high isolation between both the ports along with polarization response and pattern diversity with the acceptable value of MIMO performance parameters like envelop correlation coefficient and diversity gain.

Sensitive Hemoglobin Concentration Sensor Based on Graphene-Plasmonic Nano-structures.

In this paper, half-cylindrical-shaped rods are arranged in a row in order to form hemoglobin concentration sensors. The proposed structures can effectively detect hemoglobin concentrations in blood samples. Five individual structures based on graphene-plasmonic combinations are proposed and investigated. The proposed structures are made of different layers of Au, Ag, SiO2, and graphene. Different plates and cylindrical-shaped graphene layers are introduced in the structures to improve the functionality (absorption peak value and wavelength) of the absorber (sensor). Adding more Au layers strengthens the confinement of the incident electromagnetic waves and improves the absorption factor. Also, in the proposed structures, for improving the results, the effects of the chemical potential of graphene layers and “G” (graphene layer thicknesses) on the absorption peak and wavelength are considered. The final suggested structure indicates unity absorption peak and thus can be utilized in wide ranges of applications. As a refractive index bio-sensor, the structure is considered for detecting hemoglobin concentrations in blood samples which indicates a reasonable sensitivity factor of 570 nm/RIU.

Ultrafast all-optical metamaterial terahertz switch by exploiting nanoparticle-enriched substrate nonlinearity and graphene resonators

The emergence of graphene as a new material with several extraordinary optical properties paves a way to introduce new electromagnetic devices in the terahertz (THz) band. In this paper, we have exploited the unique qualities of graphene in combination with the optical Kerr effect in order to propose a new THz switch. In the proposed structure, two graphene layers with circular and notch ring patterns are stacked to obtain two narrowband absorption peaks at the frequencies of 3.65 and 6.5 THz with high absorption values of 92% and 97%, respectively. The imposed pump power in interaction with the nanocomposite substrate provides a controlling mechanism that makes this structure an ideal candidate for all-optical devices in the THz regime. The obtained switching time of the proposed structure is 2 ps, and the maximum extinction ratio of 83% is achieved for the pump power of 273 mW. Moreover, the absorption spectrum of the switch can be manipulated by the chemical potential of graphene layers, which is useful for electro-optical modulation application. The application of the THz switch as an all-optical tunable filter is also studied in this paper.

Electrically enhanced graphene-metal plasmonic antenna for infrared sensing

An optical graphene-metal hybrid antenna geometry, permitting the efficient tuning of plasmon resonance with unity absorption is reported. The total optical absorption and optical field enhancement are accomplished by designing a graphene-metal antenna. The antenna is comprised of gold hexagon radiator with bilayer graphene on the top, functioning as a double plasmonic resonant structure. The tunability of absorption and optical field enhancement is realized by electrical gating. The strong coupling takes place between the gold plasmons and graphene plasmons, resulting in the strong enhancement of optical fields. The proposed design is modeled in CST Microwave Studio and is simulated through FDTD solver. Moreover, the dynamic tuning of the resonance frequency and optical absorption is achieved by increasing the chemical potential of graphene layers through gate voltage. The tuning range of the designed antenna is optimized in a bandwidth starting from 30 THz to 34 THz. Although at 33 THz the antenna meets matching conditions by having high input impedance, low admittance, and almost unity absorption. The functioning bandwidth of the antenna is preferable for plasmonic applications i.e. infrared sensing and imaging, where high absorption and enhanced field characteristics are required.

A perfect electrically tunable graphene-based metamaterial absorber

In this paper, a metamaterial absorber based on graphene is proposed, designed and simulated numerically using multilayer structures. Before describing our work, the performance response of a three-layer structure is studied concerning one cylinder as a unit cell. Then, by varying the chemical potential of graphene (Fermi level of graphene) by applying external potential, the center frequency of this absorber can be adjusted. We will prove that by increasing the number of cylinders as fundamental elements in the unit cell, the bandwidth is also adjustable. Also, the center frequency can be affected by changing the dimensions of fundamental elements. Considering the above-mentioned items, the structure is investigated through increasing the graphene layers in which the absorber’s frequency response is wider respected to previous structures and the center frequency is adjustable as a result of variations in the chemical potential of graphene layers. It is shown that the proposed perfect absorber’s central frequency shift through graphene’s voltage variation is about 0.15 THz which can be increased to 0.3 THz by changing radius. In some of the introduced absorbers, the maximum value of the absorption has reached over 95%. The most important advantage of the proposed structure, which is the main purpose of designing terahertz metamaterial absorbers, is its adjustable bandwidth and center frequency, and simple fabrication.

Tunable Tamm states at the interface of a 1D graphene-based photonic crystal and a nonlinear dielectric slab

We investigate the dispersion of the Tamm states at the interface of a 1D graphene-based photonic crystal and a nonlinear dielectric slab. We use the nonlinear transfer matrix method to obtain the dispersion of the Tamm states in both the graphene induced and Bragg gaps in the terahertz region. We explore the spectral tunability of the surface modes by adjusting parameters such as the thickness of the nonlinear slab, the intensity of the surface modes, and the chemical potential of graphene layers. We reveal that the penetration depth of the graphene induced bandgap modes in the uniform environment is several times greater than that of the conventional Bragg gap modes. Also, it is shown that we can improve the penetration depth of the surface modes by adjusting the thickness of the nonlinear slab, the intensity of the Tamm states, or the chemical potential of the graphene layers.

Dual-band, Dynamically Tunable Plasmonic Metamaterial Absorbers Based on Graphene for Terahertz Frequencies

In this paper, a compact plasmonic metamaterial absorber for terahertz frequencies is proposed and simulated. The absorber is based on metamaterial graphene structures, and benefits from dynamically controllable properties of graphene. Through patterning graphene layers, plasmonic resonances are tailored to provide a dual band as well as an improved bandwidth absorption. Unit cell of the designed structure is made of four complementary square rings, on a thin grounded SiO2 layer of 5 µm thickness. Four splits are included in the square rings to provide continuity of graphene layer. Dual band absorption of 90% is provided, which frequency of peak absorption increases with increasing chemical potential of graphene layer. It is shown that with varying dimensions of the split rings an improved bandwidth absorber is also achieved, where absorption band increases with increasing graphene's chemical potential either. To better understand excitation of plasmonic resonances on the proposed structure, electric field distribution on the graphene layer as well as at the unit cell’s cross section is investigated and graphically demonstrated. Dependence of absorption on incidence and polarization angles of the incoming wave is studied and also graphically presented.

Thermal emission of one-dimensional conjugated photonic crystals heterojunction embedded with graphene

By means of characteristic matrix method, we make a theoretical investigation on the properties of thermal emission from one-dimensional conjugated photonic crystals heterojunction embedded with graphene. We find that the emissivity plots show sharp spectral peaks and narrow antenna-like angular lobes which show the excellent temporal coherence and spatial coherence. By optimizing the parameters, a close-to-unity emissivity can be achieved in our proposed structure. Furthermore, we demonstrate that the emissivity peak wavelength and emissivity values can be tuned by changing the chemical potential of graphene layer and the center wavelength of conjugated photonic crystals. The study shows that our proposed structure may find great application in infrared spectrometers and infrared gas sensors.

Transmission spectra in graphene-based octonacci one-dimensional photonic quasicrystals

In this work, we have studied theoretically the optical transmissivity spectra in graphene-based quasiperiodic dielectric multilayers made of SiO2 and TiO2 which are juxtaposed in accordance with the Octonacci sequence, with a graphene monolayer between them. Our results show that all the optical spectra are affected, and their band gaps are slightly displaced for high frequencies. We also show that the properties of fractality and self-similarity of the spectra are maintained for high frequencies. We have also analyzed the influence of the incident angle and the chemical potential, and shown that when one changes the chemical potential, there is not an appreciable difference between TE and TM transmission spectra for angles close to the normal incidence. The absorption is also calculated as a function of the frequency, and is shown that sharp peaks will appear at the edges of the allowed band frequencies when we increase the Octonacci generation number. Our model can be used in the study of polarizing filters based on photonic crystals, in order to perform an angular selectivity in the incident light. Also, it is possible to tune the chemical potential of graphene layers and set up an angular selective filter using these phenomena.

Characterization of optomechanical modes in multilayer stack of graphene sheets

Abstract

Propagation characteristics of surface plasmon polariton modes in graphene layer with nonlinear magnetic cladding

We study the dispersion characteristics of surface plasmon polariton modes guided through a graphene monolayer bounded with a nonlinear magnetic cladding and linear substrate. Nonlinear cladding with permeability provides an extra hand for controlling guided mode behavior externally. The presence of graphene layer enhances nonlinearity in the waveguide configuration thereby changing position of the self-focused peak of field components in the nonlinear medium. Also the propagation length of the fundamental mode strongly depends on the chemical potential of graphene layer. An appreciable increase in propagation length with increase in input power is observed. Phase constant and propagation length of the fundamental mode are calculated as a function of input mode power and graphene layer chemical potential over midinfrared frequencies.

Tunable Slow Light in Graphene Metamaterial in a Broad Terahertz Range

A graphene-based metamaterial with tunable electromagnetically induced transparency is numerically studied in this paper. The proposed structure consists of a graphene layer composed of H shape between two cut wires, by breaking symmetry can control EIT-like effects and by increasing the asymmetry in the structure has strong coupling between elements. It is important that the peak frequency of transmission window can be dynamically controlled over a broad frequency range by varying the chemical potential of graphene layer. The results show that high refractive index sensitivity and figure of merit can be achieved in asymmetrical structures which is good for sensing applications. We calculated the group delay and the results show we can control the group velocity by varying the S parameter in asymmetrical structure. The characteristics of our system indicate important potential applications in integrated optical circuits such as optical storage, ultrafast plasmonic switches, high performance filters, and slow-light devices.

Directional thermal emission control by coupling between guided mode resonances and tunable plasmons in multilayered graphene

Directional thermal radiation is attracting interest because of its applicability to thermal management systems and mid-infrared light sources. Plasmons in a single graphene layer are tunable by the chemical potential, while the lateral wavenumber of the plasmon dispersion is too large for the directional coupling to the far field. In this paper, we achieve directional thermal radiation by utilizing tunable plasmons in multilayered graphene. The lateral wavenumber of the plasmon is shown to be reduced as the number of graphene layers increases, and the reduction is analytically explained. The thermally excited graphene plasmon couples to the guided mode resonance in a silicon grating through evanescent waves so as to realize angular-selective far-field emission. We develop a modal analysis in order to investigate the coupling condition. In addition, the directional thermal emission including asymmetric one can be tuned by varying the chemical potential of graphene layers. The calculated emissivity obtained by changing both the chemical potential and the height of the grating suggests the control of the angular heat flux profile.

Magnetic field control of plasmon polaritons in graphene-covered gyrotropic planar waveguide.

In this Letter, we report about magnetic field switching of plasmon polaritons propagating into a planar gyrotropic waveguide covered by two graphene layers at a deeply subwavelength scale. It is shown that applying an external magnetic field may lead to energy redistribution between two waveguide surfaces. The effect value resonantly depends on the relation between waveguide size and exciting light wavelength. A change in chemical potential of graphene layers may be used for tuning the phase shift between plasmon polaritons at near-resonant wavelengths. Evident effect may be observed at low magnetic fields (less than one tesla) for wavelengths about microns on a scale of tens of nanometers. Such an effect may be used for plasmonics, photonics. and optoelectronics devices, as well as sensing applications.

Influence of graphene coating on speckle-pattern rotation of light in gyrotropic optical fiber.

In the present work, change in speckle-pattern of linearly polarized light passed through graphene-covered optical fiber placed in external magnetic field is investigated. The possibility of magnetic speckle-pattern rotation suppression and inverse speckle-pattern rotation effect is shown. This effect can be controlled by a chemical potential of graphene layer, which can be changed easily by a gate voltage, for example. For quartz optical fiber at wavelength 0.633μm, core diameter 9μm, and fiber length 5cm, an inverse rotation value of 17° is reached at chemical potential of graphene layer about 1eV and magnetic field strength 30kOe. Results of the work may be useful for different magneto-optics, opto-electronics, and photonics applications.

A Graphene-Based Hybrid Plasmonic Waveguide With Ultra-Deep Subwavelength Confinement

Reduction of propagation loss of terahertz graphene plasmon can be made by increasing the chemical potential of graphene layer, but at the cost of significantly increased modal area, which fundamentally limits the packing density on a chip. By utilizing the strong coupling between the dielectric waveguide and plasmonic modes, we propose hybrid plasmonic terahertz waveguides that not only significantly suppress the mode field confinement, but also maintain a compact modal size. A typical propagation length is 127 μm, and optical field is confined into an ultra-small area of approximately 32.6 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 3 THz. This structure also exhibits ultra-low crosstalk, which shows great promise for constructing various functional devices in future terahertz integrated circuits.