Graphene-dielectric Structure Research Articles

Sensitivity enhancement of a silver based surface plasmon resonance sensor via an optimizing graphene-dielectric composite structure.

A silver based surface plasmon resonance (SPR) sensor with dielectric-graphene composite film is presented. The influences of the dielectric layer and graphene on sensitivity and other sensing properties are theoretically calculated and then comprehensively discussed. The refractive index sensitivities for composite silver film based SPR sensors with graphene and dielectric layers could be increased by 29% and 288% more than that of monolayer silver film based SPR sensors, respectively. Further, the sensitivity could be enhanced by 202% when combining the graphene and dielectric layers together. Considering the high adsorptive capacity of graphene for biochemical molecules, the composite silver film with both a dielectric layer and graphene would have great potential application in biochemical sensing fields. Further, bovine serum albumin protein was successfully used to verify the biochemical sensing ability of the proposed SPR sensor. The shift of resonance angle is nearly 3.1 fold that of monolayer silver based SPR sensors.

Tunable TM modes in a slab waveguide including a graphene-dielectric multilayer structure

TM modes in a slab waveguide with a graphene-dielectric multilayer structure as the core and different ordinary materials as the cover and the substrate are investigated. The group velocity of TM modes can be positive, negative, and zero. The standstill of electromagnetic waves at different key waveguide thickness, which depend on the gate voltage, is successfully realized. The gate voltages or mode numbers have a greater influence on the zero and negative group velocities, but have a little influence on the positive group velocity. Besides, the power flow can be positive, zero and negative values at the key waveguide thickness. Similarly, the gate voltages or mode numbers have a little influence on the zero and negative power flows when the frequency is less than 7.0 THz, but they have a big influence on both positive and negative power flows when the frequency is larger than 7.0 THz. More interestingly, as the frequency increases from 9.0THz to 10.0THz, superpower flows of TM modes reach a dynamic balance both the core and the cladding. The tunable superpower flow properties will benefit us to construct a novel full optical switch and an ultra large optical power memory.

Controlling photonic spin Hall effect in graphene-dielectric structure by optical pumping

The photonic spin Hall effect (SHE) provides an effective way to manipulate the spin-polarized photons. However, the spin-dependent splitting is very tiny due to the weak spin–orbit coupling, and previous investigations for enhancing this phenomenon have some serious limitations (e.g. inconvenient to tune, inadequate attention in terahertz region). Therefore, controlling and enhancing the photonic SHE in a flexible way is highly desirable, especially for terahertz region. In this contribution, we propose a method to manipulate the photonic SHE by taking advantage of tunable optical properties of graphene via weak optical pumping. We find that photonic SHE of graphene-dielectric structure in terahertz region is quite sensitive to the pumping power. The spin shift for H polarized incident beam can reach its upper limitation under the optimal pumping power, which is related to the zero value of the real part of graphene conductivity. These findings may provide a new degree of freedom for the design of tunable spin-based photonic devices in the future.

Polarization conversion and phase modulation of terahertz electromagnetic waves via graphene-dielectric structure

In this paper, by using the 4 × 4 transfer matrix approach, the reflection and transmission of circularly polarized (CP) waves which are obliquely incident to the periodic structure comprising of graphene-dielectric layers are investigated. The results indicate that by applying a magnetic field in appropriate direction there exists an omni-directional band gap in the transmission spectrum either left or right CP wave. The given structure can convert the linearly polarized wave to CP wave in reflected and transmitted waves. Furthermore, the dependence of the ellipticity of the reflected and transmitted waves to the applied magnetic field and the chemical potential of the graphene is studied. Such structures can be used as a polarization converter and phase modulator in the terahertz frequency range.

Interference of Counterpropagating Waves in a “Graphene–Dielectric” Planar Lamellar Structure

The incidence of two coherent waves traveling in opposite directions on opposite surfaces of a planar layered-periodic graphene–dielectric structure has been investigated. It has been shown in the long-wave approximation that it is possible to control the intensity of the outgoing waves and absorption of radiation in the structure by changing the phase difference of the waves that are incident on the structure. There are close to optimal conditions for the intensity modulation effect in the terahertz frequency domain and modulation of the absorption capacity of the structure is possible in the near infrared and visible ranges.

TM plasmonic modes in a multilayer graphene-dielectric structure

Optical and electronic properties of multilayer systems have been extensively studied in the last years due to its potential applications in high-performance optoelectronic and photonic devices. In particular, the role of plasmonic modes is critical in such systems leading to improvements in solar cells efficiency, detection of biosensors, Raman signal enhancement, among others. In this work, we study the plasmonic modes in a multilayer system composed of graphene layers embedded within dielectric materials. The dispersion relation of plasmonic modes is obtained by calculating the poles of reflectivity using the transfer matrix method. We show the attenuated total reflection spectra for a multilayer graphene-dielectric structure, and determine the optimum distance between the prism and the multilayer system for detecting graphene plasmons in the Otto configuration. Additional to the well-known plasmonics bands, when we consider the interband and intraband contribution of graphene's conductivity, and large wavevectors parallel to graphene's plane, all plasmonic bands have an asymptotic behavior. Besides, an upper mode emerges. Finally, it is important to highlight that the number of branches in the plasmonic relation dispersion depend on the number of graphene sheets.

Faraday effect control in graphene-dielectric structure by optical pumping

Abstract At the terahertz frequency range, 2D material graphene is the most promising candidate for using as a functional part of magnetooptical devices because of discovered giant Faraday rotation and ability to dynamical properties control. In this paper magnetooptical properties of graphene were investigated. Numerical analysis on graphene-dielectric structure have shown the tunability of Faraday rotation and ellipticity in terms of amplitude and frequency by optical pumping of different radiation power and wavelength. Our work demonstrates alternative way for developing ultrafast optically tunable polarization modulators of THz wave.

Tunable strong THz absorption assisted by graphene-dielectric stacking structure

The harmonic oscillator model is used in combination with the transfer matrix method to describe the absorption of plasmonic devices with a reflector. The model demonstrates that complete absorption takes place as a result of the destructive interference between the reflection from the device surface and that from the reflector. In contrast, destructive interference between the transmission and the reflection results in zero absorption. Using a metal back reflector, the absorption of a resonance mode can be tuned from complete absorption to zero absorption by changing the Fermi level of graphene, which is the result of the shifting between the two kinds of destructive interference. A stacked graphene-dielectric structure is used to solve the problem as it can create a plasmonic bandgap within the terahertz region. The phase of the reflection can be actively tuned by changing the Fermi level of the graphene in the stacking structure. Therefore, plasmonic devices with a stacked graphene-dielectric back reflector can always show strong absorption by tuning the Fermi level of the top graphene.

Switchable hyperbolic metamaterials based on the graphene-dielectric stacking structure and optical switches design

We numerically demonstrate broadband optical switches which can control the propagation of mid-infrared waves that is incident to the interface between a switchable hyperbolic metamaterials (SHM) and air. The SHM consists of a one-dimensional periodic stacking of graphene layers and dielectric layers. The isofrequency curve of the structure can be switched between hyperbolic shape and elliptical shape by controlling the external gate voltage or the electrostatic field biasing. It is revealed that when the interface between air and the SHM is parallel to the graphene sheets, it can switch between positive refraction and total reflection; when the interface is perpendicular to the graphene sheets, it can switch between positive and negative refraction.

Long-range Tamm surface plasmons supported by graphene-dielectric metamaterials

Considering the Ohmic losses of graphene in the calculations and by obtaining exact dispersion relations of the modes, we theoretically study propagation and localization characteristics of Tamm surface plasmons supported by terminated graphene metamaterials. The metamaterials are composed of alternating layers of graphene and dielectric with subwavelength periods. We also examine the Tamm modes within the framework of long-wavelength approximation. It is shown that, in case the Ohmic losses of the graphene layers are taken into account, surface plasmons are not supported in a long-wavelength region, in which the graphene-dielectric multilayer structure behaves as a hyperbolic metamaterial. We prove that, when the metamaterial is truncated with air, by choosing sufficiently thick but still subwavelength dielectric layers, i.e., d = 300 nm, these surface waves will have a moderate propagation (localization) length that is comparable with those of a single layer of graphene. On the other hand, in case a miniaturized graphene metamaterial (10 < d(nm) < 100) is truncated by a thick cap layer (dcap = 5d) with εcap > εdielectric, it is possible to considerably improve the propagation and localization characteristics of the Tamm modes supported by the system within the 5.5–50 THz range of frequency, as compared to a single layer of graphene.

Direct growth of graphene-dielectric bi-layer structure on device substrates from Si-based polymer

To facilitate the utilization of graphene films in conventional semiconducting devices (e.g. transistors and memories) which includes an insulating layer such as gate dielectric, facile synthesis of bi-layers composed of a graphene film and an insulating layer by one-step thermal conversion will be very important. We demonstrate a simple, inexpensive, scalable and patternable process to synthesize graphene-dielectric bi-layer films from solution-processed polydimethylsiloxane (PDMS) under a Ni capping layer. This method fabricates graphene-dielectric bi-layer structure simultaneously directly on substrate by thermal conversion of PDMS without using additional graphene transfer and patterning process or formation of an expensive dielectric layer, which makes the device fabrication process much easier. The graphene-dielectric bi-layer on a conducting substrate was used in bottom-contact pentacene field-effect transistors that showed ohmic contact and small hysteresis. Our new method will provide a way to fabricate flexible electronic devices simply and inexpensively.

Photonic band gap characteristics of one-dimensional graphene-dielectric periodic structures

In this paper, we study theoretically, the transmission properties of a one-dimensional graphene-dielectric periodic structure by using the transfer matrix method. Within the framework of this method, we confirm earlier finding that a periodic structure composed of a stack of monolayer graphene sheets separated by dielectric slabs, possesses photonic band-gap (PBG) properties and supports a series of bandpass and band-stop regions at low-terahertz frequencies. Our calculations showed that the suggested structure possesses in addition to the structural Bragg gaps, a new type of band gap that exhibits a rather versatile behavior with varying angle of incidence. We find this type of band gap is omnidirectional (omni-gap) for both transverse electric (TE) and transverse magnetic (TM) polarizations. Our results show that 1D graphene-dielectric periodic structures are very good candidates for band gap engineering. Specifically, we demonstrate the existence of a band gap region for both polarizations which survives for incident angles as high as 80°. Moreover, we show how our proposed structure can also function as a highly efficient polarization splitter. It is also found that the band gaps can be tuned by tuning the properties of the graphene via a gate voltage. In order to investigate difference between the omni-gap and Bragg PBG, we plot the electromagnetic field profiles for some critical frequencies. The proposed structure is promising and can work as a gate tunable perfect stop filter which completely blocks both polarizations, and may have many other potential applications.

Tunable bulk polaritons of graphene-based hyperbolic metamaterials.

The tunable hyperbolic metamaterial (HMM) based on the graphene-dielectric layered structure at THz frequency is presented, and the surface and bulk polaritons of the graphene-based HMM are theoretically studied. It is found that the dispersions of the polaritons can be tuned by varying the Fermi energy of graphene sheets, the graphene-dielectric layers and the layer number of graphene sheets. In addition, the highly confined bulk polariton mode can be excited and is manifested in an attenuated total reflection configuration as a sharp drop in the reflectance. Such properties can be used in tunable optical reflection modulation with the assistance of bulk polaritons.

Engineered surface Bloch waves in graphene-based hyperbolic metamaterials

A kind of tunable hyperbolic metamaterial (HMM) based on the graphene-dielectric layered structure at near-infrared frequencies is presented, and the engineered surface Bloch waves between graphene-based HMM and isotropic medium are investigated. Our calculations demonstrate that the frequency and frequency range of surface Bloch waves existence can be tuned by varying the Fermi energy of graphene sheets via electrostatic biasing. Moreover, we show that the frequency range of surface Bloch waves existence can be broadened by decreasing the thickness of the dielectric in the graphene-dielectric layered structure or by increasing the layer number of graphene sheets.

Nonlinear reshaping of terahertz pulses with graphene metamaterials

We study the propagation of electromagnetic waves through a slab of graphene metamaterial composed of the layers of graphene separated by dielectric slabs. Starting from the kinetic expression for two-dimensional electric current in graphene, we derive a novel equation describing the nonlinear propagation of terahertz electromagnetic pulses through the layered graphene-dielectric structure in the presence of losses and non-linearities. We demonstrate strong nonlinearity-induced reshaping of transmitted and reflected terahertz pulses through the interaction with the thin multilayer graphene metamaterial structure.

Broadband THz Absorbers With Graphene-Based Anisotropic Metamaterial Films

We introduce a broadband THz absorber with an array of graphene-dielectric multilayered frustum pyramids on a metal sheet. The multilayered graphene-dielectric structure can be considered an effectively homogeneous metamaterial with an hyperbolic dispersion and anisotropic permittivity. Surface plasmonic waves are excited on graphene layers and the incident waves of different frequencies are absorbed at different levels of the stacked pyramid, due to the squeezing effect of the slow waves at the tapered waveguide. An absorption dip is observed and explained physically, and finally removed by adding a rectangular portion to the pyramid unit cell. High absorption with an extremely broad bandwidth from 8 THz to over 100 THz is achieved. The absorption spectrum of the present structure can be scaled down to a lower frequency (e.g. 2 THz) by increasing the size of the unit cell.