Graphene-based THz Research Articles

High radiation efficiency of coupled plasmonic graphene-based THz patch antenna utilizing strip slot ground plane removal

This paper proposes an enhancement radiation efficiency method of graphene-based patch antenna using partially strip slot ground plane removal. Graphene as a one-atom thickness (2D) supports surface plasmon polariton (SPP) of considerably decreasing wave length in comparing to the free space one. Based on this fact yields efficiency of graphene antennas is degraded. The effect of strip line ground plane removal reduces the losses due to dielectric substrate and surface waves. The proposed design has reached up to 90% at both radiation and total efficiency. Also due to using graphene, it can provide tunable conductivity derived from variable parameters. The graphene-based patch THz antenna is modeled to achieve the optimum parameter values.

A novel analytical method for designing a multi-band, polarization-insensitive and wide angle graphene-based THz absorber

In this paper, we propose a simple and analytical method to design a multi-band tunable graphene-based absorber using circuit model. The presented paper provides a flowchart and equations to design a multi-band absorber which tremendously simplifies the design procedure of a multi-band absorber. This paper only investigates the disk-shaped graphene-based absorber, but we believe the proposed method is extendable to all presented shapes of graphene which previously designers provided with a circuit model. To verify the accuracy of the presented method; we have designed several multi-band absorbers and used full-wave numerical modeling of the structure to approve our method. Also, this paper studies the reliability of our proposed model by investigating the fabrication sensitivity of the structure. Lastly, this paper investigates to see how a multi-band absorption spectra behave, in case there are any errors during the fabrication phase. The designed absorbers have potential applications in various fields of active THz metamaterials, tunable filters, and sensors. The graphene-based disk-shaped absorber has the advantage of polarization insensitivity, wide-angle absorption and ease of implementation due to its simple shape. Another advantage of using graphene in the absorber structure is that with the availability of changing the fermi energy level, it is possible to tune the absorption frequency of the multi-band absorbers.

Graphene-Based Multi-Beam Reconfigurable THz Antennas

Several configurations of multi-beam reconfigurable THz antennas based on graphene have been investigated. Two modulation mechanisms of graphene-based THz antenna are introduced, one is the reflector-transmission window model, and the other is the reflector-director model (Yagi-Uda antenna). The main parameters, such as main beam direction, resonance frequency, peak gain, and the front-to-back ratio of the proposed antenna can be controlled by adjusting the chemical potentials of the graphene in the antenna. Moreover, this paper provides an easy way to obtain complex graphene-based multi-beam antennas, showing strong potential in the design of other complex graphene-based systems, enabling nanoscale wireless communications and sensing devices for different applications.

Equivalent Resonant Circuit Modeling of a Graphene-Based Bowtie Antenna

The resonance performance analysis of graphene antennas is a challenging problem for full-wave electromagnetic simulators due to the trade-off between the computer resource and the accuracy of results. In this paper, an equivalent circuit model is presented to provide a concise and fast way to analyze the graphene-based THz bowtie antenna. Based on the simulated results of the frequency responses of the antenna, a suitable equivalent circuit of Resistor-Inductor-Capacitor (RLC) series is proposed to describe the antenna. Then the RLC parameters are extracted by considering the graphene bowtie antenna as a one-port resonator. Parametric analyses, including chemical potential, arm length, relaxation time, and substrate thickness, are presented based on the proposed equivalent circuit model. Antenna input resistance R is a significant parameter in this model. Validation is performed by comparing the calculated R values with the ones from full-wave simulation. By applying different parameters to the graphene bowtie antenna, a set of R, L, and C values are obtained and analyzed comprehensively. A very good agreement is observed between the equivalent model and the numerical simulation. This work sheds light on the graphene-based bowtie antenna’s initial design and paves the way for future research and applications.

A Dual-Band Tunable Metamaterial Near-Unity Absorber Composed of Periodic Cross and Disk Graphene Arrays

A three-dimensional dual-band perfect absorber in the THz range (3–10 THz) is theoretically investigated. The absorber is composed of periodically placed cross- and disk-shaped graphene arrays on the top of a gold layer separated by a thick SiO $_{2}$ dielectric spacer. The simulated results show two absorption peaks with near-unity absorbance ( $99.923\%$ and $99.601\%$ ) at wavelengths 43.747 $\mu$ m and 69.94 $\mu$ m, respectively. Also, the absorber is insensitive to the polarization of the incident light and has a good tolerance to the incident angle whether it is TE or TM waves. Moreover, the peak wavelengths of the absorber can be flexibly modulated by varying the Fermi level $\mu _{C}$ of graphene while no need to refabricate the structure. This paper provides a new perspective for the design of graphene-based tunable multiband perfect THz absorbers, but not limited to THz absorbers, and may also be used in other THz graphene-based photonic devices.

GRAPHENE-BASED THZ TUNABLE BANDSTOP FILTER WITH CONSTANT ABSOLUTE BANDWIDTH

GRAPHENE-BASED THZ TUNABLE BANDSTOP FILTER WITH CONSTANT ABSOLUTE BANDWIDTH

Family of graphene-assisted resonant surface optical excitations for terahertz devices.

The majority of the proposed graphene-based THz devices consist of a metamaterial that can optically interact with graphene. This coupled graphene-metamaterial system gives rise to a family of resonant modes such as the surface plasmon polariton (SPP) modes of graphene, the geometrically induced SPPs, also known as the spoof SPP modes, and the Fabry-Perot (FP) modes. In the literature, these modes are usually considered separately as if each could only exist in one structure. By contrast, in this paper, we show that even in a simple metamaterial structure such as a one-dimensional (1D) metallic slit grating, these modes all exist and can potentially interact with each other. A graphene SPP-based THz device is also fabricated and measured. Despite the high scattering rate, the effective SPP resonances can still be observed and show a consistent trend between the effective frequency and the grating period, as predicted by the theory. We also find that the excitation of the graphene SPP mode is most efficient in the terahertz spectral region due to the Drude conductivity of graphene in this spectral region.

A Graphene-Based THz Ring Resonator for Label-Free Sensing

In this paper, we report on a novel resonant THz sensor for label-free analysis. The structure consists of a silicon nitride dielectric ring resonator vertically coupled to a thin layer of graphene strip ring resonator on top. The cladding is assumed to be porous alumina on the top of the graphene strip, which enhances the interaction of the surface plasmon wave and the target molecules. Finite difference time domain analysis is used to systematically design the structure and to investigate the performance of the sensor. Our simulations show that the proposed structure has larger refractive index sensitivity and lower intrinsic quality factor with respect to the similar optical structure. It is also shown that the overall performance of the proposed structure in terms of figures of merit is superior to the performance of a similar optical structure. The proposed structure can be used for lab-on-a-chip sensing applications due to its compactness and is capable of being spectrally or spatially multiplexed for multi-analytic sensing.

A Reconfigurable Substrate–Superstrate Graphene-Based Leaky-Wave THz Antenna

A planar substrate-superstrate configuration loaded by a graphene sheet is proposed as a novel reconfigurable antenna for terahertz applications. The effects of the introduction of graphene on the dispersion and radiation properties of the fundamental leaky modes supported by the structure are analyzed in detail, with the aim of investigating the possibility of beam scanning at a fixed frequency by varying the graphene chemical potential via an electrostatic bias. After optimizing the graphene position inside the antenna substrate for achieving maximum directivity at broadside, large scanning ranges are obtained in which the normalized attenuation constants of the fundamental TE/TM leaky-mode pair exhibit small values and reduced variation, thus producing a scanning process with an essentially constant narrow beamwidth. In the same ranges, the phase constants of the TE/TM-mode pair are shown to be equalized, thus opening the possibility of designing fixed-frequency scanning antennas with conical beams and reconfigurable dual polarization.

Towards a tunable graphene-based Landau level laser in the terahertz regime.

Terahertz (THz) technology has attracted enormous interest with conceivable applications ranging from basic science to advanced technology. One of the main challenges remains the realization of a well controlled and easily tunable THz source. Here, we predict the occurrence of a long-lived population inversion in Landau-quantized graphene (i.e. graphene in an external magnetic field) suggesting the design of tunable THz Landau level lasers. The unconventional non-equidistant quantization in graphene offers optimal conditions to overcome the counteracting Coulomb- and phonon-assisted scattering channels. In addition to the tunability of the laser frequency, we show that also the polarization of the emitted light can be controlled. Based on our microscopic insights into the underlying many-particle mechanisms, we propose two different experimentally realizable schemes to design tunable graphene-based THz Landau level lasers.

Ultrafast graphene-based broadband THz detector

We present an ultrafast graphene-based detector, working in the THz range at room temperature. A logarithmic-periodic antenna is coupled to a graphene flake that is produced by exfoliation on SiO2. The detector was characterized with the free-electron laser FELBE for wavelengths from 8 μm to 220 μm. The detector rise time is 50 ps in the wavelength range from 30 μm to 220 μm. Autocorrelation measurements exploiting the nonlinear photocurrent response at high intensities reveal an intrinsic response time below 10 ps. This detector has a high potential for characterizing temporal overlaps, e.g., in two-color pump-probe experiments.

Tunable graphene antennas for selective enhancement of THz-emission

In this paper, we will introduce THz graphene antennas that strongly enhance the emission rate of quantum systems at specific frequencies. The tunability of these antennas can be used to selectively enhance individual spectral features. We will show as an example that any weak transition in the spectrum of coronene can become the dominant contribution. This selective and tunable enhancement establishes a new class of graphene-based THz devices, which will find applications in sensors, novel light sources, spectroscopy, and quantum communication devices.

The study of negative THz conductivity of graphene under the phonon scattering mechanism

The variety of optical and electronic properties of graphene attracts enormous interests. The relaxation and recombination mechanism of photogenerated electrons and holes in undoped monolayer graphene are studied with the changing pump intensities. The population inversion in graphene can lead to the negative AC conductivity in THz spectral range. And the conductivity depends on the carrier density, the carrier distribution in energy as well as the effective temperature. It indicates that the negative dynamic conductivity is associated with the THz emission and can be used in graphene-based new THz lasers.