A Study on the Dynamic Tunning Range of CVD Graphene at Microwave Frequency: Determination, Prediction and Application.

In recent years, graphene has shown great application prospects in tunable microwave devices due to its tunable conductivity. However, the electromagnetic (EM) properties of graphene, especially the dynamic tunning characteristics, are largely dependent on experimental results, and thus are unable to be effectively predicted according to growth parameters, which causes great difficulties in the design of graphene-based tunable microwave devices. In this work, we systematically explored the impact of chemical vapor deposition (CVD) parameters on the dynamic tunning range of graphene. Firstly, through improving the existing waveguide method, the dynamic tunning range of graphene can be measured more accurately. Secondly, a direct mathematical model between growth parameters and the tunning range of graphene is established. Through this, one can easily obtain needed growth parameters for the desired tunning range of graphene. As a verification, a frequency tunable absorber prototype is designed and tested. The good agreement between simulation and experimental results shows the reliability of our mathematic model in the rapid design of graphene-based tunable microwave devices.

Stretchable and self-healable spoof plasmonic meta-waveguide for wearable wireless communication system.

Microwave transmission lines in wearable systems are easily damaged after frequent mechanical deformation, posing a severe threat to wireless communication. Here, we report a new strategy to achieve stretchable microwave transmission lines with superior reliability and durability by integrating a self-healable elastomer with serpentine-geometry plasmonic meta-waveguide to support the spoof surface plasmon polariton (SSPP). After mechanical damage, the self-healable elastomer can autonomously repair itself to maintain the electromagnetic performance and mechanical strength. Meanwhile, the specially designed SSPP structure exhibits excellent stability and damage resistance. Even if the self-healing process has not been completed or the eventual repair effect is not ideal, the spoof plasmonic meta-waveguide can still maintain reliable performance. Self-healing material enhances strength and durability, while the SSPP improves stability and gives more tolerance to the self-healing process. Our design coordinates the structural design with material synthesis to maximize the advantages of the SSPP and self-healing material, significantly improving the reliability and durability of stretchable microwave transmission lines. We also perform communication quality experiments to demonstrate the potential of the proposed meta-waveguide as interconnects in future body area network systems.

Large angle beam steering THz antenna using active frequency selective surface based on hybrid graphene-gold structure.

Numerous studies have been made to design pattern reconfigurable THz antennas to achieve optimum performance for particular environmental conditions. However, it is still a challenge to achieve large angle beam steering for reconfigurable antenna in terahertz band. Here we propose a 360-degree beam steering THz antenna using active frequency selective surface (AFSS) based on hybrid graphene-gold structure. The proposed antenna consists of a THz omnidirectional monopole antenna coated with a hexagonal AFSS screen. By adjusting the chemical potential of graphene from 0 to 0.5eV, the AFSS unit cell can be switched from ON state (high transmission) to OFF state (total reflection) in terahertz, which can steer the beam direction as the monopole antenna is surrounded with six parts of AFSS screen with different ON/OFF states. In this way, the antenna can achieve beam scanning covering 360 degrees. Moreover, unlike the conventional AFSS with only two states, the reflection and transmission coefficient of the proposed AFSS are continuously variable due to the tunable chemical potential, which allows the radiation gain of antenna to be enlarged or suppressed. This antenna may serve the reconfigurable THz wireless system with flexible beam direction and gain level.

Enhanced Bandwidth of High Directive Emission Fabry-Perot Resonator Antenna with Tapered Near-Zero Effective Index Using Metasurface.

In this paper, a novel explanation on high directive emission of Fabry-Perot resonator antenna with subwavelength metasurface is proposed. Based on image theory and effective constitutive parameter retrieval, the whole Fabry-Perot resonant cavity structure composed of a single-layer metasurface with square ring element and a PEC ground plate can be acted as an effective metamaterial media with very low refractive index (near zero index). According to Snell's theory, this property can be used to enhance the directive emission. Based on this, with tapered size square ring unitcell, the overlapped bandwidth in which the effective refractive index is near to zero is obtained to widen the bandwidth of high directive emission. It is demonstrated that the maximum of directivity is nearly approaching to 19 dBi, and its 3-dB bandwidth can be improved to 19.5%. A final prototype has been fabricated and measured to validate the proposed design concept. The measured 3-dB gain bandwidth is approximately 20.3% with a peak gain of 17.9 dBi. These results indicate the feasibility of such kind of antenna for broadband and high directivity applications simultaneously.

Waveguide-coupled hybrid plasmonic modulator based on graphene.

In this paper, we propose a low-transmission-loss, high-speed, graphene-based electro-absorption modulator with a hybrid plasmonic waveguide at 1.55 µm. In the proposed device, double-layer graphene is placed on top of the horizontal hybrid plasmonic waveguide to enhance the light-graphene interaction. The adjustment of the in-plane permittivity of the anisotropy graphene causes a significant modulation of the absorption at the operating bandwidth of 0.4 THz, with modulation length of 8.5 µm and modulator footprint of 1.6 µm2. A taper silicon coupler is used for waveguide coupling, and 80% coupling efficiency is achieved. In addition, the modulation potential on a smaller footprint is further shown.

Flexible transformation plasmonics using graphene.

The flexible control of surface plasmon polaritons (SPPs) is important and intriguing due to its wide application in novel plasmonic devices. Transformation optics (TO) offers the capability either to confine the SPP propagation on rigid curved/uneven surfaces, or to control the flow of SPPs on planar surfaces. However, TO has not permitted us to confine, manipulate, and control SPP waves on flexible curved surfaces. Here, we propose to confine and freely control flexible SPPs using TO and graphene. We show that SPP waves can be naturally confined and propagate on curved or uneven graphene surfaces with little bending and radiation losses, and the confined SPPs are further manipulated and controlled using TO. Flexible plasmonic devices are presented, including the bending waveguides, wave splitter, and Luneburg lens on curved surfaces. Together with the intrinsic flexibility, graphene can be served as a good platform for flexible transformation plasmonics.

A compact structure for energy localization using a thin grounded left-handed medium slab.

A new compact structure is presented in this paper to localize electromagnetic energies using a thin grounded left-handed medium (LHM) slab. For a perfectly-matched LHM slab with negative permittivity -epsilon0 and negative permeability - micro0 backed with a conducting plane, we have shown rigorously that all electromagnetic fields excited by a current source, which is located in front of the slab at a distance of the slab thickness, are completely confined in a region between the source and the conducting plane, and the fields outside the region are zero. Hence, it is an ideal energylocalization system, and the electromagnetic energies can be localized in any small regions as required using such a system. However, it has been known that the perfectly matched LHM is unphysical and it does not exist in nature. Hence, we have further studied the lossy and retardation effects of the LHM slab on the energy localization. Most remarkably, electromagnetic waves remain strongly localized even when small losses are taken into account, as demonstrated by numerical simulations.