Xiong-Jun Shang

Actively Tunable Fano Resonance Based on a T-Shaped Graphene Nanodimer

We present the strength modulation and frequency tuning of Fano resonance by employing a graphene nanodimer formed by two coplanar perpendicular nanostrips with different dimensions. The Fano resonance is induced by destructive interference between the bright dipole mode of a short nanostrip and the dark quadrupole mode of a long nanostrip. The strength, line width, and resonance frequency of the Fano resonance can be actively modulated by changing the spatial separation of those two graphene nanostrips and the Fermi energy of the graphene nanodimer, respectively, without re-fabricating the nanostructures. The tuning of the strength and resonance frequency can be attributed to the coupling strength and optical properties of graphene, respectively. Importantly, a figure of merit value as high as 39 is achieved in the proposed nanostructures. Our results may provide potential applications in optical switching and bio-chemical sensing.

Realization of Graphene-Based Tunable Plasmon-Induced Transparency by the Dipole-Dipole Coupling

A numerical and theoretical study is presented on the realization of tunable plasmon-induced transparency (PIT) phenomenon in the three-dimensional patterned graphene nanostrips. The simulation results reveal that the PIT effect is generated due to the excitation of dark mode which can be considered a dipole. The three-level plasmonic system is employed to explain the physical mechanism of the PIT effect. Different from previous reported form (dipole-quadrupole coupling), the proposed is attributed to the dipole-dipole coupling. The PIT effect can be tuned by changing the coupling length between bright and dark mode as well as the Fermi energy of graphene. Our studies provide guidance for fabricating ultra-compact devices in practical application.

Graphene-based long-range SPP hybrid waveguide with ultra-long propagation length in mid-infrared range

A graphene-based long-range surface plasmon polariton (LRSPP) hybrid waveguide, which is composed of two identical outer graphene nanoribbons and two identical inner silica layers symmetrically placed on both sides of a silicon layer, is investigated using the finite-difference time-domain method. By combining the simulated results with the coupled mode perturbation theory, we demonstrate that the LRSPP and short-range SPP (SRSPP) modes originate from the coupling of the same modes of the two graphene nanoribbons. For the LRSPP mode, an ultra-long propagation length (~10 μm) and an ultra-small mode area (~10-7A0, where A0 is the diffraction-limited mode area) can be simultaneously achieved. This waveguide can be used for future photonic integrated circuits functional in the mid-infrared range.

Perfect spin filter and strong current polarization in carbon atomic chain with asymmetrical connecting points

The spin-dependent electron transport properties through a single-carbon atomic chain (SCAC) sandwiched between two-zigzag-graphene-nanoribbon (zGNR) electrodes are investigated by performing first-principles calculations based on the nonequilibrium Greens function (NEGF) approach in combination with spin density functional theory (DFT). Our calculations show that SCAC connecting two zGNRs with asymmetry-contacting points is a perfect spin filter in the transmission function within a large energy range. Moreover, the spin-dependent electron transmission spectra exhibit robust transport polarization characteristics and a strong current polarization behavior (almost 100%) can be found. The microscopic mechanisms are proposed for the spin-related phenomena.

Plasmon-Induced Transparency in a Surface Plasmon Polariton Waveguide with a Right-Angled Slot and Rectangle Cavity

The phenomenon of plasmon-induced transparency (PIT) is realized a in surface plasmon polariton waveguide at near-infrared frequencies. The right-angled slot and rectangle cavity placed inside one of the metallic claddings are respectively utilized to obtain bright and dark modes in a typical bright-dark mode waveguide. A PIT transmission spectrum of the waveguide is generated due to the destructive interference between the bright and dark modes, and the induced transparency peak can be manipulated by adjusting the size of the bright and dark resonators and the coupling distance between them. Subsequently, spectral splitting based on the PIT structure is studied numerically and analytically. Simulation results indicate that double electromagnetically induced transparency (EIT)-like peaks emerge in the broadband transmission spectrum by adding another rectangle cavity, and the corresponding physical mechanism is presented. Our novel plasmonic structure and the findings pave the way for new design and engineering of highly integrated optical circuit such as nanoscale optical switching, nanosensor, and wavelength-selecting nanostructure.

Broad-band and high-efficiency polarization converters around 1550 nm based on composite structures

Broad-band and high-efficiency polarization converter is an imperative component in communication systems, but its functionality often clashes with the constraint of materials. Herein we theoretically and numerically demonstrate that a broad-band and high-efficiency 90° polarization rotator around 1550 nm can be realized using an ultrathin and geometry-optimized composite structure. Based on simulation results, the reflection efficiency and operation bandwidth is up to ≈80% and ≈300 nm, respectively, for the 90° polarization rotator. With similar concept, we also demonstrate a quarter-wave plate with an efficiency of 94% and bandwidth of 110 nm. The electric filed distribution indicates that the conversion behaviors are caused by the strong magnetic coupling in the designed composite structure. Furthermore, the polarization ellipticity properties are investigated to further understand the broad-band effect of the proposed polarization convertors.

Asymmetric transmission and polarization conversion of linearly polarized waves with bilayer L-shaped metasurfaces

We numerically and theoretically investigate the optical anisotropy of ultra-thin bilayer L-shaped metal metasurfaces separated by a 200-nm-thick silicon dioxide (SiO2) substrate spacer. A broadband asymmetric transmission (AT) to forward and backward propagate electromagnetic waves can be acquired from linearly polarized waves. Additionally, narrowband cross-polarization conversion (CPC) can be realized by x linearly polarized electromagnetic illumination. The calculated results demonstrate that the full width at half maximum (FWHM) of AT is 965 nm, and the maximum value of the asymmetric parameter can reach up to 0.48. The polarization conversion rate (PCR) for CPC is more than 80%.

Numerical Investigation of a Tunable Fano-Like Resonance in the Hybrid Construction Between Graphene Nanorings and Graphene Grating

A tunable Fano-like resonance in the hybrid construction between graphene nanorings and graphene grating is proposed. The simulation results reveal that the Fano-like resonance is generated between two bright modes, which are different from the previous studies of coupling between the plasmonics bright and dark modes. The adjustability of this Fano resonance is simply researched by changing the geometry parameters of the composite structure and the Fermi-energy of graphene. Also, the dual-Fano resonance can be realized by only decreasing the thickness of the substrate spacer without employing extra resonance mode.

Multi-spectral plasmon induced transparency based on three-dimensional metamaterials

We theoretically and numerically demonstrate multi-spectral plasmon induced transparency (PIT) in three-dimensional metamaterials comprising of parallel nanorods and a vertical nanorod. By moving the vertical middle nanorod to break the structural symmetry, the structure presents single-spectral or dual-spectral PIT windows, while it exhibits multi-spectral PIT windows at the laser wavelengths with He-Ne, ruby Cr3+ and Kr, via moving the bottom nanorod. The quasi-static interaction model reveals that the near-field coupling strength between nanorods increases with the movement of the nanorod. The coupling strength between nanorods is enhanced, which makes vertical middle nanorod support dipole and quadrupole modes and further results in multi-spectral PIT by bright-dark mode coupling. This work provides a way to obtain multi-spectral PIT in an easily fabricable nanostructure, and it may achieve potential applications in a variety of fields including filters, sensing, and some other nanoplasmonic functional devices.

Plasmon-induced multilevel-transparency in two-dimensional hybrid coplanar waveguide

The optical transmission property of a hybrid coplanar waveguide consisting of three quarters of a nanoring (TQNR) and a slot cavity resonator is numerically investigated and theoretically analyzed. In this paper, the apparent multilevel plasmon-induced transparency (PIT) effect can be obtained due to the interaction between the resonance modes of the two elements. Combining the calculated magnetic field distribution with the theoretically fitted parameters, the transparency windows of all resonance modes can be clearly investigated. The results show that the second-order transparency window originates from the destructive interference between the bright and dark mode of the hybrid system, while the first- and third-order transparency windows originate from the suppression effect of the dark mode. As the assessment standard for application, the maximal values of appear at the transmission dips and their highest reaches to near 18. While the reaches to an impressive value 270 at the third-order transparent window, and the sensitivity is as high as 2650 nm RIU−1 at the first-order transparent window. This research provides a guide to the practical applications in the visible and near-infrared light region.