Tian Sang

Transmittance characteristics and tunable sensor performances of plasmonic graphene ribbons

We investigate the transmittance characteristics of graphene ribbons numerically. It is found that the transmission dips originate from the transverse and longitudinal resonances of edge graphene plasmon modes, supported by the graphene ribbon resonator. The environmental refractive index changes are detected by measuring the resulting spectral shifts of the resonant transmission dip, so the graphene ribbons can be applied to plasmonic sensor in infrared. Simulation results show that sensing performances for each resonant mode are similar, and figure of merit can be up to 6. Beside, thanks to the tunable permittivity of graphene by bias voltages, the transmittance spectra and sensor performances can be easily tuned.

Tunable Plasmon-Induced Transparency Effect in MIM Side-Coupled Isosceles Trapezoid Cavities System

We propose a plasmonic structure based on the metal-insulator-metal waveguide with the side-coupled isosceles trapezoid cavities. Both of the structures based on the side-coupled trapezoid cavities separated or connected with waveguides can realize the plasmon-induced transparency (PIT). By adjusting the structure parameters, the off-to-on PIT response can be tunably achieved. The coupled mode theory (CMT) method is used to study the PIT phenomenon and explain the transmission characteristics. This work may provide a potential way for designing highly integrated photonic devices.

Plasmonic Planar Lens Based on Slanted Nanoslit Array

The novel plasmonic lenses based on slanted nanoslits have been proposed theoretically. The slanted nanoslits with different slant angles can provide unequal propagation distances for the surface plasmon polaritons excited by incident light. The phase retardation for wavefront shaping can be obtained to realize constructive interference on a preset single spot. We can actively modulate the position of the optical focus by adjusting the slits slant angles properly. The simulation results of the finite element method are used to verify our proposals.

Transmission characteristics and transmission line model of a metal-insulator-metal waveguide with a stub modified by cuts.

We propose a structure of a metal-insulator-metal (MIM) waveguide with a stub modified by cuts. Our simulation results, conducted by the finite element method, show that the wavelengths of transmission dip vary with the position of the cuts and form the zigzag lines. A transmission line model is also presented, and it agrees with simulation results well. It is believed that our findings provide a smart way to design a plasmonic waveguide filter at the communication region based on MIM structures.

Tunable multiple channeled phenomena in graphene-based plasmonic Bragg reflectors

Plasmonic Bragg reflectors based on graphene with multiple channeled phenomena are proposed and investigated numerically. As a mid-infrared waveguide, the monolayer graphene exhibits locally variable optical properties through the modulation of electric fields. The periodical change of the effective refractive index (ERI) on graphene can be determined by applying external gate voltage. When we introduce an unmatched configuration or gate voltage, periodicity is disrupted, and a defect resonance mode is generated. At this point, the structure can be regard as a Fabry-Perot cavity. Accordingly, multiple-channel effects can be achieved by introducing cascaded multiple defects or including double symmetrical Fabry-Perot structures. This design shows applications potential in the graphene-based optoelectronic devices, particularly in the development of low-cost hyperspectral imaging sensors in mid-infrared region.

Scattering-Suppressed Plasmonic Bends and Adapters with Gradient Refractive Index Medium

We propose the designs of plasmonic bends and adapters with low scattering loss in visible region theoretically. Tens of nanometers thick gradient refractive index medium is deposited on the metallic surface, which can confine and release the surface plasmon polaritons (SPPs). When SPPs can be strongly confined the metallic surface and propagate along the corners of the plasmonics devices, the scattering loss can be dramatically suppressed. Full wave simulations based on a finite element method have been performed to validate our proposal. Compared with the same class of design, our method can be achieved only with isotropic materials.

High-sensitive transmission type of gas sensor based on guided-mode resonance in coupled gratings

Abstract A new type of high-sensitive transmission gas sensor based on the coupled gratings (CGs) and the corresponding Fabry–Pérot-like (FP-like) model for evaluating the resonance peaks are presented. The estimated locations of the FP-like resonance obtained by this theoretical model are well agreed with those of the exact results. It is shown that a narrow FP-like channel with high transmissivity occurs in the opaque background of the CGs, and its location is shifted linearly with the variation of the refractive index (RI) of the gaseous analyte. The transmission peak of the sideband can be selected as a reference, and it remains nearly fixed as the RI of the analyte is varied. Good sensing properties of the CGs sensor can be maintained, regardless of whether the two grating membranes are laterally aligned or not. The sensitivity of the CGs sensor is immune to the variation of the RI of the substrate. By selecting the higher order FP-like mode (m = 4), sensitivity as high as 748 nm/RIU with the figure of merit of 374 can be achieved.

Transmittance characteristics of the plasmonic graphene ribbon with a wing

We investigate the transmittance characteristics of graphene ribbons with a wing numerically by Finite Element Method. By conducting the dispersion relation of edge graphene plasmons (EGPs) modes and analyzing the mode distributions, it is believed that the transmission dips originate from the resonances of three kinds of EGPs modes respectively, including the symmetrical EGPs, anti-symmetrical EGPs, and EGPs of a semi-infinite sheet. By changing the width and length of the wing, the conclusion that transmission dips originate from EGPs modes is further approved. Thanks to the tunable permittivity of graphene by gate voltages, the transmittance dips can be easily tuned.

Enhancement transmission filter using a two-dimensional subwavelength periodic membrane

Enhancement transmission filter using a two-dimensional (2-D) subwavelength periodic membrane is proposed. It can be found that strong refractive-index modulation of the silicon periodic membrane can support the excitation of multiple guided-mode resonances (GMRs) in a reflection band, and every GMR relates a transmission peak on its edge, therefore the overlapping of the edges of these resonances can be tailored to create enhancement transmission. Thelocation of the transmission peak is shifted linearly with a slop of 1.51 as the period is varied. Enhancement transmission with multiple channels near 1310 nm can also be achieved using the interaction of the nondegenerate GMRs at oblique incidence.

Tunable 1 × 2 plasmonic splitter of dielectric-loaded graphene waveguide based on multimode interference

We study the multimode interference (MMI) effect in a dielectric-loaded graphene waveguide (DLGW) numerically by the finite element method. By conducting the dispersion relation of graphene plasmon (GP) modes, a 1 × 2 splitter of GPs is proposed. Structure parameters are designed on the basis of the self-imaging principle, and the calculation of electrical field distributions illustrates two-wavelength splitting. Owing to the tunable permittivity of graphene by bias voltages, the active control of wavelength routing is achieved. High extinction ratios can also be obtained, which proves good splitting performance. It is considered that our findings provide a smart way of designing a tunable plasmonic splitter in the infrared region.