Xiushan Xia

Plasmonic-induced transparency and unidirectional control based on the waveguide structure with quadrant ring resonators

In this paper, a metal–insulator–metal straight waveguide structure with quadrant ring resonators (QRRs) is theoretically presented. The transmission spectra at output ports are studied by the finite element method (FEM). The simulation results show great filtering effects at specific wavelengths. In order to unidirectionally control plasmonic flows at waveguide junctions, the original design has been expanded to T- and X-shaped waveguides with the QRRs. The results reveal that obvious Fano effects can be achieved in T-shaped systems.

Tunable plasmonically induced transparency with unsymmetrical graphene-ring resonators

The multi-wavelength tunable plasmonically induced transparency (PIT) phenomena in two-ring and three-ring systems at infrared range were theoretically and numerically investigated. In the two-ring system, changing the bias voltage of graphene ring or the separation between graphene rings would induce an off-to-on PIT optical response. An asymmetry factor has been introduced to explain the corresponding transmission spectra. In the three-ring system, by bringing in a new asymmetry factor, multiple new PIT windows would arise at the left or right side of the original PIT windows. Numerical simulation by finite element method was conducted to verify our designs. Those proposed structures hence have potential in ultra-compact graphene optoelectronic devices at the infrared range.

Multi-mode Plasmonically Induced Transparency in Dual Coupled Graphene-Integrated Ring Resonators

We propose a highly wavelength-tunable multi-mode plasmonically induced transparency (PIT) device based on monolayer graphene and graphene rings for the mid-IR region. The proposed PIT systems explore the near-field coupling and phase coupling between two graphene resonators. The multi-mode transparency windows in the spectral response have been observed in the graphene-integrated configurations. By varying the Fermi energy of the graphene, the multi-mode PIT resonance can be actively controlled without reoptimizing the geometric parameters of the structures. Based on the coupled mode theory and Fabry-Perot model, we numerically investigated the two kinds of coupling in the graphene-based PIT systems. This work may pave the ways for the further development of a compact high-performance PIT device.

Detuned Plasmonic Bragg Grating Sensor Based on a Defect Metal-Insulator-Metal Waveguide.

A nanoscale Bragg grating reflector based on the defect metal-insulator-metal (MIM) waveguide is developed and numerically simulated by using the finite element method (FEM). The MIM-based structure promises a highly tunable broad stop-band in transmission spectra. The narrow transmission window is shown to appear in the previous stop-band by changing the certain geometrical parameters. The central wavelengths can be controlled easily by altering the geographical parameters. The development of surface plasmon polarition (SPP) technology in metallic waveguide structures leads to more possibilities of controlling light at deep sub-wavelengths. Its attractive ability of breaking the diffraction limit contributes to the design of optical sensors.

Plasmonic tunable filter based on trapezoid resonator waveguide

The metal-insulator-metal waveguide structures coupled with a trapezoid cavity are proposed in theory. A variety of transmission features have been found from the simulation results, which is caused by the interference of the plasmonic modes in the trapezoid resonator. The transmission spectra can be effectively modulated by optimizing the geometrical parameters of our configurations such as the height, the upper and bottom edge lengths. Also, the coupling distance possesses great importance on the transmission characteristic. It is found that when the coupling distance is chosen to be around 20 nm, the filter works efficiently. When the trapezoid resonator is attached to the waveguide bus, the bandwidth becomes large. Besides, an effective band-stop filter can be achieved when the bottom edge length is large enough. The finite element method is carried out to verify our designs. We hope our work may open some new avenues for a high-performance plasmonic filter.

Characteristics of Plasmonic Bragg Reflectors with Graphene-Based Silicon Grating

We propose a plasmonic Bragg reflector (PBR) composed of a single-layer graphene-based silicon grating and numerically study its performance. An external voltage gating has been applied to graphene to tune its optical conductivity. It is demonstrated that SPP modes on graphene exhibit a stopband around the Bragg wavelengths. By introducing a nano-cavity into the PBR, a defect resonance mode is formed inside the stopband. We further design multi-defect PBR to adjust the characteristics of transmission spectrum. In addition, through patterning the PBR unit into multi-step structure, we lower the insertion loss and suppress the rippling in transmission spectra. The finite element method (FEM) has been utilized to perform the simulation work.

Tunable multimode electromagnetically induced absorption transmission in metal-insulator-metal resonators

The tunable multimode electromagnetically induced absorption (EIA)-like transmission was investigated in a two-ring system. In this system, by introducing asymmetry factor δi = λr - λr′, we provided several ways to modulate the EIA-like transmission spectra. An off-to-on EIA-like response could be realized by changing the radius or the refractive index of the rings. During the off-to-on process, we found the red shift and blue shift effects in the spectra are appeared and the widths of EIA-like dips are broadened. Numerical simulation by finite element method was conducted to verify our discussion. We believe all these would provide guidelines to design the useful EIA-like devices.

Modulating Plasmonic Sensor with Graphene-Based Silicon Grating

We propose a modulating plasmonic structure device which is composed of a single layer graphene above the silicon Bragg grating with the silica spacer layer. This graphene-based plasmonic modulation provides a broad stop-band with high tunability in the mid-infrared region of the transmission spectra achieved by altering the geometrical parameters of the silicon grating and the gate voltage. By engineering, a phase discontinuity into the graphene-based Bragg grating, we can selectively open a transmission window in the previous stop-band spectra. These proposed graphene-based structures are easy to fabricate and operate, which have potential applications as ultra-compact high-sensitivity sensors.

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.

Tunable multimode plasmon-induced transparency with graphene side-coupled resonators

We investigate a plasmonic graphene waveguide system based on monolayer graphene and two side-coupled graphene rings. The system exhibits multimode plasmon-induced transparency (PIT) effects in the mid-IR region. The dark-bright coupling mechanisms and coupled-resonator-induced transparency (CRIT) theory are both utilized to explain the electromagnetic responses, and the explanation is verified by simulation results of the finite-element method (FEM). By varying the Fermi level of graphene or the coupling gap, we have a means to dynamically manipulate the PIT system.