Zhen-Rong Huang

Investigation of the graphene based planar plasmonic filters

We investigate numerically the edge modes supported by graphene ribbons and the planar band-stop filter consisting of a graphene ribbon lateral coupled a graphene ring resonator by using the finite-difference time-domain method. Simulation results reveal that the edge modes can enhance the electromagnetic coupling between objects indeed and this structure realizes perfect, tunable filtering effect. Successively, the channel-drop filter is constructed. Especially, the proposed structures can be designed and the size of the ring is changed by creating non-uniform conductivity patterns on monolayer graphene. Our studies will benefit the fabrication of the planar, ultra-compact devices in the mid-infrared region.

A mid-infrared fast-tunable graphene ring resonator based on guided-plasmonic wave resonance on a curved graphene surface

We propose and numerically analyze an ultra-compact tunable plasmon ring resonator which is composed of monolayer graphene and a graphene ring. The results, calculated using the finite element method, reveal that the resonant frequency can be tuned over a wide frequency range by a small change of Fermi level in the graphene ring. Both a high quality factor and extinction ratio of the resonant mode can be achieved when the plasmonic waves in the ring and the input waveguide are properly coupled. As applications, this proposed resonator can be used to construct an ultra-compact fast-tunable filter, modulator, switch or directional coupler in the mid-infrared range.

Tunable mid-infrared plasmonic band-pass filter based on a single graphene sheet with cavities

The single graphene sheet with two cavities constructed on substrates is proposed and numerically investigated by using the finite-difference time-domain (FDTD) method. Thanks to the two introduced cavities, the sandwiched graphene strip behaves as a line-shaped plasmonic resonator. The simple single graphene sheet hence exhibits an outstanding band-pass filtering effect. The transmission spectrum is tuned dynamically not only via changing the length of the graphene strip sandwiched in cavities but also by a small change in the chemical potential of graphene. Simulation results are confirmed by the standing wave equation. In addition, the wavelength of the transmission peak can be tuned linearly by changing the substrate and the proposed structure hence has potential applications in mid-infrared plasmonic sensors. The transmission spectrum is also optimized by changing the width of the cavity. Our studies may be important for the fabrication of nano-integrated circuits for optical communication in the mid-infrared region.

Graphene-based terahertz tunable plasmonic directional coupler

We propose and numerically analyze a terahertz tunable plasmonic directional coupler which is composed of a thin metal film with a nanoscale slit, dielectric grating, a graphene sheet, and a dielectric substrate. The slit is employed to generate surface plasmon polaritons (SPPs), and the metal-dielectric grating-graphene-dielectric constructs a Bragg reflector, whose bandgap can be tuned over a wide frequency range by a small change in the Fermi energy level of graphene. As a graphene-based Bragg reflector is formed on one side of the slit, the structure enables SPP waves to be unidirectionally excited on the other side of the slit due to SPP interference, and the SPP waves in the Bragg reflector can be efficiently switched on and off by tuning the graphenes Fermi energy level. By introducing two optimized graphene-based Bragg reflectors into opposite sides of the slit, SPP waves can be guided to different Bragg reflectors at different Fermi energy levels, thus achieving a tunable bidirectional coupler.

A Graphene-Based Bandwidth-Tunable Mid-Infrared Ultra-Broadband Plasmonic Filter

The closely spaced pair of parallel graphene sheets separated by uniform dielectric gratings is proposed and investigated numerically via the finite-difference time-domain (FDTD) method. This simple structure working as plasmonic Bragg reflectors can produce an original ultra-broadband band-stop filtering effect in the mid-infrared region. The transmission spectrum is tuned dynamically not only via varying the period of the grating but also by a small change in the chemical potential of graphene without re-fabricating new structures. In addition, the bandwidth of the stopband can also be engineered by changing the refractive index of the sandwiched dielectric grating. Simulation results are confirmed by theoretical calculations. As an application, a defect is introduced into the uniform dielectric grating, and the obvious Fabry-Perot-like resonant mode hence forms in the stopband. The proposed device without doubt can be used as highly tunable ultra-broadband band-stop filters and mid-infrared optical modulators. Our studies will play a significant role in the fabrication of ultra-compact versatile integrated circuits.

Gate-Tunable mid-Infrared Plasmonic Planar Band-Stop Filters Based on a Monolayer Graphene

In this paper, the graphene ribbon bus waveguide side-coupled a coplanar short graphene strip, constructed on a monolayer graphene with substrates by spatially varying external gates, is proposed and investigated numerically by using the finite-difference time-domain (FDTD) method. Simulated results exhibit that the outstanding mid-infrared band-stop filtering effect is realized based on the edge mode resonance in the short strip acting as a perfect Fabry-Perot resonator. By changing the locations of gate voltages to vary the size of the short graphene strip, not only the Fabry-Perot resonance mode is tuned effectively but also the original rectangular-loop resonance mode appears. The changes in the coupling distance and substrates are also used for modulating the transmission spectrum. FDTD results are in excellent agreement with the coupled-mode theory (CMT). The simple structure without doubt is a real electrically controlled plasmonic device. Our studies will support the fabrication of planar nano-integrated plasmonic circuits for mid-infrared optical processing.

Plasmon resonances in a stacked pair of graphene ribbon arrays with a lateral displacement

We find that a stacked pair of graphene ribbon arrays with a lateral displacement can excite plasmon waveguide mode in the gap between ribbons, as well as surface plasmon mode on graphene ribbon surface. When the resonance wavelengthes of plasmon waveguide mode and surface plasmon mode are close to each other, there is a strong electromagnetic interaction between the two modes, and then they contribute together to transmission dip. The plasmon waveguide mode resonance can be manipulated by the lateral displacement and longitudinal interval between arrays due to their influence on the manner and strength of electromagnetic coupling between two arrays. The findings expand our understanding of electromagnetic resonances in graphene-ribbon array structure and may affect further engineering of nanoplasmonic devices and metamaterials.

Tunable, Mid-Infrared Ultra-Narrowband Filtering Effect Induced by Two Coplanar Graphene Strips

The two coplanar graphene strips coupling system supported on substrates is proposed and constructed on a monolayer graphene by spatially varying gate voltages. It is investigated numerically by using the finite-difference time-domain method. Simulation results reveal that despite of no traditional ring, disk, and rectangular geometry resonators used usually in metallic plasmonic filters, this structure based on the edge mode propagation exhibits an original, ultra-narrowband band-stop filtering effect in the mid-infrared region. This filtering effect results from the novel side-coupled resonator formed by the parallel graphene strips. The transmission spectrum is tuned and modified not only by engineering the locations of gate voltages without re-fabricating structures but also via changing substrates. Simulation results are consistent with the theoretical analysis. Our studies hence support the fabrication of ultra-compact planar plasmonic devices in nano-integrated circuits.

An ultra-compact tunable Bragg reflector based on edge propagating plasmons in graphene nanoribbon

A mid-infrared planar Bragg reflector, which is based on the fundamental edge plasmonic mode in the nanoribbons is proposed and numerically demonstrated in this paper. The simulation results calculated with the three-dimensional (3D) finite element method reveal that it shows superb wide-band filtering characteristics in the mid-infrared frequencies, and the bandwidth of stopband in the reflector can be dynamically modulated by varying the chemical potentials of corresponding nanoribbon waveguides. In addition, its band properties on the ribbon width are also analyzed. This kind of Bragg reflector exhibits extreme compactness of lateral scales and wonderful light confinement in both the longitudinal and the lateral directions, which is expected to have significant applications in constructing 3D highly integrated optical networks for signal processing.

Tunable Mid-Infrared Plasmonic Anti-Symmetric Coupling Resonator Based on the Parallel Interlaced Graphene Pair

The parallel interlaced graphene pair separated by two spatial silver strips is proposed and numerically investigated by using the finite-difference time-domain method. Thanks to the introduced silver strips, the unique plasmonic anti-symmetric coupling resonator is obtained and the outstanding mid-infrared band-pass filtering effect is achieved. The transmission spectrum is tuned dynamically not only by changing the coupling distance and but also via varying overlapping length of the double-layer graphene coupling system. In addition, without re-fabricating new structures, the transmission spectrum is also modulated over a wide wavelength range by a small change in the chemical potential of graphene utilizing external gate voltages. Simulation results are confirmed by theoretical calculations. The proposed resonator without doubt is unusual and our studies support the fabrication of active mid-infrared plasmonic nano-integrated circuits for optical processing.