Hong-Ju Li

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.

Tunable plasmon-induced transparency based on bright-bright mode coupling between two parallel graphene nanostrips

Tunable plasmon-induced transparency (PIT) is realized for the mid-infrared region only by using two parallel graphene nanostrips. The weak hybridization between the two bright modes results in the novel PIT optical response. The performance of the PIT system can be controlled by changing the geometry parameters of graphene nanostrips. At the same time, the resonance frequency of transparency window can be dynamically tuned by varying the Fermi energy of the graphene nanostrips via electrostatic gating instead of re-fabricating the nanostructures. Moreover, a figure of merit (FOM) value as high as 12 is achieved in the proposed nanostructures based on the performed sensitivity measures. Such proposed graphene-based PIT system may open up avenues for the development of compact elements such as tunable sensors, switchers, and slow-light devices.

Graphene-based mid-infrared, tunable, electrically controlled plasmonic filter

A planar plasmonic filter consisting of two graphene ribbons coupled by a graphene ring is constructed on monolayer graphene and investigated using the finite-difference time-domain method. The simulation results reveal that the edge modes indeed enhance the electromagnetic coupling between objects. The structure exhibits perfect band-pass filtering effect tuning and is optimized by means of the gate voltage. The simulation results are confirmed by the resonance theory of the ring. This structure is a real electrically controlled filter operating in the mid-infrared region. Our studies support the fabrication of ultracompact planar devices for optical processing.

Plasmonically Induced Absorption and Transparency Based on MIM Waveguides With Concentric Nanorings

In this letter, the metal-insulator-metal plasmonic waveguides shoulder-coupled two concentric nanorings are investigated both numerically and theoretically. Based on the three-level system, the extreme destructive interference between bright and dark resonators gives rise to the distinct plasmonically induced absorption response with the abnormal dispersion and novel fast-light feature. By the same principle, the dramatic plasmonically induced transparency effect with slow-light characteristic is obtained in the waveguide-resonator system with side-coupling configuration. All simulated results are confirmed by the coupled mode theory. Proposed structures will open a new avenue for controlling the speed of a light signal in ultra-compact integrated devices.

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.

Analysis of Mid-Infrared Surface Plasmon Modes in a Graphene-Based Cylindrical Hybrid Waveguide

A graphene-based cylindrical hybrid surface plasmon polariton waveguide, composed of a silicon nanowire core surrounded by a silica layer and then a graphene layer, is investigated using the finite-difference time-domain method. The analytical solutions and the numerical simulation show that an ultra-small mode area and a large propagation length can be achieved with this waveguide. Utilizing the perturbation theory of coupled mode, we demonstrate that the six lowest-order coupling modes originate from the coupling of the three lowest-order single-waveguide modes, and the m = 1 order yy-coupling mode possesses the maximum coupling length and the minimum crosstalk. This waveguide can be used for photonic integrated circuits in the mid-infrared range.

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.

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.