Sheng-Xuan Xia

Dynamically tunable plasmonically induced transparency in sinusoidally curved and planar graphene layers.

To achieve plasmonically induced transparency (PIT), general near-field plasmonic systems based on couplings between localized plasmon resonances of nanostructures rely heavily on the well-designed interantenna separations. However, the implementation of such devices and techniques encounters great difficulties mainly to due to very small sized dimensions of the nanostructures and gaps between them. Here, we propose and numerically demonstrate that PIT can be achieved by using two graphene layers that are composed of a upper sinusoidally curved layer and a lower planar layer, avoiding any pattern of the graphene sheets. Both the analytical fitting and the Akaike Information Criterion (AIC) method are employed efficiently to distinguish the induced window, which is found to be more likely caused by Autler-Townes splitting (ATS) instead of electromagnetically induced transparency (EIT). Besides, our results show that the resonant modes cannot only be tuned dramatically by geometrically changing the grating amplitude and the interlayer spacing, but also by dynamically varying the Fermi energy of the graphene sheets. Potential applications of the proposed system could be expected on various photonic functional devices, including optical switches, plasmonic sensors.

Combined theoretical analysis for plasmon-induced transparency in integrated graphene waveguides with direct and indirect couplings

By taking a graphene nanoribbon as a resonator, we have numerically and analytically investigated the spectral characteristics of plasmon-induced transparency in integrated graphene waveguides. For the indirect coupling, the formation and evolution of the transparency window are determined by the excitation of the super resonances, as well as by the destructive interference and the coupling strength between the two resonators, respectively, while for the indirect coupling, the peak transmission and corresponding quality factor can be dynamically tuned by adjusting the Fermi energy of graphene nanoribbons and the transparency peak shifts periodicity with the round-trip phase accumulated in the graphene waveguide region. Analytical results based on temporal coupled mode theory (CMT) show good consistence with the numerical calculations. Our findings may support the design of ultra-compact plasmonic devices for optical modulating.

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.

Multi-band perfect plasmonic absorptions using rectangular graphene gratings

We propose to achieve multi-band perfect plasmonic absorptions with peak absorptivity >99% via the excitation of standing-wave graphene surface plasmon polaritons using single-layer graphene-based rectangular gratings. For the case with continuous gratings, perfect absorptions are only allowed for even-order modes, while the absorptions are quite low for odd-order modes because the fields are out-of-phase. However, for gratings with bottom-open configuration, four-band perfect absorptions containing both the even- and odd-order modes can be realized, which are found to be highly sensitive to the incident angle. The simulated results agree very well with the theoretical analyses by considering the phase path of the plasmonic waves. This multi-band absorber is a promising candidate for future plasmonic devices.

Excitation of crest and trough surface plasmon modes in in-plane bended graphene nanoribbons

To achieve efficiently coupling to external light is still remaining an insurmountable challenge that graphene faces before it can play an irreplaceable role in the plasmonic field. Here, this difficulty is overcome by a scheme capable of exciting graphene surface plasmons (GSPs) in in-plane bended gratings that are formed by elastic vibrations of graphene nanoribbons (GNRs). The gratings enable the light polarized perpendicularly to the GNRs to two kinds of GSP modes, of which the field concentrations are within the grating crest (crest mode, C-M) and trough (trough mode, T-M), respectively. These two kinds of modes will individually cause notches in the transmission spectrum and permit fast off-on switching and tuning of their excitation dynamically (elastic vibration, Fermi energy) and geometrically (ribbon width). The performance of this device is analyzed by finite-difference time-domain simulations, which demonstrates a good agreement with the quasi-static analysis theory. The proposed concept expands our understanding of plasmons in GNRs and offers a platform for realizing of 2D graphene plasmonic devices with broadband operations and multichannel modulations.

Graphene Surface Plasmons With Dielectric Metasurfaces

An approach to capture light efficiently into graphene surface plasmons by patterning the sinusoidal dielectric metasurfaces above and below the graphene sheet is proposed. The presence of plasmonic resonance is demonstrated by means of an analytical model based on the transformation optics through the extraction of effective graphene conductivity, which is further revealed via a numerical study of the optical spectra as a function of grating parameters at the subwavelength scale. Besides, the resonant position is found to be sensitive to the dielectric contacted with graphene. These findings can deepen our understanding of plasmon resonances and pave the way to the design of graphene plasmonic devices like refractive index sensors.

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.

Localized plasmonic field enhancement in shaped graphene nanoribbons.

Graphene nanoribbon (GNR), as a fundamental component to support the surface plasmon waves, are envisioned to play an important role in graphene plasmonics. However, to achieve extremely confinement of the graphene surface plasmons (GSPs) is still a challenging. Here, we propose a scheme to realize the excitation of localized surface plasmons with very strong field enhancement at the resonant frequency. By sinusoidally patterning the boundaries of GNRs, a new type of plasmon mode with field energy concentrated on the shaped grating crest (crest mode) can be efficiently excited, creating a sharp notch on the transmission spectra. Specifically, the enhanced field energies are featured by 3 times of magnitude stronger than that of the unpatterned classical GNRs. Through theoretical analyses and numerical calculations, we confirm that the enhanced fields of the crest modes can be tuned not only by changing the width, period and Fermi energy as traditional ribbons, but also by varying the grating amplitude and period. This new technique of manipulating the light-graphene interaction gives an insight of modulating plasmon resonances on graphene nanostrutures, making the proposed pattern method an attractive candidate for designing optical filters, spatial light modulators, and other active plasmonic devices.

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.

Two kinds of double Fano resonances induced by an asymmetric MIM waveguide structure

Asymmetric plasmonic waveguides with a shoulder-coupled rectangle cavity are proposed and investigated numerically by using the finite-difference time-domain (FDTD) method. The symmetry breaking of the structure results in a new discrete mode supported by the cavity. The extreme interference between two discrete states and an intrinsic wide continuous state gives rise to novel double Fano resonances with symmetric and anti-symmetric configurations. Coupled-mode theory (CMT) further confirms that two Fano profiles originate from the different coupling conditions of the cavity modes with the waveguides. Moreover, the sensing characters are performed. The Fano responses with the higher sensitivity and figure of merit (FOM) up to 57 are realized. Undoubtedly, the studied structure will play an important role in the nano-integrated plasmonic devices for optical switching and sensing.