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In this Letter, we report on the phase-controlled topological plasmons in 1D graphene nanoribbons (GNRs) based on a Su−Schrieffer−Heeger (SSH) model variant. By considering the dipole–dipole mode interactions, we first study the normal SSH model by an effective Hamiltonian and calculate the Zak phase as a topological invariant, finding that it is nontrivial (trivial) when the coupling distance is bigger (smaller) than half the period. Then, we reveal that the edge modes with fields highly localized at only one side exist in the model with nontrivial topology and shows the robustness of strong field confinement and extreme frequency stability against in-plane and out-of-plane disorders. Finally, we introduce the offset SSH model variant by vertically offsetting one of the GNR in SSH unit, which allows us to greatly engineer both the width of topological gap and the number of topological windows. The underlying physics are uncovered by defining a parameter called phase difference, which reveals that the topological edge modes appear (disappear) generally near the positions where the inter-unit coupling strength is bigger (smaller) than the intra-unit coupling strength, and, more notably, the phase difference is around even (odd) multiple numbers of π, which is much different from the normal SSH model where the topological phase is simply affected by the resonator distance. In addition to opening up a possibility to explore the fundamental physics of topologically protected graphene plasmons, this work also offers potential applications of these concepts to design graphene-based plasmon devices with immunity to structural imperfections. [ABSTRACT FROM AUTHOR]

In this paper, we combine the dielectric metasurface with monolayer graphene to realize a high quality(Q)-factor quasi-BIC-based optical modulator, and the corresponding modulation performances are investigated by using the finite-difference time-domain (FDTD) method, which can be well fitting by the Fano formula based on the temporal couple-mode theory. The results demonstrate that bound states in the continuum (BIC) will turn into the quasi-BIC with high Q-factor by breaking the symmetry of every unit of the metasurface. Meanwhile, the amplitude and bandwidth of transmission based on the quasi-BIC mode can be efficiently adjusted by changing the Fermi energy (EF) of monolayer graphene, and the maximum difference in transmission up to 0.92 is achieved. Moreover, we also discuss the influence of the asymmetry degree to further investigate the modulation effect of graphene on the quasi-BIC mode. [ABSTRACT FROM AUTHOR]

A dual-band perfect optical absorber based on the graphene/hexagonal boron nitride van der Waals hybrid structure is proposed in the mid-infrared band. According to the results of the finite-difference time-domain method, dual-band perfect absorption peaks are formed at 7103 nm and 7499 nm, respectively, which also can be separately shifted in a wide wavelength region by varying some key structural parameters. Moreover, our proposed structure manifests polarization sensitivity and can maintain good optical absorption performance for wide angles of incidence. [ABSTRACT FROM AUTHOR]

We propose and investigate a tunable dual-band perfect absorber consisting of double asymmetric L-shaped graphene resonators (LSGRs) and a metal ground plane spaced by a thin SiO2 dielectric layer. The numerical results reveal that the presented absorber expresses two resonance peaks in the mid-infrared region, where their absorption coefficients are on average larger than 98.77%. The electromagnetic eigenmode resonances of a single LSGR can induce two absorption peaks with values about 50%. Assembling two LSGRs together, the absorptivity can reach up to almost 100% due to the plasmonic couplings. In addition, the design has the ability to tune the working wavelengths of the absorption peaks within a large wavelength range by changing geometric parameters and the Fermi level of the graphene layer. In addition, wavelength-selective dual-band perfect absorption peaks can be achieved in the wavelength range of interest by merely adjusting the polarization angle. This letter presents a unique route toward the realization of nanophotonic devices and has potential applications in filtering, sensing, and detecting. [ABSTRACT FROM AUTHOR]

We introduce a remarkably simple coplanar nanostructure consisting of two longer and one shorter graphene nanorods to support a Fano resonance, which arises from the coupling between bright mode and dark mode by breaking the geometrical symmetry. Simulation results, performed with the finite-difference time-domain method, show that the resonant frequency of the bright mode in the transmission spectrum can be independently tuned by changing the lateral offset of the shorter graphene nanorod, while changing its longitudinal offset can independently adjust the resonant frequency of the dark mode. Furthermore, Fano resonance spectrum can also be regulated actively by altering the Fermi energy of graphene as well as refractive index of the surrounding media. This letter proposes a unique modulation strategy of Fano resonance in metamaterials, assisting in developing potential in applications of sensor devices and some other plasmonic functional devices. [ABSTRACT FROM AUTHOR]

We numerically and theoretically demonstrate a plasmon-induced transparency (PIT) at the mid-infrared region with finite-difference time-domain method. The system consists of an optically bright dipole mode and a dark quadrupole mode, which are supported by the graphene nanoring and graphene nanostrips, respectively. The coupling between the two modes introduces transparency window and large group delays. The pronounced PIT resonance can be easily modified by adjusting the geometric parameters and the Fermi level of graphene nanostructure. Our results suggest that the demonstrated PIT effect may be applicated in the slow-light device, active plasmonic switching, and optical sensing. [ABSTRACT FROM PUBLISHER]

A novel nonreciprocal graphene waveguide with Kerr nonlinear material in mid-infrared region is proposed and the corresponding transmission characteristics are investigated. In the proposed graphene waveguide, a transmission contrast ratio up to 19.2 dB can be achieved between forward and backward transmission by using the finite-difference time-domain (FDTD) method, revealing an excellent nonreciprocal effect. Moreover, the proposed nonreciprocal graphene waveguide can also be flexibly controlled as forward transmission or backward transmission at different wavelengths by adjusting the structural parameters and input intensity. Therefore, our results can offer a new possibility and important application in highly integrated optical circuits. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM PUBLISHER]

An optical waveguide for working in mid-infrared range is highly desirable. However, such a waveguide is required to yield a long propagation distance and deep subwavelength confinement. Although the graphene-based surface plasmon polariton (SPP) waveguides have exhibited some potential in this regard, their propagation lengths are currently insufficient (∼100 μm). In this paper, we propose a graphene-based cylindrical long-range SPP (LRSPP) waveguide, which is composed of a cylindrical silicon nanowire core surrounded by an inner graphene layer, a silica layer, and an outer graphene layer from inside to out. First, the electromagnetic field equation and the dispersion equation of this waveguide are derived. Then, combining these derived equations with the numerical simulation, the propagation properties of the LRSPP and the short-range SPP modes are analyzed. The results show that the proposed waveguide with the LRSPP fundamental mode has a notable advantage for simultaneously achieving a larger propagation length (∼10 μm) and a deep subwavelength confinement ( \sim10^- 5\,A0, where A0 is the diffraction-limited mode area). This waveguide has potential for application in high-density photonic integrated circuits in the mid-infrared range. [ABSTRACT FROM PUBLISHER]

We present the strength modulation and frequency tuning of Fano resonance by employing a graphene nanodimer formed by two coplanar perpendicular nanostrips with different dimensions. The Fano resonance is induced by destructive interference between the bright dipole mode of a short nanostrip and the dark quadrupole mode of a long nanostrip. The strength, line width, and resonance frequency of the Fano resonance can be actively modulated by changing the spatial separation of those two graphene nanostrips and the Fermi energy of the graphene nanodimer, respectively, without re-fabricating the nanostructures. The tuning of the strength and resonance frequency can be attributed to the coupling strength and optical properties of graphene, respectively. Importantly, a figure of merit value as high as 39 is achieved in the proposed nanostructures. Our results may provide potential applications in optical switching and bio-chemical sensing. [ABSTRACT FROM AUTHOR]

A narrow-band plasmonic filter based on graphene waveguide with asymmetrical structure is proposed, and the corresponding transmission characteristics are investigated. The asymmetrical structure consists of two sets of five air trenches with different trench sizes. Since the Bragg wavelength could be changed with the length and depth of each air trench, the bandgap (or the passband) of the single set with five short thick air trenches and the passband (or the bandgap) of the single set with five long thin air trenches are overlapped and the cascaded transmission of the two structures is very low in a certain wavelength range. By using the finite-difference time-domain (FDTD) method, a narrow-band transmission peak with 3-dB bandwidth of 0.12 μm could be obtained in the mid-infrared region around 6.9 μm. [ABSTRACT FROM AUTHOR]

We analyze the plasmonic resonant properties in engineered coplanar graphene nanoribbons by using the finite-difference time-domain (FDTD) method. A numerical analysis is introduced to calculate the coupling strength from frequency. Using this method, the coupling strengths of the first-order plasmon mode are calculated in two kinds of models, one is built with different doping concentration in graphene nanoribbons, the other is created with increasing period. As emulational examples of application of the method, a continuously tuning of the number of transmission dips from one to four is achieved for the first time, which is realized by severally varying the Fermi level of nanoribbons via electrostatic gating. Meanwhile, the novel hybridized plasmon resonances are realized based on periodic graphene nanoribbon pairs. The present paper is helpful for designing coupled plasmonic structures, which demonstrates much promise for applications in graphene-based optoelectronic nanodevices. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM PUBLISHER]

We theoretically demonstrate the properties of surface plasmon polaritons (SPPs) propagating along a 90° sharp bending graphene parallel pair and the mode conversion phenomenon around the corner. They are illustrated numerically using the finite-difference time-domain (FDTD) method. As applications, a couple of bending graphene sheets are employed to construct Mach–Zehnder interferometers, T-shaped splitters and switches, and two-Y-shaped splitters and switches. By means of a gate voltage, these devices present novel optical properties and yield a new way to obtain tightly confined and low loss SPPs for nanophotonic networks. Undoubtedly, the proposed devices will have significant application prospects in ultra-compact integrated circuits for optical communications and switching. [ABSTRACT FROM AUTHOR]

An amendment to this paper has been published and can be accessed via the original article. [ABSTRACT FROM AUTHOR]

In order to remarkably enhance its absorption capability, a tunable dual-band graphene-based perfect absorber is proposed. By using the finite-difference time-domain (FDTD) simulations, dual-band perfect absorption peaks are realized in the optical communication regime based on the critical coupling effect. By changing the lateral displacement of the two different holes, multiple absorption peaks could also be achieved. Furthermore, by manipulating the widths of air trenches, the wavelengths of absorption peaks could be shifted, while the values of absorption peaks are almost unchanged. Moreover, our proposed absorber also demonstrates polarization-dependence. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this paper, we propose a structure composed of two graphene waveguides and dual coupled graphene ring resonators (GRRs) to achieve a plasmon-induced absorption (PIA) effect. A three-level plasmonic system and a temporal coupled mode theory (CMT) are utilized to verify the simulation results. Moreover, a double-window-PIA effect can be conveniently attained by introducing another GRR with proper parameters to meet more specific acquirement in optical modulation process. The pronounced PIA resonances can be tuned in a number of ways, such as by adjusting the coupling distance between the GRRs and the couplings between the GRR and the waveguide, and tuning the radius and the Fermi energy of the GRRs. Besides, the produced PIA effect shows a high group delay up to − 1 . 87 ps, exhibiting a particularly prominent fast-light feature. Our results have potential applications in the realization of THz-integrated spectral control and graphene plasmonic devices such as sensors, filters, ultra-fast optical switches and so on. [ABSTRACT FROM AUTHOR]