Near-field coupled plasmonic systems generally achieve plasmonically induced transparency (PIT) using only one-way bright–dark mode coupling. However, it is challenging to realize such well-designed devices, mainly because they depend significantly on the polarization direction. We exploit surface plasmons supported by two crossed layers of graphene nanoribbons (GNRs) to achieve dynamically tunable PIT, where each GNR operates as both the bright and dark modes simultaneously. The proposed PIT can result from either one-way bright–dark mode interactions or bidirectional bright–bright and bright–dark mode hybridized coupling when the polarization is perpendicular/parallel or at an angle to the GNRs, respectively. Additionally, identical ribbon widths yield polarization-insensitive single-window PIT, whereas different ribbon widths produce polarization-dependent double-window PIT. We examine the proposed technique using plasmon wave functions and the transfer matrix method; analytical and numerical results show excellent agreement. This study can provide physical insight into the PIT coupling mechanisms and advance the applicability and versatility of PIT-based sensing platforms and other active devices.

Plasmonically induced transparency in double-layered graphene nanoribbons

Control of light transmission and reflection through nanostructured materials has led to demonstration of metamaterial absorbers that have augmented the performance of energy harvesting applications of several optoelectronic and nanophotonic systems. Here, for the first time, a broadband plasmonic metamaterial absorber is fabricated using two-dimensional titanium carbide (Ti3C2Tx) MXene. Arrays of nanodisks made of Ti3C2Tx exhibit strong localized surface plasmon resonances at near-infrared frequencies. By exploiting the scattering enhancement at the resonances and the optical losses inherent to Ti3C2Tx MXene, high-efficiency absorption (∼90%) for a wide wavelength window of incident illumination (∼1.55 μm) has been achieved.

Highly Broadband Absorber Using Plasmonic Titanium Carbide (MXene)

The plasmon-induced transparency (PIT), which is destructive interference between the superradiation mode and the subradiation mode, is studied in patterned graphene-based terahertz metasurface composed of graphene ribbons and graphene strips. As the results of finite-difference time-domain (FDTD) simulation and coupled-mode theory (CMT) fitting, the PIT can be dynamically modulated by the dual-mode. The left (right) transmission dip is mainly tailored by the gate voltage applied to graphene ribbons (stripes), respectively, meaning a dual-mode on-to-off modulator is realized. Surprisingly, an absorbance of 50% and slow-light property of 0.7 ps are also achieved, demonstrating the proposed PIT metasurface has important applications in absorption and slow-light. In addition, coupling effects between the graphene ribbons and the graphene strips in PIT metasurface with different structural parameters also are studied in detail. Thus, the proposed structure provides a new basis for the dual-mode on-to-off multi-function modulators.

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Dual-Mode On-to-Off Modulation of Plasmon-Induced Transparency and Coupling Effect in Patterned Graphene-Based Terahertz Metasurface.

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.

Multi-band perfect plasmonic absorptions using rectangular graphene gratings.

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.

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

Transition-metal dichalcogenides with exceptional electrical and optical properties have emerged as a new platform for atomic-scale optoelectronic devices. However, the poor optical absorption resists their potential applications. The novel method of critical coupling with guided resonances is proposed to realize total absorption of light in monolayer MoS2 both theoretically and numerically. Simulated results illustrate that the perfect absorption with critical coupling is achieved by choosing suitably the ration of the hole radius to the period of the photonic crystal slab, and that the tunability of absorption peaks is obtained by a small change in the period and the thickness of the slab. Intriguingly, such device manifests the unusual polarization-insensitive feature and the good absorption stability over a wide angle range of incidence. The total absorption in monolayer MoSe2, WS2, and WSe2 is realized handily by the same principle. Hence, our results may open up new possibilities for improving the light-matter interaction in monolayer transition-metal dichalcogenides and find utility in wavelength-selective photoluminescence and photodetection.

Total absorption of light in monolayer transition-metal dichalcogenides by critical coupling

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.

Investigation of the graphene based planar plasmonic filters

Investigation of multiband plasmonic metamaterial perfect absorbers based on graphene ribbons by the phase-coupled method

In this paper, the periodic double-layer graphene ribbon arrays placed near a metallic ground plate coated by a dielectric layer are proposed and analyzed by the coupled-mode theory (CMT) to predict the perfect absorption response in the mid-infrared region. Numerical simulations of the finite-difference time-domain (FDTD) method confirm this effect and give the underlying physical origin. The anti-symmetric dipole-dipole coupling mode is supported by the double-layer graphene ribbons and acts as the electrical resonance to suppress the reflection, because of the impedance matching. The transmission from this system is restricted by the ultra-thick metallic ground plate. All incident electromagnetic energy is efficiently confined in the interlayer between graphene ribbons and the metallic plate, and the dramatic narrowband perfect absorption peak with the FWHM (full width at half maximums) of 300 nm hence is achieved. The spectral position of the absorption peak can be dynamically tuned by a small change in the chemical potential of graphene, in addition to varying geometrical parameters of the absorber. Meanwhile, this device exhibits good absorption stability over a wide angle range of incidence around ± 60° at least. Such absorber will benefit the fabrication of mid-infrared nano-photonic devices for optical filtering and storage.

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Tunable graphene-based mid-infrared plasmonic wide-angle narrowband perfect absorber

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.

Hong-Ju Li

Papers

Total absorption of light in monolayer transition-metal dichalcogenides by critical coupling

Investigation of the graphene based planar plasmonic filters

Investigation of multiband plasmonic metamaterial perfect absorbers based on graphene ribbons by the phase-coupled method

Tunable graphene-based mid-infrared plasmonic wide-angle narrowband perfect absorber

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