A Nano‐Imaging Study of Graphene Edge Plasmons with Chirality‐Dependent Dispersions

Abstract

confinements [5] and substantially low loss, [6] and can be actively controlled via external stimuli. [7][8][9][10] These virtues make the GPs highly attractive for tunable nanophotonics devices. In addition to the sheet plasmon modes which propagate within the 2D plane, there exists another type of plasmon modes, that is, 1D edge plasmon modes propagating along the graphene edge. [11] In comparison to the sheet modes, the edge modes can exhibit enhanced electromagnetic field confinements. [12] Monolayer graphene is terminated by two types of edge with different chiralities, that is, zigzag and armchair edges. [13,14] These two edges give rise to a variety of intriguing localized electronic states that are associated with superconductivity, [15] localized magnetism, [16][17][18] and topological states, [19] among others. Moreover, the physical and chemical properties on the edges are strongly dependent on the termination type. For example, zigzag edge can result in a band of zero-energy modes that is absent in the armchair case. [20] Furthermore, the two edges exhibit different electronic density of states and vacuum potential barriers, making their charge-transfer behaviors and molecule adsorption abilities different from each other. [20,21] These characteristics are anticipated to endow the 1D edge plasmon modes with topology-specific behaviors that are not observed in the sheet modes. [22] Moreover, the evolution of the edge plasmon modes in response to external stimuli, such as electrical gating, chemical doping, and magnetic field, can in turn help reveal the fine electronic structure at these two edges. A few studies have identified that plasmon behaviors associated with these two edges are different because of the edge-specific electronic band structure. However, they only focused on the impact of chirality on the reflections of the sheet plasmons modes at the edges. [22] A direct characterization and comparison of the 1D edge plasmons propagating along the zigzag and armchair edges are still incomplete.Here, we present the study on 1D edge plasmon modes in a monolayer graphene. Using real-space nano-imaging, we for the first time manage to visualize and compare the edge plasmon modes propagating respectively along the zigzag and armchair edges. The results show that in the pristine graphene, the plasmon wavelengths of zigzag and armchair edges are the same and shorter than those of the sheet modes, suggesting a stronger electromagnetic field confinement. Furthermore, upon chemical doping via HNO 3 exposure, the plasmon wavelengths Graphene plasmons (GPs) have a variety of potential applications in nanophotonics and optoelectronics, such as room temperature mid-infrared photodetectors, sensors, and modulators, due to their ultrahigh electromagnetic field confinements. In particular, the GPs at the edges of a monolayer graphene have been demonstrated to exhibit superior electromagnetic field confinements compared to the sheet plasmons. The graphene is terminated by two types of edges with different chiralit...

Manipulating polaritons at the extreme scale in van der Waals materials

“…2a) 28 . Upon chemical doping, the wavelength of zigzag edge-oriented PPs possesses a more pronounced increment, due to the metallic zigzag edge having higher charge density (n) of the metallic zigzag edge than that of the semiconducting armchair edge 82 .…”

Enhanced Circular Dichroism by F-Type Chiral Metal Nanostructures

“…The CD is strongly linked to the absorption loss of the metallic material in the micro nanostructure, which is represented by the imaginary component of its refractive index. Metal nanostructures, in general, exhibit higher chiral properties compared to natural chiral molecules and have extensive applications in the design of nanosensors based on surface plasmon resonance (SPR) effects [6], nanomanipulation [7], near-field trapping technology [8], nanoscale imaging [9], and biosensing [10].…”

Role of Local Conductivities in the Plasmon Reflections at the Edges and Stacking Domain Boundaries of Trilayer Graphene

“…Figure b displays the IR-sSNOM (region marked as a box in Figure a) of the sample at 952 cm –1 , containing two flakes of TLG (named flake1 and flake2). The image shows fringe patterns near the edges and the ABA-ABC boundaries, representing the interference ( 2 Re [ ψ̃ 0 ψ̃ r * ] ) between the tip-launched ( ψ̃ 0 ) and reflected ( ψ̃ r ) SPP waves at the tip position. ,,,− ,, In Figures –, all of the results are shown as the intensity (|s 2 | 2 ) rather than the amplitude |s 2 |, because the interference (oscillatory) component of the signal can be more directly extracted from the intensity than from the amplitude.…”

“…) between the tiplaunched ( 0 ) and reflected ( r ) SPP waves at the tip position. 1,2,7,[13][14][15]38,39 In Figures 3−5, all of the results are shown as the intensity (|s 2 | 2 ) rather than the amplitude |s 2 |, because the interference (oscillatory) component of the signal can be more directly extracted from the intensity than from the amplitude.…”