Electron‐Induced Chirality‐Selective Routing of Valley Photons via Metallic Nanostructure

Abstract

Valleytronics in 2D transition metal dichalcogenides has raised a great impact in nanophotonic information processing and transport as it provides the pseudospin degree of freedom for carrier control. The imbalance of carrier occupation in inequivalent valleys can be achieved by external stimulations such as helical light and electric field. With metasurfaces, it is feasible to separate the valley exciton in real space and momentum space, which is significant for logical nanophotonic circuits. However, the control of valley‐separated far‐field emission by a single nanostructure is rarely reported, despite the fact that it is crucial for subwavelength research of valley‐dependent directional emission. Here, it is demonstrated that the electron beam permits the chirality‐selective routing of valley photons in a monolayer WS2 with Au nanostructures. The electron beam can locally excite valley excitons and regulate the coupling between excitons and nanostructures, hence controlling the interference effect of multipolar electric modes in nanostructures. Therefore, the separation degree can be modified by steering the electron beam, exhibiting the capability of subwavelength control of valley separation. This work provides a novel method to create and resolve the variation of valley emission distribution in momentum space, paving the way for the design of future nanophotonic integrated devices.

Cathodoluminescence Nanoscopy: State of the Art and Beyond

“…More recently, the chirality-selective routing of valley photons was reported in a study of hybrid nanostructure constituting of monolayer WS 2 and nanoantennas. 18 As depicted in Figure 4d, CL polarimetry was applied to demonstrate the eolotropic exciton emissions from different valleys under defined excitation positions.…”

“…The precise excitation on the middle of the long edge generates the electrical dipole and quadrupole momentums with well-controlled phases and amplitudes, thereby inducing the directional splitting of photons with different spins via far-field interference of multipole moments. More recently, the chirality-selective routing of valley photons was reported in a study of hybrid nanostructure constituting of monolayer WS 2 and nanoantennas . As depicted in Figure d, CL polarimetry was applied to demonstrate the eolotropic exciton emissions from different valleys under defined excitation positions.…”

“…The electron beam excitation positions with different colors are marked in the SEM image. Adapted from ref . Copyright 2023 John Wiley and Sons.…”

“…Beyond panchromatic and hyperspectral 2D CL imaging, which has been the dominant historical application, more degrees of freedom in CL emission are exploited in nanophotonic research. Polarization-resolved CL has been introduced such that the degree of linear and circular polarization of the emitted CL light can be determined. − In addition, the momentum distribution of CL emission, i.e., the direction in which light is emitted, can be recorded by angle-resolved CL. − By combining these two techniques and performing polarization studies in momentum space, it is viable to fully define the polarization state of CL emission and explore polarization-dependent phenomena with great spatial and angular resolution. − With the advent of ultrafast electron microscopy, time-resolved CL has emerged as a powerful technique for fundamental studies of the dynamics of electron–matter interaction with simultaneous nm–ps–meV space–time–energy resolution, for which it is appealing in the research of exciton/carrier dynamics − and single-photon sources. − On the basis of the synchronized laser and ultrafast electron pulse excitation, the prototypical pump–probe CL spectroscopy has also been reported, enabling analyses of the electron–light–material energy-transfer process. Furthermore, conventional space-resolved CL imaging and spectroscopy have made impressive advances in cathodoluminescence excitation spectroscopy, delocalized imaging, 3D tomography, , and nanothermometry for semiconductors …”