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

Deep-Subwavelength Conversion of Localized Excitons to Trions in 2D Semiconductor Heterostructures Observed by Cathodoluminescence Nanoscopy

Localized excitons in 2D semiconductors are pivotal for advancements in quantum information science. Despite their importance, conventional optical excitation methods, bounded by diffraction limits, inadequately investigate exciton states at the deep-subwavelength scale. In this work, we employ the cathodoluminescence (CL) technique, known as electron beam excitation with spatial resolution that far exceeds the diffraction limit of light, to uncover the localized excitons in h-BN/WS2/h-BN heterostructures, facilitating a deep-subwavelength conversion of two distinct excitonic states and amplifying the excitonic emission intensity by a factor of 11 within plasmonic hybrid configurations. These enhancements and precise excitations yield a detailed imaging of the transformation from localized excitons to trions at the nanometer scale. Based on density functional theory (DFT) and the finite-element method (FEM), we identify a funneling effect prompted by localized alterations in the valence and conduction bands of WS2 as the mechanism behind the exciton-state transformations. The findings address major challenges in near-field imaging and transformation of localized excitonic states, enhancing methodologies for probing excitonic behaviors in confined geometries. This advancement enables potential applications in the development of deep-subwavelength excitonic lasers, optoelectronic circuits, and next-generation quantum information technologies.

Single-Molecule-Sensitive Three-Dimensional Atomic Heterostructures with Extreme Light-Matter Coupling

Three-dimensional heterostructures (3DHS) with controlled compositions and tuned properties are highly desired for fundamental studies and applications in optoelectronics, nanocatalysis, clean energy, and biomedicine. However, conventional nanostructure engineering is hindered by challenges such as poor structural control, time- and energy-intensive processes, the use of hazardous and expensive chemicals, and harsh conditions. Here, we report plasma-assisted epitaxy (PAE) engineering of a metal-organic 3DHS with extreme light-matter interaction for rapid single-molecule-level sensing. Plasmonic-active 3DHS composed of structure-tuned gold-silver core-shell nanoparticles (AuAgCSNPs) was precisely engineered using stable and scalable microplasma-enabled nanofabrication under ambient conditions. The engineered AuAgCSNP-based 3DHS possessed exceptional Raman enhancement under suitable laser excitation, leading to single-molecule detection of SARS-CoV-2 spike proteins in simulated human saliva via surface-enhanced Raman scattering (SERS). The developed plasma fabrication method allows the production of centimeter-scale SERS-active metal-organic 3DHS on disposable, flexible, lightweight, and cost-effective substrates, thereby opening a new avenue for next-generation biosensing, nanoelectronics, nanocatalysis, and biomedical applications.

Influence of resonant plasmonic nanoparticles on optically accessing the valley degree of freedom in 2D semiconductors

The valley degree of freedom in atomically thin transition metal dichalcogenides, coupled with valley-contrasting optical selection rules, holds great potential for future electronic and optoelectronic devices. Resonant optical nanostructures emerge as promising tools for controlling this degree of freedom at the nanoscale. However, their impact on the circular polarization of valley-selective emission remains poorly understood. In our study, we explore a hybrid system where valley-specific emission from a molybdenum disulfide monolayer interacts with a resonant plasmonic nanosphere. Contrary to the simple intuition that a centrosymmetric nanoresonator mostly preserves the degree of circular polarization, our cryogenic experiments reveal significant depolarization of the photoluminescence scattered by the nanoparticle. This striking effect presents an ideal platform for studying the mechanisms governing light-matter interactions in such hybrid systems. Our full-wave numerical analysis provides insights into the key physical mechanisms affecting the polarization response, offering a pathway toward designing novel valleytronic devices.

Valley-Related Multipiezo Effect and Noncollinear Spin Current in an Altermagnet Fe2Se2O Monolayer

An altermagnet exhibits many novel physical phenomena because of its intrinsic antiferromagnetic coupling and natural band spin splitting, which are expected to give rise to new types of magnetic electronic components. In this study, an Fe2Se2O monolayer is proven to be an altermagnet with out-of-plane magnetic anisotropy, and its Néel temperature is determined to be 319 K. The spin splitting of the Fe2Se2O monolayer reaches 860 meV. Moreover, an Fe2Se2O monolayer presents a pair of energy valleys, which can be polarized and reversed by applying uniaxial strains along different directions, resulting in a piezovalley effect. Under the strain, the net magnetization can be induced in the Fe2Se2O monolayer by doping with holes, thereby realizing a piezomagnetic property. Interestingly, noncollinear spin current can be generated by applying an in-plane electric field on an unstrained Fe2Se2O monolayer doped with 0.2 hole/formula unit. These excellent physical properties make the Fe2Se2O monolayer a promising candidate for multifunctional spintronic and valleytronic devices.

Cathodoluminescence Nanoscopy: State of the Art and Beyond

Cathodoluminescence (CL) nanoscopy is proven to be a powerful tool to explore nanoscale optical properties, whereby free electron beams achieve a spatial resolution far beyond the diffraction limit of light. With developed methods for the control of electron beams and the collection of light, the dimension of information that CL can access has been expanded to include polarization, momentum, and time, holding promise to provide invaluable insights into the study of materials and optical near-field dynamics. With a focus on the burgeoning field of CL nanoscopy, this perspective outlines the recent advancements and applications of this technique, as illustrated by the salient experimental works. In addition, as an outlook for future research, several appealing directions that may bring about developments and discoveries are highlighted.