Nahid Talebi

Phonons in hexagonal boron nitride (hBN) possess exceptionally strong oscillator strengths, giving rise to hyperbolic phonon polaritons and enabling robust phonon‐mediated nonlinear interactions. Defect states in hBN are known to interact strongly with lattice vibrations, yet the role of defects in enabling inelastic optical scattering through otherwise Raman‐inactive, infrared (IR)‐active phonon modes remains largely unexplored. Here, we demonstrate a highly efficient inelastic phonon scattering mechanism in which the general selection rules are violated, leading to a down‐converted laser emission mediated by IR‐active phonons in defect‐engineered hBN, via stimulated Raman scattering. We further show that the phonon landscape of hBN can be tailored through substrate choice, enabling multiple inelastic scattering processes and sequential down‐conversion of the excitation laser frequency. These results establish a general strategy for nonlinear defect–phonon scattering and tailoring the nonlinear‐optical selection rules in hBN.

Nonlinear optical processes in low‐dimensional materials are often weak or symmetry forbidden, limiting their use in nanoscale light sources and on‐chip frequency conversion. Here, we show that combining two weakly nonlinear systems—anisotropic borophene and excitonic zinc oxide—yields an enhanced and resonant nonlinear response. In borophene–ZnO heterostructures, cathodoluminescence reveals a two‐orders‐of‐magnitude enhancement in the near‐infrared region and at half its wavelength, due to an enhanced nonlinear absorption process. Strikingly, this enhancement exhibits a remarkable in‐plane anisotropy, owing to the hyperbolic optical response of borophene. Under tunable near‐infrared excitation, a clear second‐harmonic signal emerges with quadratic power dependence and strong resonance near 800 nm. We attribute these observations to nonlinear plasmon–exciton coupling, which reshapes the excitonic response and enables efficient hybrid pathways for frequency conversion. These results establish anisotropic plasmon–exciton hybridization as a route to controlling nonlinear optical responses in low‐dimensional heterostructures.

Excitons in two-dimensional Ruddlesden–Popper perovskites (RPPs) exhibit large and tunable binding energies, making them promising candidates for optoelectronic applications. In particular, long-range exciton energy transfer in these materials holds potential for light-harvesting technologies and nanoscale interconnects. Here, using cathodoluminescence spectroscopy, we demonstrate that exciton energy can be transferred over ultralong distances─up to 150 μm─in heterostructures composed of hexagonal boron nitride (hBN) and RPPs. This transfer is enabled by efficient exciton coupling to defect centers in hBN and subsequent defect–defect interactions. This mechanism not only facilitates long-range energy transfer but also leads to enhanced luminescence intensity, narrower emission line widths, extended exciton lifetimes, and reduced electron beam-induced degradation. Owing to the high density of emitters within the hBN layers, the investigated van der Waals heterostructure emerges as a robust and stable hybrid platform. Our findings enable room-temperature excitonic devices with enhanced performance, including quantum transducers, light-harvesting systems, and optoelectronic interconnects.

Examining and controlling the interaction between semiconductor quantum qubits and their environment can boost semiconductor quantum technologies, which have many applications in table-top quantum computing hardware. Electron beams in electron microscopes have opened up a new avenue for the quantum-coherent probing of semiconductor excitations and strong-coupling effects. Here, I provide a brief overview of recent advancements in electron-beam probes for investigating quantum coherence in semiconductors and two-dimensional materials, complemented by my perspective on using electron beams to manipulate the entanglement and correlations between quantum systems.

Cathodoluminescence, i.e., the radiation caused by the interaction of high‐energy electron beams with matter, has gained a major interest in the analysis of minerals, semiconductors, and plasmonic resonances in nanoparticles. This radiation can either be coherent or incoherent, depending on the underlying interaction mechanism of electrons with nanostructured matter. Thanks to their high spatial resolution and large spectral excitation bandwidth, the evanescent near‐field of a moving electron in a scanning electron microscope is used to probe locally photonic modes at the nanoscale, e.g., exciton or plasmon polaritons. The properties of these excitations can be analyzed through both spectral and temporal statistics of the emitted light. Here, we report on the technical design and implementation of a novel fiber‐based cathodoluminescence detector for a scanning electron microscope. Moreover, we present first characterization measurements to prove the ability for raster scanning the cathodoluminescence emission using optical fibers with 3D printed micro‐optics. The functionality and flexibility of this fiber‐based detector is highlighted by resolving the spatial far‐field distribution of the excited light, as well as cathodoluminescence spectroscopy and time‐correlated single photon counting. Our findings pave the way for a better understanding of the characteristic of the light emitted from electron beams interacting with nanostructures and 2D materials.

Free-electron interactions with light and matter have long served as a cornerstone for exploring the quantum and ultrafast dynamics of material excitation. In recent years, this paradigm has evolved from a classical description of radiation and acceleration toward a fully quantum framework, transforming our understanding of light-matter interactions at the single-electron level. These advances have opened new opportunities in high-resolution imaging, ultrafast spectroscopy, interferometry, and the coherent shaping of electron wavepackets. This review surveys stimulated interactions between slow electrons and light, encompassing free-space and near-field mediated mechanisms. We discuss how free-space optical fields coherently modulate electron momentum and energy, and how near-field coupling in nanophotonic and plasmonic structures enables strong, phase-matched, efficient momentum exchange with the electron wavepacket. We further describe electron recoil, which is significant in the slow-electron regime, and temporal and spatial wavepacket shaping that enhances coupling efficiency and extends access to quantum-coherent regimes. Building on these foundations, we outline emerging frameworks including hybrid optical-electrostatic modulation, ponderomotive laser-based aberration correction, and optical electron interferometry. By unifying these developments, stimulated electron-light interactions provide a versatile route to precise beam control, quantum-state engineering, and tailored light-matter coupling, with implications for ultrafast spectroscopy, nanoscale metrology, attosecond pulse generation, electron-photon entanglement, and the creation of nonclassical states of light.

Excitons – bound electron–hole pairs – are central to the optical properties of semiconducting van der Waals materials, offering a platform for exploring strong light‐matter interactions, Bose‐Einstein condensation, superradiance, nanolasing, and energy harvesting applications. While strong exciton–photon coupling is typically realized using high‐quality optical cavities, such architectures often limit scalability and integration with van der Waals heterostructures and photonic circuitry. In this review, we focus on emerging pathways for coherent exciton energy transfer that circumvent these limitations. Specifically, we examine two classes of self‐sustained hybrid light‐matter states: self‐hybridized exciton–polaritons, which arise from intrinsic excitonic interactions with photons in layered materials, and plasmon‐enhanced exciton–polaritons (or plexcitons), enabled by strong coupling between excitons and plasmon polaritons or Bloch plasmon waves. We discuss key experimental approaches, including electron beam excitation, far‐field optical spectroscopy, and scanning‐probe techniques, that allow the direct observation of Fabry‐Pérot resonances, guided polariton modes, and their propagation and dephasing dynamics. Furthermore, we provide a broader overview on exciton energy transfer, contrasting incoherent diffusion with coherent, delocalized transfer mechanisms. Finally, we highlight design strategies and material platforms that are paving the way toward long‐range, room‐temperature coherent exciton transport in van der Waals systems, with implications for quantum optics, integrated photonics, and excitonic circuitry.

The Smith–Purcell effect enables electromagnetic radiation across arbitrary spectral ranges by phase‐matching the diffraction orders of an optical grating with the near‐field of a moving electron. In this work, we introduce a novel approach using a helically shaped waveguide, where phase‐matching is achieved through guided light within a helical optical fiber fabricated via two‐photon polymerization using a 3D printer. Our results demonstrate that radiation from these structures precisely satisfies the phase‐matching condition and is emitted directionally at specific angles, contrasting with the broad angular distribution characteristic of the traditional Smith–Purcell effect. Helical electron‐driven photon sources establish a new paradigm, enabling 3D‐printed structures to control electron‐beam‐induced radiation and, inversely, to facilitate light‐induced efficient electron beam shaping and acceleration.

Cathodoluminescence spectroscopy has recently emerged as a novel platform for nanoscale control of nonclassical features of light. Here, we propose a theoretical model for cathodoluminescence from a multilevel quantum emitter. Employing a master equation approach and treating the electron-beam excitation as an incoherent broadband field source, we show that quantum interference can arise between the different relaxation pathways. The induced interference can significantly modify the time-dependent spectra resulting in the enhancement or suppression of cathodoluminescence. We find that the excitation rate, initial state of the emitter, and excited level spacing play a crucial role in determining the influence of interference. Our findings shed light on electron-beam-induced quantum interference in cathodoluminescence and provides a theoretical basis for exploring quantum optical phenomena in electron-driven multilevel systems.

Over the past century, continuous advancements in electron microscopy have enabled the synthesis, control, and characterization of high-quality free-electron beams. These probes carry an evanescent electromagnetic field that can drive localized excitations and provide high-resolution information on material structures and their optical responses, currently reaching the sub-Å and few-meV regime. Moreover, combining free electrons with pulsed light sources in ultrafast electron microscopy adds temporal resolution in the subfemtosecond range while offering enhanced control of the electron wave function. Beyond their exceptional capabilities for time-resolved spectromicroscopy, free electrons are emerging as powerful tools in quantum nanophotonics, on par with photons in their ability to carry and transfer quantum information, create entanglement within and with a specimen, and reveal previously inaccessible details on nanoscale quantum phenomena. This Roadmap outlines the current state of this rapidly evolving field, highlights key challenges and opportunities, and discusses future directions through a collection of topical sections prepared by leading experts.