Marc R. Bourgeois

This paper describes room‐temperature polariton lasing with high circular polarization from plasmonic Kagome lattice cavities strongly coupled to colloidal semiconducting quantum wells. By shrinking and expanding the trimer unit cells in Al nanoparticle lattices, the inversion symmetry is broken, resulting in the spin‐momentum locking of cavity photons. Circularly polarized lattice resonances emerged from crossings of different diffraction orders, denoted as T points, along the Γ‐K direction in momentum space and whose degree of circular polarization is higher than those of high‐symmetry K‐points. Deformed Kagome lattice cavities combined with CdSe nanoplatelets enabled the formation of exciton‐polaritons. Spin‐selectivity from cavity modes resulted in control over the handedness of circular polarization as well as direction of photoluminescence from lower polaritons. Six lasing beams from T points has alternating handedness, low thresholds (8 µJ cm ^{− 2} ), and high degrees of circular polarization (≈0.7). The anticipation that spin‐momentum locking via plasmonic Kagome lattices can be extended to other non‐Bravais metasurfaces with hexagonal symmetries and opens prospects for exploring optical analogues of spin and valley physics.

Chiral phonons possessing valley pseudoangular momentum (PAM) underlie a diversity of quantum phenomena of fundamental and applied importance, but are challenging to probe directly. We show that deficiencies of typical momentum-resolved electron energy loss measurements that make it impossible to distinguish the PAM of chiral phonons can be overcome by introducing pinwheel free electron states with well-defined PAM. Transitions between such states generate 2D periodic arrays of in-plane field vortices with polarization textures tailored to selectively couple to desired chiral mode symmetries.

Spectroscopies utilizing free electron beams as probes offer detailed information on the reciprocal-space excitations of 2D materials such as graphene and transition metal dichalcogenide monolayers. Yet, despite the attention paid to such quantum materials, less consideration has been given to the electron-beam characterization of 2D periodic nanostructures such as photonic crystals, metasurfaces, and plasmon arrays, which can exhibit the same lattice and excitation symmetries as their atomic analogues albeit at drastically different length, momentum, and energy scales. Because of their lack of covalent bonding and influence of retarded electromagnetic interactions, important physical distinctions arise that complicate interpretation of scattering signals. Here we present a fully-retarded theoretical framework for describing the inelastic scattering of wide-field electron beams from 2D materials and apply it to investigate the complementarity in sample excitation information gained in the measurement of a honeycomb plasmon array versus angle-resolved optical spectroscopy in comparison to single monolayer graphene.

Recent advancements in abilities to create and manipulate the electron's transverse wave function within the transmission electron microscope (TEM) and scanning TEM have enabled vectorially resolved electron-energy-loss (EEL) and -gain (EEG) measurements of nanoscale and quantum material responses using pre- and postselected free-electron states. This newfound capability is prompting renewed theoretical interest in quantum-mechanical treatments of inelastic-electron-scattering observables and the information they contain. Here we present a quantum-mechanical treatment of the inelastic scattering of free electrons between pre- and postselected transverse states with fully retarded electron-sample interactions for both spontaneous EEL and continuous-wave laser-stimulated EEG measurements. General expressions for the state-resolved energy-loss and -gain rates are recast in forms amenable to numerical calculation using the method of coupled dipoles. We numerically implement our theory within the electron-driven discrete dipole approximation code and use it to investigate specific examples that highlight its versatility regarding the number, size, geometry, and material composition of the target specimen as well as its ability to describe matter-wave diffraction from finite nanoscopic targets.

Advances in the ability to manipulate free-electron phase profiles within the electron microscope have spurred development of quantum-mechanical descriptions of electron energy loss (EEL) processes involving transitions between phase-shaped transverse states. Here, we elucidate an underlying connection between two ostensibly distinct optical polarization analogs identified in EEL experiments as manifestations of the same conserved scattering flux. Our work introduces a procedure for probing general tensorial target characteristics including global mode symmetries and local polarization.

Free-electron-based measurements in scanning transmission electron microscopes (STEMs) reveal valuable information on the broadband spectral responses of nanoscale systems with deeply subdiffraction limited spatial resolution. Leveraging recent advances in manipulating the spatial phase profile of the transverse electron wavefront, we theoretically describe interactions between the electron probe and optically stimulated nanophotonic targets in which the probe gains energy while simultaneously transitioning between transverse states with distinct phase profiles. Exploiting the selection rules governing such transitions, we propose phase-shaped electron energy gain nanospectroscopy for probing the 3D polarization-resolved response field of an optically excited target with nanoscale spatial resolution. Considering ongoing instrumental developments, polarized generalizations of STEM electron energy loss and gain measurements hold the potential to become powerful tools for fundamental studies of quantum materials and their interaction with nearby nanostructures supporting localized surface plasmon or phonon polaritons as well as for noninvasive imaging and nanoscale 3D field tomography.

Exciton polaritons (EPs) are ubiquitous light-matter excitations under intense investigation as test beds of fundamental physics and as components for all-optical computing. Owing to their unique attributes and facile experimental tunability, EPs potentially enable strong nonlinearities, condensation, and superfluidity at room temperature. However, the diffraction limit of light and the momentum content of fast electron probes preclude the characterization of EPs in nanoscale structured cavities exhibiting energy-momentum dispersion. Here we present fully relativistic analytical theory and companion numerical simulations showing that these limitations can be overcome to measure EPs in periodic nanophotonic cavities on their natural energy, momentum, and length scales via lattice electron energy gain spectroscopy. With the combined high momentum resolution of light and nanoscale spatial resolution of focused electron beams, lattice electron energy gain spectroscopy can expose deeply subwavelength EP features using currently available monochromated, aberration-corrected scanning transmission electron microscopes.