Electron spectroscopy using free electrons in electron microscopes (EM) probably entered the field of nano-optics at the end of the 20 th century, with among other the pivotal paper of Yamamoto 1 showing the mapping of plasmonic modes with deep sub-wavelength resolution.Since then, the field has kept growing exponentially, with applications from plasmons, phonons or exciton mapping at near atomic resolution, to quantum optics of nanomaterials or of the free electrons themselves 2 . These results have been boosted by constant disruptions in technology – monochromation, fs sources of pulsed electrons, high efficiency light injection and detection system in the EM – and theory – introduction of concepts of optics or nano-optics in the realm of EM, such as EMLDOS or quantum statistics, to name a few.
In this talk, I will try to cover some of these recent results. I will first present a rapid overview of recent development in nanooptics with free electron beams. I will then focus on two major issues in the fields. The first one relates to the study of high-quality factor photonic cavities, which are limited by the spectral resolution of common EM techniques. I will show how we can use a new type of spectroscopy that allies the spectral resolution of lasers to the spatial resolution of free electrons to resolve these photonic cavity modes 3 (see figure). The second relates to the relation between the energy transferred from a free electron to a nanomaterial, as detected by electron energy loss spectroscopy (EELS) and the subsequent emission of light (aka cathodoluminescence, CL). The relation between the two events has been for a long time totally elusive. We will see that the coincident, nanosecond-resolved measurement of these two signals brings our understanding a step further by unveiling the fate of optical excitations, from their creation through absorption to their annihilation through emission 4 .
References:
1. Yamamoto, N., Araya, K. & García de Abajo, F. J. Photon emission from silver particles induced by a high-energy electron beam. Phys. Rev. B – Condens. Matter Mater. Phys. 64, 2054191–2054199 (2001).
2. Polman, A., Kociak, M. & García de Abajo, F. J. Electron-beam spectroscopy for nanophotonics. Nat. Mater. 18, 1158–1171 (2019).
3. Auad, Y. et al. µeV electron spectromicroscopy using free-space light. Nat. Commun. 14, 4442 (2023).
4. Varkentina, N. et al. Cathodoluminescence excitation spectroscopy: Nanoscale imaging of excitation pathways. Sci. Adv. 8, (2022).
Laboratoire de Physique des Solides, CNRS/Université Paris Saclay, Orsay