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).