Biography:

Mathieu Kociak is researcher at the Centre National de la Recherche Scientifique (CNRS). After a Ph. D on superconductivity in carbon nanotubes at the Université Paris Sud, he made a post doc in Meijo university, Japan, on in situ TEM transport measurements on carbon nanotubes. After a second post-doc dedicated to magnetic force microscope design in Saclay, he joined the  Laboratory for Solid States Physics (LPS) in Orsay as a junior researcher, then a research director, in the  STEM group. His main research interests include the study of the correlations between the structure, the optical and electronic properties of individual nanoobjects, that he tackles through a combination of instrumental developments in electron microscopy, experiments on the STEM and theory of the electron/matter/photon interaction. He is currently working especially on nanooptics with fast electrons using EELS and nanocathodoluminescence (STEM-CL). He has transferred his STEM-CL technology to the Attolight compagny. He is the STEM group responsible since July 2021. He is the scientific leader of CHROMATEM, a ultra-high energy resolution electron microscopy project, and the deputy director of the french electron microscopy network METSA. Mathieu's awards include the Guinier Prize of the french Physical Society (2002), the quadrennial FEI-EM award (2012) of the European Microscopy Society, the Innovation Prize of the university Paris-Sud (2014) and the Agar Medal of the Royal Society of Microscopy (2015).

LECTURE: "Electron energy loss spectroscopy and cathodoluminescence in the scanning transmission electron microscope"

In this lecture, I will introduce the use of electron energy loss spectroscopy (EELS) and cathodoluminescence (CL) in the scanning transmission electron microscope (STEM) for nanooptics applications. After a short presentation a STEM technologies and general applications, I will introduce the bare minimum theoretical and instrumentation concepts necessary to understand the result of STEM-EELS and CL experiments. I will then present a representative set of experiments in the literature illustrating the interest of STEM-EELS and CL for the study of a variety of photonic and optical materials, from plasmonic nanoparticles to single photon emitters.

TALK: "Enhancing electron spectroscopies using light"

The emergence of new monochromation technologies¹, which are at the basis of the microscope we are about to inaugurate, triggered a vast field of research. In particular, it has allowed the mapping of phonon-polaritons at the nanometric scale², and phonons at the atomic scale³. We will illustrate this aspect with a recent work on mapping three-dimensional phonon-polaritons⁴.

However, the use of these technologies for the study of optical phenomena is still beginning. This is probably because due to the spectral resolutions available. They make trivial addressing the historical field of application of EELS, plasmons mapping⁵. But they are at the limit necessary to approach broader fields of nanooptics, like studying of excitons or photonic modes.

With this in mind, we will first present recent results on the study of excitons in transition metal dichalcogenides⁶ (TMDs) and whispering modes in dielectric spheres⁷. We will see how cathodoluminescence can improve the spectral resolution far below that of EELS, ~ 5 meV, and getting finer physical hints. However, the CL is not sufficient for modes with very large quality factors (larger than ~few hundred). We will present electron energy gain spectroscopy measurements at sub-meV resolution in a monochromated microscope, that allow resolving such modes⁸. On the other hand, the knowledge of the CL and the EELS of the same object brings access to otherwise unknown quantities, such as the Stokes shifts at the nanometer scale^6,7. Beyond,  we will see that the coincident 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 destruction through emission⁹.

References

1. Krivanek, O. L. et al. Vibrational spectroscopy in the electron microscope. Nature 514, 209–212 (2014).
2. Lagos, M. J., Trügler, A., Hohenester, U. & Batson, P. E. Mapping vibrational surface and bulk modes in a single nanocube. Nature 543, 529–532 (2017).
3. Hage, F. S., et al. Single-atom vibrational spectroscopy in the scanning transmission electron microscope. Science (80-. ). 367, 1124–1127 (2020).
4. Li, X. et al. Three-dimensional vectorial imaging of surface phonon polaritons. Science (80-. ) 371, 1364–1367 (2021).
5. Simon, T. et al. Aluminum Cayley trees as scalable, broadband, multiresonant optical antennas. Proc. Natl. Acad. Sci. 119, 1–6 (2022).
6. Bonnet, N. et al. Nanoscale Modification of WS 2 Trion Emission by Its Local Electromagnetic Environment. Nano Lett. 21, 10178–10185 (2021).
7. Auad, Y. et al. Unveiling the Coupling of Single Metallic Nanoparticles to Whispering-Gallery Microcavities. Nano Lett. 22, 319–327 (2022).
8. Auad, Y. et al., µeV electron spectromicroscopy using free-space light. Nature Communications (in press) (2023).
9. Varkentina, N. et al. Cathodoluminescence excitation spectroscopy: nanoscale imaging of excitation pathways. Sci. Adv. 8, (2022).