Enlightening electrons: detecting and injecting light in a TEM

Using free electrons, such as those generated in a transmission or scanning electron microscope, to study the optical and physical properties of materials is not new. Indeed, in the past, techniques such as cathodoluminescence (CL) have been used in scanning electron microscopes (SEMs) or electron energy loss spectroscopy (EELS) in transmission electron microscopes (TEMs), respectively to trace the emission properties or the dielectric constant of bulk materials.
However, the field has undergone a rather radical change in the last 15 years, thanks to remarkable technological advances. We can mention the constant improvement of the spectral resolution in EELS, the arrival of increasingly more sensitive CL detectors, the development of time-resolved techniques or the possibility of injecting light into a microscope. These advances have been accompanied by conceptual ones, such as the introduction of concepts previously reserved for optics or quantum optics (electromagnetic local density of state, temporal autocorrelation function, Rabi oscillations ... ) to describe optical experiments with free electrons.
In this talk, I will discuss the use of a light injection and detection system in a TEM. I will try to highlight the advantages and disadvantages of such a system when it is used for CL, compared to a CL system installed in a SEM. I will insist on the study of materials, in particular for optics or quantum optics, on the understanding of the electron-matter interaction when this technique is coupled with EELS, as well as the perspectives in particular for bio-microscopy and for time-resolved TEM.
I will also introduce light injection in TEM and in particular its advantage to perform the recently developed electron energy gain spectroscopy. I will show how such a system can break, by several orders of magnitude, the limits in terms of spectral resolution imposed by even the best monochromator technology.