Abstract text (incl. figure legends and references)
Introduction
The wealth of information collected by electron spectroscopies, such as EELS, CL, PINEM, or EEGS, on the physical or material properties of nanobjects is now beyond dispute1.
It is pretty amazing to see how A. Howie's once-promised ability to research nanooptics with free electrons has developed into a reality that surpasses even his exhilarating predictions from almost 25 years ago.
Objectives
I wish to address two crucial questions among the new possibilities provided by cutting edge electron monochromators, light injection and detection tools, electron detectors, and lasers.
The first is technical: how can we maintain the famed spatial resolution of the electron microscope while achieving the sub-meV spectral resolution required to study excitations like excitons or photonic modes?
The second question is more conceptual: how can we comprehend an optical excitation's life cycle, from its creation as a result of interacting with a free electron to its light emission?
Materials and Methods
I'll discuss CL, EELS, and EEGS experiments that try to provide answers to these questions.
A high-precision, high-efficiency light injection and detection technology is the common factor among them all.
Although it has been utilized here in conjunction with a time-resolved EELS direct detector or a highly monochromated STEM, its design has also found utility in other techniques, such as scanning tunneling microscopes.
Results
I shall therefore demonstrate that EEGS can be several orders of magnitude better resolved than EELS, much in the vein of the work of Henke et al., but here proven on arbitrary photonic samples, despite the very high monochromaticity of modern electron microscopes beams (30 meV at 200 keV).
We will see how the combination of sub-10 meV EELS and high efficiency CL in the context of 2D semiconducting structures enables us to illustrate a direct analogy between macroscopic optical absorption and luminescence on the one hand, and EELS and CL on 2D semiconducting materials on the other.
We'll focus on remarkable differences like the direct observation of trions in CL but not in EELS.
However, at this point, it is still unclear what causes these physical distinctions.
Therefore, beyond correlation, we will demonstrate that the coincidence between an absorption event at a certain energy (as revealed by EELS) and a potential emission event at a different energy (as revealed by CL) can shed light on the various routes from absorption to emission, ultimately revealing the fate of an optical excitation.
Conclusions
It is now possible to open numerous locks in nanooptics by combining high spectral resolution EELS, CL, and EEGS. Having the appropriate light injection and detection equipment definitely helps.
Although the use of electron coincidence techniques and light (emission or absorption) techniques is still in its infancy, there is little doubt that they will be very useful in a variety of physics domains, from materials science to quantum technologies.