Invited: COUPLING OF ELECTRON BEAMS TO HIGH-QUALITY OPTICAL CAVITIES: CO-INCIDENCE ELECTRON MICROSCOPY AND ELECTRON SPECTROSCOPY WITH MICRO-EV PRECISION

The recent few years have seen a surge of novel microscopy techniques stemming from combining the best of two worlds – the ultrahigh temporal and spectral precision of photonics with the superb resolution of electron microscopy. The scientific approach to this merge was meandering. The first step was the laser triggering of transmission electron microscope (TEM) cathodic beams, allowing for electron pulses with sub-picosecond durations (1 ps = 10^-12 s). It was utilized for capturing laser-driven optical modes with sub-wavelength resolution in a process called PINEM (photon-induced nearfield electron microscopy). These pulsed electrons then facilitated ultrafast electron microscopy, producing a large body of scientific work on rapid dynamical processes captured with unprecedented effective frame rate. Furthermore, the electron wavefunction within these ultrafast TEMs (UTEMs) can be subjected to quantum-coherent manipulations by intense laser fields. The short electron pulse can be fully engulfed within a picosecond optical pulse with high peak intensity. The electron subjected to these modulations is restructured to an energy-ladder state, corresponding to sub-femtosecond duration (1 fs = 10^-15 s).

This talk will present recent paradigmatic turns, conceptually and experimentally, investigating the interaction of high-quality (high-Q) optical micro-resonators with electrons in the TEM. Starting with some theory, I will describe my suggestions for imprinting electron beams with quantum-optical information by entangling them with cavity-confined photons or laser-dressing them with optical coherence and transferring it to a sample downstream. The second part of the talk will show the experimental milestones achieved by a team from Germany, Switzerland, and Israel. We first used round microresonators, simple glass spheres, to bind the photons of a pulsed laser long enough that their combined effect amplifies the modulation of the wavefunction of the ultrashort electron pulse. Then, we developed a high-Q cavity that binds photons for several nanoseconds, similar to the average time separating consecutive electrons in continuous beams (1 eˉ/ns = 160 pA). Thus, we facilitated the investigation of electron-light interaction in standard, continuous-beam TEMs. The cavity is based on commercial silicon-photonics fabrication techniques and can be used as-is in any exiting TEM. These fiber-integrated photonic circuits demonstrate novel microscopy modalities of photonic states based on the efficient in- and out-coupling of light to- and from the resonator. By tuning the incoming optical wavelength and monitoring the energy-filtered micrograph, we show that energy gain electron spectroscopy (contrary to EELS) can reach micro-eV resolution, defined by the resonator linewidth. For comparison, the current record is a few mili-eV, three orders of magnitudes broader (!). We used the high coupling and collection efficiency of our integrated photonic chips for correlated electron-photon microscopy, that is, we count the electrons that lost energy equivalent to one photon only when it is co-detected with a generated single photon. Thus, we can reduce imaging noise and false-positive counting.

These examples of enhanced microscopy modalities powered by light-electron correlations are particularly timely amidst the rising global interest in quantum technologies and may hint at a rich set of phenomena right around the scientific corner.