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We investigate how free-electrons can excite modes in nearby photonic waveguides. Using particle-in-cell simulations, we explore how a free-electron packet can couple energy into multiple, velocity-matched modes of an adjacent silicon waveguide. 

The interaction between free electrons and photons in electron microscopes offers unique opportunities for microscopy and quantum science. For example, modulating electron beams with multiple laser excitations, researchers have demonstrated a novel near-field electron microscope, capable of probing electromagnetic excitations on the nanometer spatial scale and in the attosecond (10 −18 s) temporal range [see D. Nabben et al., Nature, 619, 63 (2023)]. Additionally, it has recently been demonstrated that the interaction between free electrons and photons in an electron microscope can be quantum coherent, and furthermore, this quantum coherence could potentially be leveraged for heralded sources of single electrons and photons [see A. Feist et al., Science, 377, 777 (2022)]. Although promising, these innovations in free-electron-photon interactions have thus far suffered a significant limitation: they require high-energy (>100-ke V) electron beams. Accordingly, these demonstrations have taken place in energetic (and expensive) transmission electron microscopes (TEMs). TEMs are a logical setting for these experiments, as their high-energy electrons can be velocity-matched to co-propagating photons in dielectric waveguides. However, achieving such velocity-matching between photons in conventional dielectric waveguides and electrons is not feasible for the low electron energies (<30-keV) in more common scanning electron microscope (SEMs). 

We investigate dielectric waveguides with subwavelength-scale modulation for applications in free-electron-photon interactions. We show that such waveguides are capable of supporting low-loss modes that can efficiently couple to co-propagating, <10-keV electrons. 

We illuminate nanoantenna-based, metal-silicon-metal photodetectors with ultrafast, mid-infrared laser pulses. We record the current versus pulse energy response and observe microamp-level currents, low effective nonlinearities, and strong-field signatures.