Single-pass stochastic cooling for electron microscopy

Abstract

Scientific applications of free electrons, such as in electron microscopes and particle accelerators, are limited by the brightness of available electron sources. Accelerators can use phase space cooling to surpass these source limitations, whereas electron microscopes can only rely on particle filtering to increase beam coherence at the cost of reduced brightness. We show that strong electron-light coupling in nanophotonic structures makes it possible to achieve single-pass stochastic cooling inside an electron microscope for a lossless improvement of longitudinal emittance. This approach is laser-free and compatible with normal CW operation of electron microscopes. We derive a fully quantum mechanical model of stochastic cooling valid in the strong coupling regime, finding an analytic asymptotic formula for the optimal cooled energy spread $σ_{E_f}\simeq0.88(\hbarω)^{2/3}σ_{E_0}^{1/3}$, with $\hbar ω$ the cooling photon energy and $σ_{E_0}$ the initial energy spread. Numerical simulations confirm that the energy spread reduction is reflected in an increase in the purity of the electron's quantum state and a reduction of the longitudinal phase space area. Our work paves a path towards even brighter beams for electron microscopy, promising higher image quality, enhanced spectral resolution, and a route to reaching fundamental quantum limits on electron beam phase space area.