Free-electron interactions with light and matter have long served as a cornerstone for exploring the quantum and ultrafast dynamics of material excitation. In recent years, this paradigm has evolved from a classical description of radiation and acceleration toward a fully quantum framework, transforming our understanding of light-matter interactions at the single-electron level. These advances have opened new opportunities in high-resolution imaging, ultrafast spectroscopy, interferometry, and the coherent shaping of electron wavepackets. This review surveys stimulated interactions between slow electrons and light, encompassing free-space and near-field mediated mechanisms. We discuss how free-space optical fields coherently modulate electron momentum and energy, and how near-field coupling in nanophotonic and plasmonic structures enables strong, phase-matched, efficient momentum exchange with the electron wavepacket. We further describe electron recoil, which is significant in the slow-electron regime, and temporal and spatial wavepacket shaping that enhances coupling efficiency and extends access to quantum-coherent regimes. Building on these foundations, we outline emerging frameworks including hybrid optical-electrostatic modulation, ponderomotive laser-based aberration correction, and optical electron interferometry. By unifying these developments, stimulated electron-light interactions provide a versatile route to precise beam control, quantum-state engineering, and tailored light-matter coupling, with implications for ultrafast spectroscopy, nanoscale metrology, attosecond pulse generation, electron-photon entanglement, and the creation of nonclassical states of light.
The interaction between free electrons and light has been a fundamental concept in physics for more than a century, from the earliest scattering experiments 1,2 to the development of electron microscopy and diffraction. 3,4 In its traditional formulation, this interaction was described within a classical framework, where electrons were treated as point-like particles deflected by electromagnetic fields. [5][6][7] While this approach accounted for many early observations, it overlooked the intrinsic wave nature of electrons, [8][9][10] for describing phenomena such as quantum-path interferences, quantum nonlinearity, and decoherence. In recent years, innovations in electron microscopy have further transformed nanoscience by enabling atomic-scale insights into biological, chemical, and semiconductor materials. 11 With all this progress, the quantum-coherent interaction of free electrons and photons is currently evolving into a booming research field. It started with the advances of photon-induced near-field electron microscopy (PINEM) in transmission electron microscopes (TEMs) by integrating a femtosecond laser pulse with an electron microscope two decades ago. 8,12 Since then, electron microscopes have been developed not only as devices for investigating the ultrafast dynamics of material excitations such as localized plasmons, 13 and phonon polaritons, 14 but also into an apparatus to test the frontiers of quantum science, such as strong-coupling effects, 15 electron-photon entanglement, 16 ultrafast charge oscillations, 17,18 and nonequilibrium optical excitations. 14,19 These advances have opened new ways for probing plasmon resonances, 15,20,21 exciton dynamics, 22 and phonon behavior, 14 thus driving breakthroughs in electron holography, 23,24 phase retrieval, 25 attosecond pulse trains, 25,26 and wave packet shaping. [27][28][29] To date, most investigations of stimulated electron-photon interactions have employed fast, relativistic electrons in the 70-300 keV range. These high-energy probes have been essential for mapping optical near-fields and quantifying light-matter coupling strengths. Extending this research to slower, low-energy electrons, however, opens a distinct regime in which recoil effects become significant and interaction times are substantially prolonged. Building on this conceptual and instrumental foundation, recent experiments have begun to explore these effects using much less energetic electrons in scanning electron microscopes (SEMs) 20,30 and point-projection microscopes (PPMs). 18,31 In this slow-electron regime (below 30 keV), the reduced electron velocity enhances phase synchronization and extends the interaction time so that even moderate optical fields can induce a pronounced energy-momentum exchange. The recoil effect becomes substantial, [32][33][34] giving rise to asymmetric sidebands, transverse deflections, and richer quantum-coherent pathways. Taken together, these features make slow electrons ideally suited for investigating strong coupling, recoil-sensitive dynamics, [33][34][35] and programmable wavefunction shaping in experimentally accessible and compact systems.
Understanding the fundamentals of these interactions requires a fully quantum-mechanical framework, achieved by combining time-dependent Schrödinger-equation solvers with Maxwellbased electromagnetic field models. 34 Such approaches reveal a rich landscape of new physics, from strong-coupling signatures 20 reminiscent of cavity quantum electrodynamics to quantumpath interactions. 36 Ultrafast energy gating, in addition, enables femtosecond-resolved imaging of transient phenomena such as carrier transport, phonon generation, and plasmon oscillations. 15,19 Drawing on concepts from quantum optics, 10,15,37 researchers have developed methods to manipulate electron wave functions with these confined fields. 38,39 The coherence of an electron beam is characterized by two orthogonal components: longitudinal (temporal) and transverse (spatial) coherence. Longitudinal coherence describes the phase relationship along the direction of propaga- [70][71][72][73][74] Dominant recoil; low mean free path; high sensitivity to surface potentials. Low-Energy 0.1-5 keV ULV-SEM 20,35,55 Recoil dynamics are dominant; high sensitivity to electromagnetic fields. Medium-Energy 5-30 keV SEM 30,50,75 Valid non-recoil approximation for thin films and nanostructures; used for beam shaping. High-Energy > 30 keV TEM, STEM, UEM [8][9][10] Relativistic electrons; negligible recoil; standard PINEM framework.
tion and is inversely proportional to the energy spread (∆E) of the electrons, defining the temporal coherence of the wavepacket. In contrast, transverse coherence refers to the phase relationship between different points across the beam’s wavefront, relating the spatial coherence directly to the source size and beam divergence. 40 These parameters are fundamental in determining spatial resolution and interference contrast in ele
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