Abstract
The interaction between light and free electrons not only provides a promising pathway toward the miniaturization of electron accelerators, but also enables rich quantum effects that can drive frontier applications such as quantum sensing and quantum light sources with high spatial and temporal resolution. This talk will include recent progress in the direction of free-electron—light interaction [1]  with an emphasis on the topics of nanophotonic particle accelerators and quantum optical applications based on free electrons.
Electron accelerators are essential in science, medicine, and industry. Dielectric laser accelerators (DLAs), which use laser-driven fields in dielectric nanostructures, can achieve acceleration gradients over an order of magnitude higher than conventional radio-frequency systems, enabling significantly reduced device size. However, key challenges remain, especially the low throughput imposed by subwavelength apertures. To overcome these challenges, we introduced a photonic crystal DLA with multiple electron channels [2]. By engineering the underlying photonic crystal, uniform acceleration fields are achieved across channels, increasing current by orders of magnitude. Beyond acceleration, DLAs enable new capabilities in ultrafast electron science. They support compact pulse compression from picosecond to femtosecond via optical beat-note modulation, offering performance comparable to terahertz techniques with improved efficiency [3].
In the quantum region, light can modulate the wavefunction of free electrons. We investigated the efficient modulation of the free-electron wave function using free-space optical beams [4]. Moreover, we studied how resonant modulation of the free electron can enhance the interaction between the free electron and a two-level atom and probe the atomic coherence [5]. Furthermore, a large coupling between free electrons and photons is generally desired for free-electron-based quantum optical applications, including sensing and quantum light generation. We derived an upper bound for the coupling coefficient describing the free-electron—photo interaction [6]. The upper bound can provide guidance to reach the strong coupling between free electrons and photons.

References
[1] F. J. García de Abajo, A. Polman, C. I. Velasco, M. Kociak et al., ACS Photonics 12, 4760 (2025).
[2] Z. Zhao, D. S. Black, R. J. England, T. W. Hughes, Y. Miao, O. Solgaard, R. L. Byer, and S. Fan, Photonics Research 8, 1586 (2020).
[3] Z. Zhao, K. J. Leedle, D. S. Black, O. Solgaard, R. L. Byer, and S. Fan, Phys. Rev. Lett. 127, 164802 (2021).
[4] Z. Zhao, Y. Fang, M. Uludağ, and P. Hommelhoff, arXiv:2510.24939 (2025).
[5] Z. Zhao, X.-Q. Sun, and S. Fan, Phys. Rev. Lett. 126, 233402 (2021).

[6] Z. Zhao, Phys. Rev. Lett. 134, 043804 (2025).