YANG GROUP @ HKU PHYSICS
Free-electron-light interaction
The interaction between free electrons and optical environments gives rise to a multitude of spontaneous and stimulated radiative processes. These processes constitute an invaluable diagnostic platform in microscopy and spectroscopy, and are foundational for free-electron radiation sources spanning the whole electromagnetic spectra, dielectric accelerators, and free-electron-based quantum light generation and manipulation. We are also interested in the interplay between free electrons and photonic topology.
Some selected representative works are as follows.

Maximal Quantum Interaction between Free Electrons and Photons
Achieving strong quantum coupling between free electrons and photons is critical for advancing quantum information, photon entanglement, and free-electron radiation technologies.
We establish the upper limit of the spontaneous coupling strength gQ and provide a universal framework to maximize it, deriving optimal conditions for electron and photon energies. We identify two key regimes: one favoring fast electrons with high-energy photons and the other favoring slow electrons with low-energy photons.
A simple golden rule for maximizing the interaction is obtained: kd = 0.4064 for a given electron photon separation d.
This work generalizes our previous work on the single-frequency limit of free-electron-light interaction (Nature Physics, 2018)
Synthetic gain for electron-beam spectroscopy
Electron-beam microscopy and spectroscopy, with atomic-scale resolution, are vital in nanoscale science. Free electrons, as supercontinuum light sources, interact with phonons, plasmons, and other excitations, complicating spectral isolation amid complex experimental backgrounds.
We introduce synthetic complex frequency waves, created by coherently superposing real-frequency waves from free electrons. These waves provide virtual gain to counteract material
losses, enhancing spectral features.
Measurements on various nanostructures confirm that this approach reveals buried resonances and improves hyperspectral imaging, demonstrating its diagnostic potential in electron-beam spectroscopy and in resolving entangled multiple-photon-electron quantum events.


Photonic flatband resonances for free-electron radiation
In this work, we show that flatbands facilitate the transfer of momentum from free electrons to a continuum of photonic modes and vice versa, thereby enhancing their interaction strength and the resulting radiation. We conduct radiation measurements using silicon-on-insulator photonic crystal slabs to confirm the predictions.