Exceptional-point-enabled unidirectional Smith-Purcell radiation from a non-Hermitian metasurface.

Smith-Purcell radiation (SPR) from a periodic grating typically exhibits symmetric emission for electrons propagating in opposite directions, even when the structure lacks mirror symmetry. Here, we demonstrate that introducing material loss into a phase-gradient metasurface creates a non-Hermitian system with an exceptional point (EP), thereby enabling unidirectional SPR. At the EP, charged particles (e.g., electrons) incident from the +x direction yield suppressed radiation (i.e., near-zero radiation), whereas incidence from the -x direction produces strong, broadband radiation comparable to that of the lossless counterpart. This unidirectional response is intrinsically locked to the sign of the phase gradient and reverses when the gradient is flipped. Our results establish a passive, gain-free platform for harnessing exceptional points to tailor free-electron radiation, paving the way for on-chip unidirectional THz emitters, electron-beam velocity sensors, and asymmetric free-electron devices.