Unidirectional manipulation of Smith-Purcell radiation by phase-gradient metasurfaces.

Here, we present a new, to the best of our knowledge, approach to control Smith-Purcell radiation (SPR) via phase-gradient metasurfaces (PGMs), i.e., periodic grating structures with gradient phase modulation. We show that the phase gradient and the parity design of the PGM can efficiently manipulate higher order diffraction to achieve perfect unidirectional SPR, which significantly alters the SPR in the spectrum and the spatial distribution beyond traditional understanding. Specifically, the even-parity PGM results in incidence-free unidirectional radiation, while the odd-parity PGM enables incidence-locking unidirectional radiation. This unidirectional SPR is very robust, ensured by the parity-dependent diffraction rule in PGMs. A modified formula is presented to reveal the relationship between the radiation wavelength and the radiation angle. Our findings offer a new way to control the electromagnetic radiation of moving charged particles (CPs) with structured materials, which may lead to novel applications in tunable, efficient light sources and particle detectors.