Directional Smith-Purcell radiation via coupled high-index gratings

The emission of light when an electron passes by a periodic photonic structure, called Smith-Purcell radiation (SPR), creates new types of adjustable light sources. Here, we investigate directional or asymmetric SPR from a high-index dielectric double-grating structure. By introducing a lateral shift between the two gratings, we break the symmetry and enable directional emission through interference between coupled Bloch modes, thus without the need for a mirror-type structure as in previous designs. Using full-wave simulations and eigenmode analysis, we identify the modes responsible for radiation and describe their coupling properties. A coupled mode theory (CMT) model is developed to capture the transition between symmetric and asymmetric regimes, offering fast and intuitive insight into the emission behavior. This work highlights a compact and efficient approach for controlling SPR directionality in dielectric photonic structures.