Loss-induced spatial division of Smith-Purcell radiation from even-parity phase gradient metasurfaces

Smith-Purcell radiation is a fundamental electromagnetic phenomenon that occurs when charged particles are near a periodic structure. Conventionally, the intrinsic loss of the structure reduces its radiation efficiency. Here, we show an exception to this understanding by exploring the interaction of the charged particles with a lossy even-parity phase gradient metasurface. We demonstrate that the intrinsic loss can enable precise control of the SPR spatial distribution while keeping the total radiation energy constant. Remarkably, increasing the loss induces a transition from unidirectional radiation in the lower half-space to a critical state with equal intensities in both half-spaces, and eventually to a regime where radiation in the upper-space dominates. These findings provide an approach to manipulating the SPR effect, with potential applications in free-electron radiation devices and related sources.