Near-field coherent and focused free-electron radiation based on ordered structures with functional units

Smith–Purcell radiation (SPR) has emerged as a compelling platform for exploring light–matter interactions and realizing tunable free-electron light sources. As the demand for compact, high-performance emitters grows, there is increasing interest in structurally reconfigurable SPR systems that operate in the near-field regime—where enhanced light confinement, subwavelength field shaping, and spatial focusing become accessible. However, conventional SPR designs, which treat gratings as homogeneous and indivisible structures, lack the fine-grained tunability required for coordinated spectral and spatial control and inherently support parasitic surface modes. Here, we fill this key gap by introducing an innovative design paradigm. Specifically, we disassemble traditional grating structures into a programmable array of discrete functional units, simulate the electromagnetic response of each unit via CST particle-in-cell simulations, and ultimately assemble these pre-characterized units into a reconfigurable grating. This design paradigm embeds spectral and spatial control at the unit level, enabling frequency locking through Doppler compensation, energy convergence via directional alignment, and suppression of surface-bound modes by breaking Bloch symmetry. Additionally, this design paradigm allows near-field SPR to achieve coherent and focused emission without reliance on external optics. Furthermore, our grating structure demonstrates robustness against variations in electron velocity and electron position. Our results pave the way for developing on-chip terahertz sources and programmable free-electron-based light sources.