High-Power Terahertz Emission from Picosecond Nano-Plasma Switching Driven by Secondary Electron Emission Avalanche.

Developing compact, high-power sources in the terahertz (THz) gap (0.1-10 THz) remains a critical challenge in modern electronics, and nano-plasma device (NPD) represents a promising pathway to close this gap. Here, we advance the fundamental understanding of NPDs by showing that an ultra-dense (∼1025 m-3) electron sheet, confined within ∼10 nm above the substrate surface, drives the picosecond nano-plasma switching through secondary electron emission avalanche (SEEA) on the substrate surface. This mechanism was validated by analytical theories, particle-in-cell simulations, and THz experiments, showing that a minimum electric field of ∼108 V/m is required for SEEA initiation, which covers an ∼100 nm gap within ∼ 0.1 ps, switches the NPD on via ionization avalanche within ∼ 1 ps, and produces a 0.4 THz signal with 2 W power. The switching speed increases with higher pressure. Model-guided device optimization identified the high-secondary-emission-yield substrates that favor THz generation, and defined the optimum nanogap length range that ensures both discharge initiation and emission within the THz range. This work demonstrates NPD as a feasible and scalable design for watt-level, on-chip THz generation, and suggests possible future integration with triboelectric nanogenerators as well as potential implications for vacuum nanoelectronic platforms.