Surface plasmons play a critical role in enhancing the quality of photoemission currents from electron sources, which have the potential to drive the development of the next generation of free electron devices. Classical theory suggests that the maximum enhancement of the photoemission current can be achieved when the input laser frequency matches the intrinsic resonance frequency of the metal surface plasmon structure. However, classical theory does not account for quantum effects, such as electron transfer between different structures and orbital hybridization. When quantum effects are considered, like the plasmon structure is at an angstrom-scale distance from the electron emission source, new resonant mechanisms like orbital hybridization and charge-transfer plasmons emerge. The photoemission process exhibits anomalous behavior, and the optimal incident laser frequency significantly deviates from the predictions of classical theory.

This work studies the photoemission current behavior of a plasmon-enhanced emitter composed of a gold nanoparticle plasmon structure and a sodium atom electron source using time-dependent density functional theory (TDDFT). We analyze the underlying quantum mechanisms. The theoretical study shows that as the distance between the gold nanoparticle and the sodium atom decreases, the maximum laser excitation frequency of the emitter gradually deviates from the intrinsic resonance frequency of the gold nanoparticle, which contradicts the classical theory. Compared to the photoemission current excited at the intrinsic frequency, the charge-transfer plasmon-enhanced current increases by a factor of 2.07. Further, as the two structures approach each other, the enhancement of the emission current is sequentially dominated by three interaction mechanisms: near-field enhancement, orbital hybridization, and charge-transfer plasmons. This reveals the corresponding system resonance modes and enhancement mechanisms, providing theoretical guidance for the practical construction and experimentation of plasmon-enhanced photoemission electron sources.

This work was published in the journal Physical Review A on April 30, 2024 (DOI: 10.1103/PhysRevA.109.043119). Ph.D. candidate Zhexuan Wang is the first author of the paper, and Professor Fang Liu is the corresponding author.