Quantum Efficiency of Photoemission Induced by Few-Cycle Optical Fields*

Photoemission driven by femtosecond optical fields from solid-state nanostructures is an important research area with broad applications in particle accelerators, high-power high-frequency electromagnetic radiation sources, time-resolved electron microscopy, streaking spectroscopy, and strong-field nano-optics [1-3]. The development of ultrafast lasers has enhanced our ability to study photoelectron emission—a phenomenon that unveils the complex dynamics between optical fields and electrons in solids. While quantum efficiency (QE), which quantifies the number of photoelectrons emitted per absorbed photon, is a key metric for characterizing photocathodes, its behavior under ultrashort pulse excitation remains less explored.

In this work, we investigate photoemission at the metal-vacuum interface induced by a Gaussian-profile femtosecond laser pulse and a static electric field by exactly solving the one-dimensional (1D) time-dependent Schrödinger equation (TDSE) [4]. The results show a substantial increase of QE as the pulse duration approaches a few optical cycles under low laser intensities. Additionally, the influence of the carrier-envelope phase (CEP) diminishes when the laser pulse is reduced to sub-cycle duration, in contrast to optical field emission at high laser intensities. The findings enhance our understanding of photoemission mechanisms driven by few-cycle optical field pulses and provide insights for the development of ultrafast optics enabled devices and systems.

[1] S. Hüfner, Photoelectron Spectroscopy: Principles and Applications (Springer Science & Business Media, 2013).

[2] C. Hernandez-Garcia, et al., Electron sources for accelerators, Physics Today 61, 44 (2008).

[3] L. Jin, Y. Zhou, and P. Zhang, Direct density modulation of photo-assisted field emission from an RF cold cathode, Journal of Applied Physics 134, 074904 (2023).

[4] Y. Luo, Y. Zhou, and P. Zhang, Few-cycle optical-field-induced photoemission from biased surfaces: An exact quantum theory, Phys. Rev. B 103, 085410 (2021).