Enhanced quantum efficiency in photoemission driven by few-cycle optical fields

We theoretically show that the quantum efficiency (QE) of photoemission can be dramatically enhanced by reducing the laser pulse duration to a few optical cycles. Using an exact quantum theory for ultrafast photoemission, we find that QE can increase by several orders of magnitude as the pulse length decreases from tens of cycles to a few cycles, at fixed low laser intensities. The corresponding photon order of the multiphoton process is also significantly reduced, indicating a shift in the underlying emission mechanism. We find that the carrier-envelope phase strongly modulates the photoelectron spectrum and QE in the few-cycle regime; however, counterintuitively, its influence diminishes for subcycle pulses. The results open up opportunities for enhancing and controlling ultrafast electron emission through optical pulse shaping in the low-intensity regime, with potential applications in attosecond science, ultrafast electron sources, and light-wave-driven quantum devices. The macroscopic enhancement of photoemission efficiency driven by few-cycle optical fields may be considered as yet another example of macroscopic quantum phenomena.