Mixed-Color Multiphoton Transitions as Additional Quantum Channels for Electron Photoemission*

We demonstrate mixed-color electron photoemission from tungsten nanotips. In the experiment, second-harmonic photons were introduced to modify the multiphoton emission process. A twofold increase in quantum efficiency results from the opening up of an additional three-photon quantum channel. The super-additive photoelectron signal can be controlled by input power, field polarization, and pulse overlap. The results of our study provide new prospects for quantum photonics, multiphoton microscopy, and spin-polarized electron sources.\\ We acknowledge supports from NSF, Grant Number 1306565, 1430519.