An Ultrafast Switch for Electron Emission

Abstract

We demonstrate coherent control of multiphoton and above-threshold photoemission from a single solidstate nanoemitter driven by a fundamental and a weak second harmonic laser pulse. Depending on the relative phase of the two pulses, electron emission is modulated with a contrast of the oscillating current signal of up to 94%. Electron spectra reveal that all observed photon orders are affected simultaneously and similarly. We confirm that photoemission takes place within 10 fs. Accompanying simulations indicate that the current modulation with its large contrast results from two interfering quantum pathways leading to electron emission. DOI: 10.1103/PhysRevLett.117.217601 Ionization by two-color laser fields with well-defined relative phase allows one to tune and control electronic dynamics on the (sub)femtosecond time scale. By virtue of the synthesized laser field, the energy and angular distributions of emitted electrons can be manipulated. Two-color pulses have been used in investigations of above-threshold ionization of atoms [1][2][3], controlled dissociative ionization [4][5][6], dichroism in ionization [7,8], and orientation of molecules [9]. Recently, they have also been applied to control interference fringes in the momentum distribution of electron emission [10][11][12]. In this Letter we demonstrate exquisite coherent emission control by simultaneous interaction of fundamental (ω) and second harmonic (2ω) femtosecond laser pulses with condensed matter. Specifically, we control photoemission from an individual nanotip.While nanotips are nowadays routinely used as electron sources in high-resolution electron microscopes [13], their superb transverse coherence known from dc field emission has only recently been observed in photoemission [14]. Field enhancement at the apex of nanotips [15] confines and enhances electron emission, thus enabling the study of strong-field physics with moderate laser intensities well below the damage threshold [16][17][18][19][20][21]. Laser-induced photoemission from tips (for extensive reviews, see Ref.[22]) has already been employed in pulsed electron guns for electron diffraction experiments [23,24], while arrays of nanotips have been fabricated and explored as electron sources for compact coherent x-ray production [25][26][27]. Recently, a ω − 2ω experiment was performed on a silicon tip array, but no phase-resolved signal indicative of coherent control was reported [27].In our experiment, atomic-scale in situ control over the sample surface results in a well-defined nanoemitter that surpasses the limitations of focal averaging and inhomogeneous broadening. We show that electron emission induced by a strong fundamental pulse can be enhanced or suppressed with a contrast of up to 94% when superimposing a weak second harmonic pulse. This scheme thereby allows efficient coherent control of the nanotip photocurrent utilizing the metal-vacuum interface to suppress emission in one direction. Accompanying simulations suggest that the strong modulation can be explained in te...