High-order above-threshold photoemission from nanotips controlled with two-color laser fields

Abstract

We investigate the process of phase-controlled high-order above-threshold\nphotoemission from sharp metallic nanotips under bichromatic laser fields.\nExperimental photoelectron spectra resulting from two-color excitation with a\nmoderately intense near-infrared fundamental field (1560 nm) and its weak\nsecond harmonic show a strong sensitivity on the relative phase and clear\nindications for a plateau-like structure that is attributed to elastic\nbackscattering. To explore the relevant control mechanisms, characteristic\nfeatures, and particular signatures from near-field inhomogeneity, we performed\nsystematic quantum simulations employing a one-dimensional nanotip model.\nBesides rich phase-dependent structures in the simulated above-threshold\nionization (ATI) photoelectron spectra we find ponderomotive shifts as well as\nsubstantial modifications of the rescattering cutoff as function of the decay\nlength of the near-field. To explore the quantum or classical nature of the\nobserved features and to discriminate the two-color effects stemming from\nelectron propagation and from the ionization rate we compare the quantum\nresults to classical trajectory simulations. We show that signatures from\ndirect electrons as well as the modulations in the plateau region mainly stem\nfrom control of the ionization probability, while the modulation in the cutoff\nregion can only be explained by the impact of the two-color field on the\nelectron trajectory. Despite the complexity of the phase-dependent features\nthat render two-color strong-field photoemission from nanotips intriguing for\nsub-cycle strong-field control, our findings support that the recollision\nfeatures in the cutoff region provide a robust and reliable method to calibrate\nthe relative two-color phase.\n