  We investigate the process of phase-controlled high-order above-threshold
photoemission from sharp metallic nanotips under bichromatic laser fields.
Experimental photoelectron spectra resulting from two-color excitation with a
moderately intense near-infrared fundamental field (1560 nm) and its weak
second harmonic show a strong sensitivity on the relative phase and clear
indications for a plateau-like structure that is attributed to elastic
backscattering. To explore the relevant control mechanisms, characteristic
features, and particular signatures from near-field inhomogeneity, we performed
systematic quantum simulations employing a one-dimensional nanotip model.
Besides rich phase-dependent structures in the simulated above-threshold
ionization (ATI) photoelectron spectra we find ponderomotive shifts as well as
substantial modifications of the rescattering cutoff as function of the decay
length of the near-field. To explore the quantum or classical nature of the
observed features and to discriminate the two-color effects stemming from
electron propagation and from the ionization rate we compare the quantum
results to classical trajectory simulations. We show that signatures from
direct electrons as well as the modulations in the plateau region mainly stem
from control of the ionization probability, while the modulation in the cutoff
region can only be explained by the impact of the two-color field on the
electron trajectory. Despite the complexity of the phase-dependent features
that render two-color strong-field photoemission from nanotips intriguing for
sub-cycle strong-field control, our findings support that the recollision
features in the cutoff region provide a robust and reliable method to calibrate
the relative two-color phase.
