Two-color phase-controlled photoemission from a zero-dimensional nanostructure

We demonstrate that multi-photon photoemission including above-threshold multiphoton orders from a nanotip can be coherently controlled with the optical phase between two light fields. By focusing 74 fs drive pulses at 1560 nm and their second harmonic at 780 nm onto the tip and changing the optical phase between the two colors, we observe an emission current modulation of up to 97.5 %. Additionally, electron energy spectra reveal a homogeneous modulation of all multiphoton orders. Hence, the electron current can be strongly increased (by a factor of 3.7) or almost completely turned off due to interference between two different quantum channels in the material. We argue that the extremely high degree of coherence evidenced by this near-unity current modulation depth is due to the confinement of the local field enhancement at the nanotip. The nano-rod effect allows to apply large DC fields, adding a further degree of freedom to investigate the modulation contrast of the photoemitted electron yield. We show that for an increasing DC electric field a non-cooperative distribution of electron emission leads to a decrease in modulation contrast.