Ionization and trajectory control in strong-field photoemission from tungsten needle tips with a two-color laser field

Two-color laser fields with well-defined relative phase allow probing and controlling electronic dynamics on the sub-femtosecond time scale. With two-cycle fundamental pulses we can reach the strong-field regime of photoemission at nanometer sharp tungsten needle tips. When superimposing a weak second harmonic field, we expect a strong modulation of the emission yield\footnote{F\"{o}rster et al., PRL \textbf{117}, 217601 (2016)}$^{,}$\footnote{Paschen et al., J. Mod. Opt. \textbf{64}, 10-11, 1054 (2017)}, and, in the strong-field regime, also trajectory modifications\footnote{Seiffert et al., J. Phys. B. \textbf{51}, 134001 (2018)}. Here we show our experimental findings for field-driven and ionization-related electron dynamics in energy spectra as a function of the phase between fundamental and second harmonic. The comparison with time-dependent Schr\"{o}dinger equation and simple-man's model simulations shows excellent agreement. This allows us to define characteristic markers and use them to disentangle ionization from trajectory modifications, giving insight into the rescattering mechanism by the fundamental field alone and its modification by the second harmonic, in the nearfield and at the surface of a nanoscale needle tip.