Influence of microscopic and macroscopic effects on attosecond pulse generation using two-color laser fields

Attosecond pulses and pulse trains generated by high-order harmonic generation are finding broad applications in advanced spectroscopies and imaging, enabling sub-femtosecond electron dynamics to be probed in atomic, molecular and material systems. To date, isolated attosecond pulses have been generated either by using very short few-cycle driving pulses, or by using temporal and polarization gating, or by taking advantage of phase-matching gating. Here we show that by driving high harmonics with a two-color linearly polarized laser field, the temporal window for time-gated phase matching is shorter than for the equivalent singe-color driving laser. As a result, we can generate quasi-isolated attosecond pulses with a peak width of ∼ 450 as using relatively long 26 femtosecond laser pulses. Our experimental data are in good agreement with theoretical simulations, and show that the phase matching window decreases by a factor of 4 - from four optical cycles in the case of a single-color fundamental driving laser, to one optical cycle in the case of two-color (ω-2ω) laser drivers. Finally, we also demonstrate that by changing the relative delay between the two-color laser fields, we can control the duration of the attosecond bursts from 450 as to 1.2 fs.National Science Foundation (NSF) (1125844); Air Force Office of Scientific Research (FA9550-16-1-0121); REA (328334); Junta de Castilla y León (SA046U16); MINECO (FIS2013-44174-P, FIS2016-75652-P)