Robust enhancement of high harmonic generation via attosecond control of ionization

Summary form only given. Advancements in high harmonic generation (HHG) have led to the development of table-top XUV and soft X-ray light sources for attosecond science [1]. However, the very low conversion efficiency of HHG, particularly for longer wavelength driving laser fields [2], poses a significant practical limitation for the use of these sources in many experimental applications. We show that a two colour driving field produces a considerable enhancement of the ionization rate compared to a single colour field, leading to huge increases in the HHG efficiency. We use a tunable mid-IR (1300-1600 nm) source as a driving field and a weaker 800 nm beam as an assisting field. By systematically varying the field parameters we observe increases in HHG efficiency of over two orders of magnitude. The enhancement is achieved via subcycle control of the tunnel ionization dynamics in the bichromatic driving field.