Terahertz and higher-order Brunel harmonics: from tunnel to multiphoton ionization regime in tailored fields

Brunel radiation appears as a result of a two-step process of photo-ionization and subsequent acceleration of electron, without the need of electron recollision. We show that for generation of Brunel harmonics at all frequencies the subcycle ionization dynamics is of critical importance. Namely, such harmonics disappear at low pump intensities when the ionization dynamics depends only on the slow envelope (so called multiphoton ionization regime) and not on the instantaneous field. Nevertheless, if the pump pulse contains incommensurate frequencies, Brunel mechanism does generate new frequencies even in the multiphoton ionization regime.