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

Abstract

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.

Terahertz radiation generation by three-color laser pulses in air filament

“…Nevertheless, since we expect that intensity clamping plays the critical role in the dynamics, we take it into account in our model in a simplified way. In our so called local current (LC) model the so called Brunel radiation EBr(t) is generated by the photoionization-based plasma current J(t) as , where g is a geometry-dependent factor [21,22]. The model assumes that the free electrons are born (in process of ionization) with zero velocity.…”

“…In order to enhance THz generation efficiency it was proposed to use the longer wavelength driver pulses [14][15][16], which later was demonstrated experimentally by using a tunable IR light source [17]. In addition, there were both experimental and theoretical reports [18][19][20] of THz generation by twocolor driver pulses with the shift of one of the frequencies from the exact second harmonic (thus making frequency ratio "optically incommensurate" [21]) which showed the THz frequency shifts and also a spectral broadening [22]. Recently, efficient THz radiation generation has been demonstrated using two-color laser scheme with uncommon frequency ratios (such as 1:4 and 2:3) [23][24][25].…”

Symmetry Breaking and Strong Persistent Plasma Currents via Resonant Destabilization of Atoms

“…As a result, persistent current arises and does not disappear after the end of the pulse. The step-like nature of tunnel ionization is of critical importance for this method: in the deep multiphoton regime, as the sub-cycle steps in the ionization dynamics gradually disappear, the method appears to fail [12].…”

“…It is also known that the change of the macroscopic free current J due to ionization can produce radiation (Brunel harmonics) [5,10,12,[26][27][28]. More specifically, if we assume that plasma arises in a small spatial spot, the corresponding field at the observer point will be governed by the expression:…”

“…As mentioned above, ionization creates harmonics of the pump field only in the case of sub-cycle ionization dynamics. If the ionization event is much slower than the optical cycle (typically associated with the multiphoton ionization regime), the nonlinearity is "too slow" to create harmonics [12]. From Eq.…”