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

## TL;DR

This paper investigates the generation of Brunel harmonics across different ionization regimes, emphasizing the importance of subcycle ionization dynamics and demonstrating new frequency generation with incommensurate fields.

## Contribution

It reveals the critical role of subcycle ionization dynamics in Brunel harmonic generation and shows that incommensurate fields can produce new frequencies even in the multiphoton regime.

## Key findings

- Brunel harmonics depend on subcycle ionization dynamics.
- Harmonics vanish at low intensities in multiphoton regime.
- Incommensurate fields generate new frequencies in multiphoton regime.

## 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.