High order Brunel harmonics and supercontinuum formed by a weak optical pump in presence of a strong terahertz field

TL;DR

This paper demonstrates that a strong terahertz field can enhance Brunel harmonics generated by a weak optical pump, leading to a broad supercontinuum capable of producing ultrashort, phase-insensitive pulses.

Contribution

It reveals that a strong THz field suppresses recollision harmonics, enabling efficient high-order Brunel harmonics and supercontinuum generation with weak optical fields.

Findings

Strong THz fields suppress recollision-based harmonics.

High-order Brunel harmonics form a coherent supercontinuum.

Supercontinuum can produce 100-attosecond pulses.

Abstract

Brunel harmonics appear in the optical response of an atom in process of laser-induced ionization, when the electron leaves the atom and is accelerated in the strong optical field. In contrast to recollision-based harmonics, the Brunel mechanism does not require the electron returning to the core. Here we show that in the presence of a strong ionizing terahertz (THz) field, even a weak driving field at the optical frequencies allow for generating Brunel harmonics effectively. The strong ionizing THz pump suppresses recollisions, making Brunel dominant in a wide spectral range. High-order Brunel harmonics may form a coherent carrier-envelope-phase insensitive supercontinuum, compressible into an isolated pulse with the duration down to 100 attoseconds.