High-order harmonics with frequency-varying polarization within each harmonic

TL;DR

This paper predicts high-order harmonics with continuously varying polarization within each harmonic, enabling new ultrafast chiral phenomena exploration.

Contribution

It introduces polarization-fan harmonics generated by counter-rotating bichromatic drivers, a novel polarization structure in high-order harmonics.

Findings

Polarization varies significantly within each harmonic

Harmonics are nearly circular, linear, then circular with opposite helicity

Process is phase-matched and maintains polarization during propagation

Abstract

We predict high-order harmonics in which the polarization within the spectral bandwidth of each harmonic varies continuously and significantly. For example, the interaction of counter-rotating circularly-polarized bichromatic drivers having close central frequencies with isotropic gas leads to the emission of polarization-fan harmonics which are nearly circularly-polarized in one tail of the harmonic peak, linear in the center of the peak and nearly circular with the opposite helicity in the opposite tail. Polarization fan harmonics are obtained as a result of multiple (at least two) head-on recollisions of electrons with their parent ions occurring from different angles. The process can be phase-matched using standard methods (e.g. pressure tuning phase matching) and maintains the single-atom polarization property through propagation. These polarization-fan harmonics may be used for exploring non-repetitive ultrafast chiral phenomena, e.g. dynamics of magnetic domains, in a single shot