Optical chirality in high harmonic generation

TL;DR

This paper explores the role of optical chirality in high harmonic generation, introducing new measures to understand chiral light-matter interactions in nonlinear optics and proposing a tri-circular pump for tailored attosecond pulses.

Contribution

It extends optical chirality theory to nonlinear processes, especially in high harmonic generation, by defining new quantities for multi-chromatic pumps and their generated pulses.

Findings

Polarization OC of attosecond pulses matches pump in monochromatic cases.

Multi-chromatic pumps have rapidly varying polarization OC.

Proposes tri-circular pump for tunable elliptical attosecond pulses.

Abstract

Optical chirality (OC) - one of the fundamental quantities of electromagnetic fields - corresponds to the instantaneous chirality of light. It has been utilized for exploring chiral light-matter interactions in linear optics, but has not yet been applied to nonlinear processes. Motivated to explore the role of OC in the generation of helically polarized high-order harmonics and attosecond pulses, we first separate the OC of transversal and paraxial beams to polarization and orbital terms. We find that the polarization-associated OC of attosecond pulses corresponds to that of the pump in the quasi-monochromatic case, but not in multi-chromatic pump cases. We associate this discrepancy to the fact that the polarization OC of multi-chromatic pumps vary rapidly in time along the optical cycle. Thus, we propose new quantities, non-instantaneous polarization-associated OC, and timescale-weighted polarization-associated OC, that link the chirality of multi-chromatic pumps and their generated attosecond pulses. The presented extension to OC theory should be useful for exploring various nonlinear chiral light-matter interactions. For example, it stimulates us to propose a tri-circular pump for generation of highly elliptical attosecond pulses with a tunable ellipticity.