Magic high-order harmonics from a quasi-one-dimensional hexagonal solid

TL;DR

This paper reports a novel phenomenon of selective high-order harmonic generation in a hexagonal solid, revealing symmetry-dependent harmonic suppression that could enable crystal-structure characterization.

Contribution

It uncovers a new type of harmonic generation in a quasi-one-dimensional hexagonal solid, showing selective harmonic orders due to symmetry effects, which is a novel finding in HHG research.

Findings

Harmonics are generated only at 1st, 5th, 7th, and 11th orders under circular polarization.

Magic harmonics are exclusive to circular polarization and hexagonal symmetry.

Symmetry analysis explains the suppression of certain harmonic orders.

Abstract

High-order harmonic generation (HHG) from atoms is a coherent light source that opens up attosecond physics, but it is the application of HHG to solids that brings much of excitement for the last decade. Here we report a completely new kind of harmonics in a quasi-one-dimensional and hexagonal barium titanium sulfide: Under circularly polarized laser excitation, harmonics are generated only at first, fifth, seventh and eleventh orders. These magic harmonics appear only with circularly polarized light, not with linearly polarized light. Neither cubic nor tetragonal cells have magic harmonics even with circularly polarized light. Through a careful group-theory analysis, we find that two subgroups of symmetry operations unique to the hexagonal symmetry cancel out third and ninth harmonics. This feature presents a rare opportunity to develop HHG into a crystal-structure characterization tool for phase transitions between hexagonal and nonhexagonal structures.