Second harmonic generation as a probe of broken mirror symmetry

TL;DR

This paper demonstrates that rotational-anisotropy second harmonic generation (RA-SHG) is a highly sensitive and versatile technique for detecting and differentiating broken mirror symmetry in crystalline materials, with broad applicability.

Contribution

It introduces RA-SHG as a method to detect and distinguish broken mirror symmetry and applies it to 1T-TaS2, revealing bulk symmetry breaking in its charge density wave phase.

Findings

RA-SHG detects broken mirror symmetry in 1T-TaS2.

RA-SHG differentiates between opposite chiral mirror symmetry-broken structures.

Evidence of bulk mirror symmetry-breaking in the incommensurate phase of 1T-TaS2.

Abstract

The notion of spontaneous symmetry breaking has been used to describe phase transitions in a variety of physical systems. In crystalline solids, the breaking of certain symmetries, such as mirror symmetry, is difficult to detect unambiguously. Using 1$T$-TaS$_2$, we demonstrate here that rotational-anisotropy second harmonic generation (RA-SHG) is not only a sensitive technique for the detection of broken mirror symmetry, but also that it can differentiate between mirror symmetry-broken structures of opposite planar chirality. We also show that our analysis is applicable to a wide class of different materials with mirror symmetry-breaking transitions. Lastly, we find evidence for bulk mirror symmetry-breaking in the incommensurate charge density wave phase of 1$T$-TaS$_2$. Our results pave the way for RA-SHG to probe candidate materials where broken mirror symmetry may play a pivotal role.