The "chirality" (left-right asymmetry) of a substance is a fundamental property that induces the rotation of light—a phenomenon known as optical activity. Raman optical activity (ROA) is a distinct manifestation of this effect, observed as a difference in Raman scattering intensity between left- and right-circularly polarized light. Traditionally, ROA was believed to occur only in systems where inversion or time-reversal symmetry is broken, such as in chiral molecules or magnetic materials.
In this study, however, a research team reports significant ROA in an achiral crystal exhibiting "ferroaxial order." This unique state is characterized by the rotational arrangement of atoms and can persist even under inversion symmetry—a condition previously thought to forbid such activity. Furthermore, symmetry analysis and first-principles calculations confirmed that this phenomenon occurs within the scope of the electric dipole approximation, a finding that defies conventional expectations. This research proves that optical activity can emerge even in achiral materials, fundamentally expanding our understanding of the interaction between light and matter.
Figure: Conceptual illustrations of Raman optical activity in ferroaxial material.
Papers
Journal: Physical Review Letters
Title: Raman Optical Activity Induced by Ferroaxial Order in NiTiO3
Authors: Gakuto Kusuno, Takeshi Hayashida, Takayuki Nagai, Hikaru Watanabe, Rikuto Oiwa, Tsuyoshi Kimura, and Takuya Satoh
DOI: 10.1103/wrv8-4f7k