Ferrorotationally ordered materials possess a planar chiral lattice of electric dipoles that are a promising platform for exhibiting directionally asymmetric response functions. A key characteristic of such order is that enantiomorph conversion occurs when the solid is flipped by ${180}^{\circ }$ about an in-plane axis. Using second-harmonic interferometry, we show here that when circularly polarized light is incident on ${\mathrm{MnTiO}}_3$, the generated harmonic intensity depends on whether the incident light is parallel or antiparallel to the ferrorotational axis. For a particular photon helicity, constructive interference occurs in one direction while destructive interference occurs in the other. Our observations represent a fundamentally new optical effect, associated with circularly polarized light in ferrorotational solids, that is distinct from conventional optical activity observed in three-dimensional chiral and Faraday media.

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• Chirality
• Non-reciprocal propagation
• Nonlinear optics
• Multiferroics
• Lattice models in condensed matter
• Optical techniques