Directionally asymmetric nonlinear optics in planar chiral MnTiO$_3$

TL;DR

This paper demonstrates a novel directionally asymmetric nonlinear optical response in planar chiral MnTiO$_3$, revealing new effects in ordered solids that could lead to innovative optical devices.

Contribution

It reports the first observation of directionally asymmetric second harmonic generation in a ferro-rotational ordered material, MnTiO$_3$, due to planar chirality.

Findings

Nonlinear signal shows directional asymmetry with circularly polarized light.

Conversion efficiency varies depending on light orientation relative to FR axis.

Uncovers a new optical effect in ferro-rotational ordered solids.

Abstract

Planar chiral structures possess a two dimensional handedness that is associated with broken mirror symmetry. Such motifs span vast length scales; examples include certain pinwheel molecules, nautilus shells, cyclone wind patterns and spiral galaxies. Although pervasive in nature, it has only recently been found that condensed matter systems can exhibit a form of planar chirality through toroidal arrangements of electric dipoles, known as ferro-rotational (FR) order. A key characteristic of such order is that enantiomorph conversion occurs when the solid is flipped by 180 degrees about an in-plane axis. Consequently, ferro-rotationally ordered materials may exhibit directionally asymmetric response functions, even while preserving inversion and time-reversal symmetry. Such an effect, however, has yet to be observed. Using second harmonic interferometry, we show here that when circularly polarized light is incident on MnTiO$_3$, the generated nonlinear signal exhibits directional asymmetry. Depending on whether the incident light is parallel or anti-parallel to the FR axis, we observe a different conversion efficiency of two right (left) circularly polarized photons into a frequency-doubled left (right) circularly polarized photon. Our work uncovers a fundamentally new optical effect in ordered solids and opens up the possibility for developing novel nonlinear and directionally asymmetric optical devices.