Breaking symmetry with light: photo-induced chirality in a non-chiral crystal

TL;DR

This paper demonstrates that light can induce and control chirality in a non-chiral, piezoelectric crystal by resonantly exciting vibrational modes, creating chiral structures with specific handedness.

Contribution

It introduces a method to generate and manipulate chirality in non-chiral materials using resonant light excitation of vibrational modes.

Findings

Chirality can be photo-induced in non-chiral BPO₄ crystal.

Resonant excitation of vibrational modes creates chiral structures.

Photo-induced chirality exhibits rotary power comparable to chiral quartz.

Abstract

Chirality is a pervasive form of symmetry that is intimately connected to the physical properties of solids, as well as the chemical and biological activity of molecular systems. However, its control with light is challenging, because inducing chirality in a non-chiral material requires that all mirrors and all roto-inversions be simultaneously broken. Electromagnetic fields exert only oscillatory forces that vanish on average, mostly leading to entropy increase that does not break symmetries, per se. Here, we show that chirality of either handedness can be generated in the non-chiral piezoelectric material BPO$_4$, in which two compensated sub-structures of opposite handedness coexist within the same unit cell. By resonantly driving either one of two orthogonal, doubly degenerate vibrational modes at Terahertz frequency, we rectify the lattice distortion and exert a displacive force onto the crystal. The staggered chirality is in this way uncompensated in either direction, inducing chiral structure with either handedness. The rotary power of the photo-induced phases is comparable to the static value of prototypical chiral alpha-quartz, limited by the strength of the pump laser pulse.