Transient birefringence of liquids induced by terahertz electric-field torque on permanent molecular dipoles

TL;DR

This study demonstrates that intense terahertz pulses can directly excite reorientational modes in liquids via molecular dipoles, causing significant transient birefringence and enabling new insights into liquid molecular dynamics.

Contribution

It provides direct experimental evidence of terahertz-induced reorientational modes in liquids through coupling to permanent dipoles, a novel approach in soft-matter science.

Findings

Transient birefringence up to ten times higher than optical excitation

Resonant THz excitation couples to molecular dipoles and reorientational modes

Potential for improved molecular alignment and studying collective motions

Abstract

Microscopic understanding of low-frequency molecular motions in liquids has been a longstanding goal in soft-matter science. So far, such low-frequency motions have mostly been accessed indirectly by off-resonant optical pulses. A more direct approach would be to interrogate the dynamic structure of liquids with terahertz (THz) radiation. Here, we provide evidence that resonant excitation with intense THz pulses is capable of driving reorientational-librational modes of aprotic polar liquids through coupling to the permanent molecular dipole moments. We observe a hallmark of this enhanced coupling: a transient optical birefringence up to an order of magnitude higher than obtained with optical excitation. Our results open up the path to applications such as efficient molecular alignment and systematic study of the coupling of rotational motion to other collective motions in liquids.