Ultrafast Faraday rotation probe of chiral phonon-polaritons in LiNbO₃

A strong magnetic response in materials after time reversal symmetry breaking excitation of circular ionic motion may open avenues for ultrafast control. By combining a pair of perpendicularly polarized terahertz pulses with the right relative delay, we create a chiral terahertz driving field to excite chiral phonon-polaritons in LiNbO₃. The magnitude of the ultrafast Faraday rotation probe matches what we would expect from an internal magnetization of ∼10 [Formula: see text] per unit cell, which would require an external magnetic field in excess of 10 Tesla to produce. The Faraday rotation signal switches direction when the input terahertz pulse is changed from left- to right-circular polarization, indicating a change in magnetization direction. In addition, we improve upon previous experiments by using a differential chopping scheme to remove signals arising from linearly polarized terahertz components that can contaminate the Faraday signal. Models show that chiral atomic motion combines with the inverse Faraday effect to induce a magnetic moment in nonmagnetic LiNbO₃.