Coherent Steering of Order via Lattice Resonances

Discovering methods that facilitate ultrafast and minimally-dissipative switching of spontaneous ordering represents one of the most prominent research directions in modern condensed matter physics. In recent years, breakthrough experiments have revealed that circularly-polarized femtosecond pulses, in the visible spectral range, can non-thermally perturb magnetization via the ultrafast inverse Faraday effect. However, the associated pathway of energy flow from light to electrons to spins - incurs substantial parasitic energy losses while simultaneously restricting the functional duration and amplitude of the spin stimulus to the highly-limited lifetime and strength of the photo-excited electrons. Aiming to unveil an alternative and potentially superior method for the selective switching of magnetic order, this project will explore the possibility of manipulating and ultimately reversing magnetization using left- or right-handed circularly-polarized optical phonons driven at resonance. To coherently pump such phonons, this project will exploit the intense and narrow-band infrared light pulses delivered by free-electron lasers. The ensuing rotational motion of ions, in a manner analogous to the ultrafast Barnett effect, is predicted to temporarily create a magnetic moment that could be sufficiently strong enough, by virtue of the longer lifetime and non-linear character of optical phonons, to drive large-amplitude permanent reorientation of magnetization. By constructing state-of-the-art multi-color pump-probe techniques, operational in both stroboscopic and single-shot modes, the existence, character and universality of this never-seen-before source for magnetic recording and processing of data will be discovered. While the challenging and high-risk experiments proposed here explore largely-uncharted physics, they could reveal a disruptive new tool that enables the highly-efficient, ultrafast and directional switching of spontaneous order.