Speaker
Description
What is a twisted light, i.e. an X-ray beam carrying orbital angular momentum? Can the “twist” of a light beam move atoms inside a crystal like it can optically tweeze nano-particles [1]? We explore this question in GeTe, a simple ferroelectric in which tiny displacements of Ge and Te atoms determine the direction of the electric polarization [2]. Instead of applying a conventional voltage, we illuminate the material with an X-ray beam carrying orbital angular momentum, an unusual beam whose wavefront twists like a corkscrew around a dark vortex core. To see how the atoms respond, we use X-ray standing waves, which act as an atomic-scale ruler capable of resolving changes in atomic positions deep inside the crystal. The experiment therefore becomes an unusual form of pump–probe measurement: the structured X-ray vortex drives the ferroelectric state, while the coherent X-ray standing wave simultaneously reveals the resulting atomic rearrangement. By changing the handedness of the vortex, we can follow how the Ge and Te atoms move through the ferroelectric energy landscape and how the polarization is switched. The experiment shows how the spatial structure of light, and not just simply its intensity or polarization, can become a new knob for controlling matter at the atomic scale.
References
[1] Dienerowitz, M. & Dholakia, K. Transfer of orbital angular momentum from an optical vortex beam to a nanoparticle. Topologica 2, 008–008 (2009).
[2] O. Caha, V.Holý, J. Krempaský et al., Optical Vortex Control of Lone-Pair Ferroelectric Order, in preparation
