Juraj Krempasky
Optical vortex beams, distinguished by their helical phase fronts, carry orbital angular momentum (OAM), in addition to their spin angular momentum (SAM). This unique "twist" in the light's structure leads to complex light-matter interactions, driven by their spin and spatial characteristics. The topological charge, defined by the quantum number l, quantifies the OAM per photon, allowing twisted light to engage in interactions fundamentally different from conventional beams, which possess only SAM linked to polarization. When focused, these twisted beams exhibit entanglement between SAM and OAM through spin-orbit interactions [1], enabling angular momentum redistribution on a micron scale—a crucial factor for applications like optical tweezers and spanners [2]. However, exploration of nano-scale OAM spin-Hall effects (SHE), for example, in materials with broken inversion symmetry, is still lacking. Here, we present direct evidence of the functional properties of non-paraxial OAM wavefields at atomic resolution using an x-ray standing wave experiment on a 5 nm thick α-GeTe Rashba-type semiconductor. We demonstrate controllable and displacive poling of the Ge-Te bonds in response to selected OAM settings by varying topological charges. Our dynamical diffraction simulations of focused OAM wavefields at the Ge-K edge, generated by a spiral zone plate, reveal an electric field gradient that aligns with the ferroelectric order of α-GeTe, which we ascribe to SHE. This gradient depends on the OAM topological charge and modulates the direction of spin-dependent shifts, providing a tunable mechanism for ferroelectric control. To support our findings, we present systematic variations in diffracted intensity, chemical shifts at the Ge-K edge, and pronounced dichroism in the Ge-K NEXAFS spectra under specific OAM settings. Backed by ab initio NEXAFS scattering calculations, our results offer compelling evidence for OAM-driven all-optical control, facilitating maximum possible displacive ferroelectric switching in α-GeTe.
[1] K. Y. Bliokh et al., Spin-orbit interactions of light, Nature Photonics, 9, 2015[2] N. B. Simpson et al., Optics Letters. Vol. 22, No. 1, 1997