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Colloquium – Chen-Ting Liao (University of Colorado)
Probing topologies in magnets and light: hedgehogs, skyrmions, optical vortices, and beyond
Broken symmetries are ubiquitous in physics and are associated with ferromagnetism, nonreciprocal effects, optical activity, and vortex beams of photons and electrons. In some cases, symmetry breaking also leads to the formation of various topological excitations or defects of a continuous field. In the first part of the talk, I will show how we developed state-of-the-art x-ray nanoscale microscopic and femtosecond time-resolved scattering methods to probe topological spin textures. Our new x-ray imaging method provides world-record 3D vector imaging at 10nm resolution and more than 150nm penetration depth. We then used it to study emergent topological magnetic monopoles (i.e., magnetic hedgehogs or Bloch points)  and magnetic skyrmion lattice. In the second part of the talk, we will look into singular optics and topologies in light. Examples include the creation and characterization of time-varying  and space-time vortices of light . Novel scattering and imaging methods enabled by them will be introduced, including the potential applications using mode-entangled (also called classically entangled) light. Lastly, the potential pathways to generate quantum entangled and squeezed x-rays will be introduced, which could serve as a new quantum sensing tool to probe quantum materials such as spin liquids in the future.
References:  Nature Nanotechnology, in press (2023);  Science 364, eaaw9486 (2019);  Nature Photonics 15, 608 (2021).