Resonant Helical Dichroism in Twisted Dielectric Metastructures.

Circular dichroism, arising from interactions with light fields of opposite spin angular momentum, has become a fundamental tool for molecular characterization. Meanwhile, helical dichroism (HD)─the dichroic response to vortex beams carrying opposite orbital angular momentum (OAM)─offers an alternative approach for probing chiral molecules and photonic structures. Previous demonstrations of HD have been limited to nonresonant light-matter interactions with chiral micro- and nanostructures, leaving the realization of resonant helical dichroism largely unexplored. Here, we present the design and implementation of twisted dielectric metastructures, composed of an array of rotated silicon trimer nanostructures harnessing nonlocal photonic modes with a high-quality factor of several dozen that enable strong resonant HD for OAM values up to 10. We experimentally demonstrate resonantly enhanced HD for strongly focused OAM beams with the magnitude of topological charges from 1 to 3. Our work suggests a route to use resonant metastructures for control of structured OAM beams with applications in molecular sensing, optical imaging, nonlinear optics, and optical data storage.