Harnessing structured-light-matter interaction: non-paraxial quantum key distribution and modal control across scales

Structured light—light with spatially varying properties, such as amplitude, phase, polarization, and angular momentum—offers powerful capabilities for encoding information and probing matter across scales. We explore structured-light-matter interactions from two angles: (i) engineering the material and (ii) tailoring the input light field. First, we present an inversely designed vector beam decoder (VBD) metasurface that enables encoding and decoding of non-paraxial fields with 3D polarization, and facilitates high-dimensional quantum key distribution with non-paraxial light. Second, at the nanoscale, we introduce a theoretical framework for controlling multipolar excitations in nanoparticles using tightly focused Generalized Cylindrical Vector Beams (GCVBs). By tuning beam parameters, we selectively excite or suppress electric and magnetic modes. These results demonstrate how structured light enables both advanced optical control and quantum functionality in metasurfaces and nanoscale systems.