Multi-level upconversion polarization enabled by programmable plasmons

The active control of upconversion polarization in lanthanide-doped nanocrystals through plasmon-photon coupling enables ultracompact nonlinear photonic devices for polarization-encoded optical communication and information processing. However, current plasmonic nanostructures used for this purpose suffer from limited tunability and insufficient sensitivity to polarization, making effective control challenging. Here, we introduce an upconversion plasmonphore platform that overcomes the limitations of isotropic upconversion nanocrystals by utilizing anisotropic gap-plasmon-mode-supported metasurfaces. This platform allows the precise control of plasmon-enhanced excitation polarization and plasmon-coupled emission. When excited with linearly polarized light, the hybrid nanoplatform can switch between four upconversion polarization states, enabling multi-level photonic outputs in parallel or orthogonal configurations. We also demonstrate an information multiplexing scheme using this platform. Our numerical and experimental results not only shed light on nonlinear light-matter interactions and luminescence anisotropy at the nanoscale but also facilitate the development of novel nonlinear polaritonic nanodevices for polarization-based integrated photonics.