Dual-Mode Polarization Control with Quasi-Bound States in the Continuum

Full control of light polarization is one of the most sought-after functionalities in nanophotonics since it allows the replacement of bulky optical components like wave retarders. Here, the study reports the theoretical and experimental demonstration of an ultra-compact dual-mode frequency selective polarization controller that leverages the topological features of symmetry protected quasi-bound states in the continuum (q-BICs) supported by a silicon-based nanostructure. Thanks to q-BIC resonances, arbitrarily polarized incoming light can be converted into linearly polarized light without resorting to the local phase tuning mechanisms that characterize optical metasurfaces. Moreover, the dual-mode operating regime allows to select the transmitted polarization without modifying the device orientation, therefore overtaking the concept of the wire grid polarizer. The experimental findings show that the proposed meta-polarizer possesses an extinction ratio of ≈40 dB for two linear cross-polarization excitations. These results pave the way for a novel class of ultra-compact devices that can be used to compensate unwanted birefringence in optical fibers or to control polarization in complex media environments.