Dual-Band Polarization Control with Pairwise Positioning of Polarization Singularities in Metasurfaces.

Emerging applications of metasurfaces in classical and quantum optics are driving the need for precise polarization control of nearly-degenerate, high quality (Q)-factor modes. However, current approaches to creating specifically polarized pairs of modes force a trade-off between maintaining high Q factors and robustness. Here, we solve this challenge by employing pairwise generation, annihilation, and positioning of polarization singularities, derived from symmetry-guaranteed pairs of symmetry-protected bound states in the continuum. We experimentally demonstrate this design paradigm in silicon metasurfaces with mode splittings of ≈20  nm, mode splitting deviations as low as 1 nm, and Q factors up to 200. This approach opens new avenues for enhancing metasurface performance across a diverse range of applications, including sensing, modulating, nonlinear mixing, and generating quantum light.