Hybrid-integrated nonreciprocal polarization rotator based on dielectric metasurfaces and a magnet-free thin-film Faraday rotator

Integrated nonreciprocal polarization control is essential for optical isolators and circulators that suppress back-reflection in photonic integrated circuits. In this study, we propose and experimentally demonstrate a hybrid-integrated metasurface-based nonreciprocal polarization rotator (MNPR) consisting of a metalens (ML), a magnet-free Faraday rotator (MFFR), and a bifunctional metasurface (BFMS). The ML provides reciprocal beam collimation and focusing, while the BFMS combines the same wavefront-shaping function with half-wave-plate-like 45° polarization rotation. The MFFR provides nonreciprocal 45° Faraday rotation without an external magnetic field. By aligning the beam path and polarization axes of the three components, the MNPR preserves the input polarization in the forward direction and converts it into the orthogonal polarization in the backward direction. The fabricated MNPR operates over the C-band (1527–1565 nm). At 1550 nm, the MNPR exhibits polarization extinction ratios of 21.3 ± 1.6 dB and 21.2 ± 0.7 dB and insertion losses of 6.4 ± 0.5 dB and 6.4 ± 0.1 dB in the forward and backward directions, respectively. After integration with a silicon waveguide chip, the MNPR preserves its nonreciprocal behavior, with insertion losses of 12.6 dB and 12.5 dB in the forward and backward directions, respectively. Structural optimization strategies are presented to reduce the insertion loss, suggesting its potential as a polarization-control building block for future on-chip optical isolators and circulators.