"Möbius" microring resonator

A category of optical microring resonator, which is analogous to the Möbius strip, is proposed. The "Möbius" microring resonator allows the conversion between modes with different polarizations in the ring, and light must circulate two cycles to be converted back to the original polarization status, which is similar to a Möbius strip. This topology structure of polarization makes the free spectral range half of that which corresponds to the cavity round trip. The eigenmodes of this microring are hybridizations of different polarizations, and the breaking of the rotation invariance of the ring makes the transmission related to the polarization of input light and the relative rotation of the cavity. Our work opens the door towards the photonic devices with nontrivial mode topology and provides another way to engineer photonic structures for fundamental studies.