Active spin-selection and nonreciprocal transmission via nonlocal chiral guided resonance in magneto-optical meta-waveguides.

Achieving intrinsic chirality and giant nonreciprocity for circularly polarized light (CPL) is of great importance in optical communication, quantum information, and advanced imaging. Here, we report a tunable magneto-optical meta-waveguide that can achieve dual narrow-band nonreciprocal transmission of CPL via nonlocal chiral guided resonances (GRs). We show, by breaking time-reversal symmetry, that the polarization states of two GRs at the Ґ point can be simultaneously modulated from linear to circular polarization by applying external magnetic fields, enabling dual-band unidirectional transmission of CPL with a contrast ratio up to 48 dB. Calculation results further reveal that high-quality (Q) chiral GRs are governed by spin-selective electric quadrupole and magnetic dipole resonances with a Q factor of 105. Moreover, the wavelength of asymmetric transmission can be tuned by modifying structural parameters, while giant transmission contrast remains higher than 35 dB. Our design offers versatility for advanced chiral wave manipulation, promising applications in optical communication, ultra-narrow spin filters, and nonreciprocal devices.