Magnetic Exciton-Based On-Chip Optical Attenuators Using CrSBr

Two-dimensional magnetic semiconductors, in which excitonic resonances are coupled directly to the underlying spin order, offer a fundamentally distinct route to active photonic functionality unattainable in conventional dielectrics. The van der Waals antiferromagnet CrSBr exemplifies this class: its sharp excitons exhibit large oscillator strengths and energies that are continuously and reversibly tunable through the magnetic state of the material, producing dramatic field-induced changes in the permittivity. Here, we exploit this magneto-excitonic response to realize compact, magnetically tunable optical attenuators in standard silicon nitride waveguides. We demonstrate up to 2.5 dB/μm continuous attenuation from CrSBr flakes under in-plane magnetic fields below 1 T. We further show that the device response depends nontrivially on the CrSBr thickness through coupled-mode physics with the polarity of the field response inverting between the perturbative and strongly coupled regimes. This work highlights two-dimensional magnetic semiconductors as a promising materials platform for compact, continuously reconfigurable control of guided light.