Nonreciprocal Electro-Optic Intermodal Scattering With Momentum Engineered RF Waves

Spatiotemporal modulation approaches have been often employed as alternatives for producing optical nonreciprocity without magneto-optic materials. Unidirectional inter-modal scattering, enabled by either acousto-optic or electro-optic (EO) modulation, is a promising method in this category as it can directly modify optical dispersions and even enables linear nonreciprocal photonic devices in the strong coupling limit. Although EO approaches are often preferred for their practicality, it is challenging to generate the large spatiotemporal momentum required for inter-modal phase matching without EO drive schemes involving multiple drive stimuli. Here, we demonstrate highly selective nonreciprocal inter-modal EO scattering enabled by a single high-index radiofrequency (RF) traveling wave stimulus. Our experimental demonstration is performed on a thin-film lithium niobate-integrated photonics platform in which we engineer a slow-wave radiofrequency (SWRF) transmission line with an effective RF index $>9$ that natively generates the required RF momentum while simultaneously maintaining strong RF-optical mode overlap. By additionally engineering the interaction length, we achieve a directional $\sim 20$ dB nonreciprocal scattering contrast. The SWRF architecture provides a scalable route to magnetic-free nonreciprocity and establishes momentum-engineered RF waves as a powerful tool for next-generation, fully integrated nonreciprocal photonic systems.