Bound-state-in-continuum guided modes in a multilayer electro-optically active photonic integrated circuit platform.

In many physical systems, the interaction with an open environment leads to energy dissipation and reduced coherence, making it challenging to control these systems effectively. In the context of wave phenomena, such lossy interactions can be specifically controlled to isolate the system, a condition known as a bound-state-in-continuum (BIC). Despite the recent advances in engineered BICs for photonic waveguiding, practical implementations are still largely polarization- and geometry-specific, and the underlying principles remain to be systematically explored. Here, we theoretically and experimentally study low loss BIC photonic waveguiding within a two-layer heterogeneous electro-optically active integrated photonic platform. We show that coupling to the slab wave continuum can be selectively suppressed for guided modes with different polarizations and spatial structure. We demonstrate a low-loss same-polarization quasi-BIC guided mode enabling a high extinction Mach-Zehnder electro-optic amplitude modulator within a single Si