Dispersion engineering of topological bound states in the continuum in photonic crystal slabs for sensing, nonlinear optics, and self-collimated emission

Bound states in the continuum (BICs) in dielectric photonic crystal slabs offer a powerful route to extreme field enhancement and beam shaping in silicon photonics. We present a silicon nitride (Si3N4) on silicon platform where quasi-BICs in photonic crystal slabs enable a broad family of functionalities in a CMOS-compatible environment. By operating close to these quasi-BICs, we demonstrate applications ranging from high-sensitivity, ultra-narrowband response for sensing, to regimes of enhanced and even supercritical coupling associated with giant nonlinear upconversion and self-collimated in-plane emission with narrow angular spread, up to topological effects such as intrinsically chiral transmission. These results show that BIC physics can be systematically exploited in Si3N4 to realize highly directive and spectrally selective integrated devices.