In-plane excitation of a topological nanophotonic corner state at telecom wavelengths in a cross-coupled cavity

In silicon photonics,the cavity mode is a fundamental mechanism to design integrated passive devices for on-chip optical information processing.Recently,the corner state in a second-order topological photonic crystal(PC)rendered a global method to achieve an intrinsic cavity mode.It is crucial to explore such a topological corner state in silicon photonic integrated circuits(PICs)under in-plane excitation.Here,we study both theoretically and experimentally the topological nanophotonic corner state in a silicon-on-insulator PC cavity at a telecommunications wavelength.In theory,the expectation values of a mirror-flip operation for the Bloch modes of a PC slab are used to characterize the topological phase.Derived from topologically distinct bulk polarizations of two types of dielectric-vein PCs,the corner state is induced in a 90-deg-bend interface,localizing at the corner point of real space and the Brillouin zone boundary of reciprocal space.To implement in-plane excitation in an experiment,we fabricate a cross-coupled PC cavity based on the bend interface and directly image the corner state near 1383 nm using a far-field microscope.Finally,by means of the temporal coupled-mode theory,the intrinsic Q factor of a cross-coupled cavity(about 8000)is retrieved from the measured transmission spectra.This work gives deterministic guidance and potential applications for cavity-mode-based passive devices in silicon PICs,such as optical filters,routers,and multiplexers.