Etchless photonic integrated circuits enabled by bound states in the continuum: tutorial

We provide a detailed tutorial demonstrating how the principle of “bound states in the continuum” (BICs) enables ultralow-loss guiding and routing of photons in photonic integrated circuits fabricated with an etchless process. Here, BICs refer to the nondissipative transverse magnetic (TM) polarized bound modes that exist in the transverse electric (TE) polarized continuum. First, we provide a theoretical analysis of BICs based on the coupling between the TM bound modes and the TE continuum, which is next verified by numerically simulated waveguide propagation loss of the TM bound modes for different waveguide geometries. Then, we present the experimental details, which include fabrication processes and characterization methods for various types of BIC-based integrated photonic devices. Finally, we discuss the superiority and versatility of the BIC-based integrated photonic platform, which can be adopted for different thin-film substrates, for different wavelength ranges, and for heterogeneous integration with different functional materials.