Manipulation of topological valley kink states in an ultrathin substrate-integrated photonic circuitry

Valley degrees of freedom, providing a novel way to increase the capacity and efficiency for information storage and processing, become an important instrumental for future photonics. However, due to the inter-valley scattering, current realizations of the topological valley kink states are restricted to zigzag boundaries, strongly limiting the development and applications of valley photonics. Besides, the previous topological photonic crystals suffer from either large thickness or insufficient electromagnetic shielding, leading to incompatibility with the standard integrated waveguide circuits. Here, we experimentally demonstrate the realization, engineering and manipulation of valley-polarized topological kink states at generic boundaries in an ultrathin substrate-integrated photonic circuitry with nearly negligible inter-valley scattering. Valley-resolved topological kink states are manipulated by tailoring the boundary geometry between two photonic crystals of opposite valley Hall effects, yielding tunable edge spectrum yet robust valley-polarized transport. Such salient properties can be exploited to design functional substrate-integrated photonic devices, such as disorder-immune waveguides with high transmission, robust photonic delay line, and geometry-dependent topological channel intersections. Our systematic study provides a new route for the manipulation of valley degrees of freedom in the substrate-integrated circuitry, and may work as a novel integration platform for information processing with disorder-insensitivity, easy access, and light weight.