Topological valley quasi-bound states in the continuum for radiatively accessible robust transport.

Topological valley photonics enables on-chip light transport robust against backscattering, but valley edge states typically lie outside the light cone and cannot be directly excited from free space. Existing devices with radiative functionality mainly exploit on-chip-to-free-space emission through strongly leaky modes, where enhanced radiative coupling compromises transport performance. Achieving free-space-to-on-chip excitation while preserving low-loss topological transport therefore remains a challenge. Here, we experimentally demonstrate topological valley quasi-bound states in the continuum (QBICs) in terahertz valley photonic crystals. By introducing geometric modulation and Brillouin-zone folding, valley edge states are folded into the light cone and transformed into topological valley QBICs with controllable radiative coupling while preserving robust valley transport. Using time-domain terahertz near-field imaging, we directly visualize their transport and demonstrate tunable propagation lengths together with robust transport through sharp bends. We further realize on-chip routing functionalities based on splitter and junction architectures, illustrating free-space-to-on-chip excitation and subsequent signal processing. Our work establishes topological valley QBICs as a platform for interfacing free-space radiation with low-loss topological transport, opening new opportunities for integrated terahertz devices.