Observation of topological large-area pseudospin-momentum locking waveguide states

Unlike conventional topological edge states confined at a domain wall between two topologically distinct media, the recently proposed large-area topological waveguide states in three-layer heterostructures, which consist of a domain featuring Dirac points sandwiched between two domains of different topologies, have introduced the mode width degree of freedom for more flexible manipulation of electromagnetic waves. Up to now, the experimental realizations of photonic large-area topological waveguide states are exclusively based on quantum Hall and quantum valley-Hall systems. In this work, we propose a new way to create large-area topological waveguide states based on photonic quantum spin-Hall system and observe their unique feature of pseudospin-momentum locking unidirectional propagation for the first time in experiments. Moreover, due to the new effect provided by the mode width degree of freedom, the propagation of these large-area quantum-spin-Hall waveguide states exhibits unusual strong robustness against defects, e.g., large voids with size reaching several unit cells, which has not been reported previously. Our work not only completes the last member of such states in the photonic quantum Hall, quantum valley-Hall and quantum spin-Hall family, but also provides further opportunities for high-capacity energy transport with tunable mode width and exceptional robustness in integrated photonic devices and on-chip communications.