Interference and switching effect of topological interfacial modes with geometric phase

We investigate interference between topological interfacial modes in a semiconductor photonic crystal platform with Dirac frequency dispersions, which can be exploited for interferometry switch. It is demonstrated both theoretically and experimentally that in a two-in/two-out structure with four topological waveguides, geometric phases of the two-component spinor wave functions of topological photonic modes accumulate at turning points of waveguides, which govern the interferences and split the electromagnetic energy into two output ports with relative power ratio tunable by the relative phase of inputs. We show that this photonic phenomenon is intimately related to the spin-momentum locking property of quantum spin Hall effect, and results from the symphonic contributions of three phase variables: the spinor phase and geometric phase upon design, and the global phase controlled from outside. The present findings open the door for manipulating topological interfacial modes, thus exposing a facet of topological physics. The topology-driven interference can be incorporated into other devices and is expected to have far-reaching impacts on advanced photonics, optomechanics, and phononics applications.