Observation of wavefront dislocation as bulk evidence of topological transition in a 1D microwave photonic insulator
From amphidromic points of tides to singular optics, wave phenomena commonly involve topology. In particular, the phase singularities of classical wave fields can lead to wavefront dislocations in the interference patterns. In quantum mechanics, the U(1) gauge invariance also enables manifestations of the wave function phase singularities in the context of topological phases of matter. If their topological properties are defined from the wave functions delocalised in the bulk, their main experimental manifestation is the existence of gapless excitations localised at the boundaries. Here we show wavefront dislocation as direct evidence of the phase singularity of the delocalised wave functions and observe it through standing-wave interference in a 1D microwave photonic insulator. By bridging the bulk topology of insulators to a ubiquitous wave phenomenon, we open a promising route to investigate the topological phases near topological defects in real-space interference.