Observation of Photonic Mobility Edge Phases

Inverse Anderson localizations in lower dimensions predict that, as the hopping rates increase, all localized eigenmodes transition to extended states. Here, through the implementation of a mosaic quasiperiodic photonic waveguide lattice, we experimentally demonstrate a distinctive scenario, where the intermediate-energy eigenmodes become extended, while the low- or high-energy eigenmodes remain localized, leading to the emergence of energy-dependent Anderson localization transitions and mobility edge phases. Our experiment is enabled by developing an adiabatic procedure to prepare the photonic lattice into the zero-energy, lower and upper middle-energy, and ground and highest excited eigenmodes and subsequently measuring their localization properties. Moreover, we also experimentally investigate nonequilibrium quench dynamics for photons and show that photonic Loschmidt echoes can identify the appearance of mobility edge phases. Our study thus opens new avenues for investigating energy-dependent photonic Anderson localizations and harnessing photons to explore intriguing nonequilibrium physics.