Coexistence of photonic and phononic corner states in a second-order topological phoxonic crystal

Recently, higher-order topological insulators (HOTIs) have been extended from the electronic system to classical wave systems. Beyond the conventional bulk-boundary correspondence, HOTIs can host zero-dimensional topologically protected corner states, which show the strong field localization and robustness against fabrication flaws. Here, we propose a second-order topological phoxonic crystal (PXC) based on a two-dimensional (2D) square lattice, of which different unit cell choices can show either a topologically trivial or non-trivial band structure characterized by the 2D Zak phase. The proposed PXC supports the coexistence of photonic and phononic topological corner states, and their robustness to disorders and defects is numerically demonstrated. Our work opens a venue for achieving simultaneous confinement of photons and phonons, which is potentially useful for exploring the interaction of photonic and phononic second-order topological states and for designing novel topological optomechanical devices. Published under an exclusive license by AIP Publishing.