Photonic Square‐Root Second‐Order Topological Bound States in the Continuum

Abstract Second‐order topological bound states in the continuum (SOTBICs) have received widely attention in photonics due to theirs counterintuitive localized properties. A model of square‐root SOTBICs is proposed that not only inherit topological features from its parent Hamiltonian, but also support multiple corner states with different propagation constants, a unique feature not found in its parent Hamiltonian. Using femtosecond laser‐writing technology, a photonic nontrivial 2D edge‐centered SSH lattice is fabricated and observe two types of localized corner states with distinct phase distribution. Interestingly, by using an auxiliary waveguide with defined refractive index and coupling into the array, it is experimentally demonstrated that corner states are embedded but do not hybridize with bulk states of the lattice. Additionally, it is experimentally verified that these square‐root SOTBICs are robust against disorders as long as the essential symmetries remain preserved. The work marks the instance of SOTBICs with multiple corner states in photonic lattices.