Observation of multiple topological bound states in the continuum in the photonic bilayer trimer lattice.

A topological bound state in the continuum (TBIC) is a novel topological phase that has attracted significant attention. Different from conventional topological insulators (TIs), where boundary states reside within gaps, TBICs can support unconventional boundary states that remain isolated from the surrounding bulk states. In this work, we experimentally demonstrate multiple TBICs in photonic bilayer trimer lattices using femtosecond laser writing technology. By modulating the interlayer coupling between two trimer chains, we observe the emergence of two distinct types of TBICs. Moreover, we experimentally achieve the coexistence of in-gap topological states and TBICs and demonstrate the transformation between them. Our work unveils new insights into the flexible construction of TBICs, and this method can be easily applied to other one-dimensional topological structures, offering promising avenues for further research.