A new class of higher-order topological insulators that localize energy at arbitrary multiple sites

Z-classified topological phases lead to a larger-than-unity number of topological states. However, these multiple topological states are only localized at the corners in nonlocal systems. Here, first, we rigorously prove that the multiple topological states of nonlocal Su-Schrieffer-Heeger (SSH) chains can be inherited and realized by local aperiodic chains with only the nearest couplings. Then, we report a new class of higher-order topological insulators constructed with the local aperiodic chains, which can have any integer number of 0D topological states localized at arbitrary positions in the whole domain of the insulators, including within the bulk. The 0D topological states are protected by the local topological marker in each direction, instead of the bulk multipole chiral numbers in existing work. We experimentally demonstrate these multimode topological states in designed acoustic coupled-resonators systems. Our work enables multiple combinations of localized corner-bulk topological states, which leads to programmable lasers and sasers by selecting the excitation sites without altering the structure, and thus opens a new avenue to signal enhancement for computing and sensing.