in condensed-matter physics and classical waves due to their adherence to unconventional
bulk-boundary correspondence. A three-dimensional second-order topological insulator can
support one-dimensional modes along its hinges (dubbed “hinge states”). Here, we present
a simple and direct method to construct chiral hinge modes based on a Chern-insulator
stack. We analyze the existence of the hinge modes through the nontrivial quadrupole …