Twisted Quadrupole Topological Photonic Crystals.

Topological manipulation of waves is at the heart of cutting‐edge metamaterial research. Quadrupole topological insulators were recently discovered in 2D flux‐threading lattices that exhibit higher‐order topological wave trapping at both the edges and corners. Photonic crystals (PhCs), lying at the boundary between continuous media and discrete lattices, however, are incompatible with the present quadrupole topological theory. Here, quadrupole topological PhCs triggered by a twisting degree‐of‐freedom are unveiled. Using a topologically trivial PhC as the motherboard, it is shown that twisting induces quadrupole topological PhCs without flux‐threading. The twisting‐induced crystalline symmetry enriches the Wannier polarizations and leads to the anomalous quadrupole topology. Versatile edge and corner phenomena are observed by controlling the twisting angles in a lateral heterostructure of 2D PhCs. This study paves the way toward topological twist photonics as well as the quadrupole topology in the quasi‐continuum regime for phonons and polaritons. [ABSTRACT FROM AUTHOR]