Topological insulators in longitudinally driven waveguides: Lieb and Kagome lattices

Abstract: Topological insulators are studied via tight-binding approximations of longitudinally driven photonic lattices with three lattice sites per unit cell. Two cases are considered in detail: Lieb and Kagome lattices. The lattice is decomposed into three sublattices each of which are allowed move independently of one another. Emphasis is placed on periodic driving induced by laser-etched helical coils along the direction of propagation. The linear Floquet bands are constructed for various inter-sublattice rotation patterns such as: different radii, different frequency, phase offset and quasi one-dimensional motion. Depending on the nature of the band structure, bulk spectral bands with nonzero Chern number are found to support topologically protected edge states which can move unidirectionally. In this case, the modes move scatter-free around defects due to underlying topological protection. Intriguing mode dynamics are found including bi-directional topological modes and bulk-edge leakage i.e. excitation of bulk modes at a defect for edge modes with dispersion frequencies nearby the bulk bands. Finally, certain nonlinear edge modes are also found to propagate unidirectionally and scatter-free around lattice defects.