Unified approach to floquet lattices, topological insulators, and their nonlinear dynamics

A unified method for analyzing the dynamics and topological structure associated with a class of Floquet topological insulators is presented. The method is applied to a system that describes the propagation of electromagnetic waves through the bulk of a two-dimensional lattice that is helically driven in the direction of propagation. Tight-binding approximations are employed to derive reduced dynamical systems. Further asymptotic approximations, valid in the high-frequency driving regime, yield a time-averaged system which governs the leading order behavior of the wave. From this follows an analytic calculation of the Berry connection, curvature, and Chern number via analyzing the local behavior of the eigenfunctions near the critical points of the spectrum. Examples include honeycomb, Lieb, kagome, and 1/5-depleted lattices. In the nonlinear regime, novel equations governing slowly varying wave envelopes are derived. For the honeycomb lattice, numerical simulations show that for relatively small nonlinear effects a striking spiral pattern occurs; as nonlinearity increases, localized structures emerge, and for somewhat higher nonlinearity the waves appear to collapse.