Period-doubled Floquet solitons

Floquet solitons are shape-preserving nonlinear wave packets exhibiting periodic micromotion during propagation. Here we propose and experimentally demonstrate a family of period-doubled Floquet solitons in the bulk of a photonic topological insulator. Unlike the traditional Floquet states, the wave function of a period-doubled soliton repeats itself after two periods, apart from an overall phase factor associated with the Floquet quasienergy. Our experimental system consists of periodically modulated honeycomb lattices of optical waveguides fabricated by femtosecond laser writing. We employ a Kerr nonlinearity in which self-focusing gives rise to spatial lattice solitons. Our photonic system constitutes a powerful platform for probing the interplay of time-periodic driving, topology, and nonlinearity in a highly tunable way.