Floquet π mode engineering in non-Hermitian waveguide lattices

Floquet topological systems exhibit rich physics associated with quasienergy band structures and new topological states; nevertheless, they are usually explored in Hermitian systems. Recent studies have shown the capability of non-Hermiticity in engineering topological states, while the interplay of Floquet topological phases and non-Hermiticity remains unclear. Here, we reveal that the non-Hermitian modulation can induce the phase transitions between trivial and nontrivial topological Floquet states. Our study theoretically predicts that the non-Hermitian modulation can create a Floquet π mode in an originally topological trivial system according to the reopening of quasienergy band gap (i.e., the π gap), which is well confirmed experimentally in the silicon waveguide platform. Our approach shows the powerful capability of non-Hermitian modulation in engineering topological modes in Floquet photonics systems and would inspire different possibilities in optical field manipulation in open systems.