  Conventional topological photonic insulators typically have narrow nontrivial
band gaps truncated by broad dispersive bulk bands, resulting in limited edge
mode transmission bandwidths that can be exploited for potential applications.
Here we propose and demonstrate the first Floquet-Lieb topological photonic
insulator with all flat bands which can support continuous edge mode
transmission across multiple Floquet-Brillouin zones. This supercontinuum of
edge states results from the orthogonality between the flat-band modes and the
edge states, allowing for continuous excitation of the latter without
scattering into the bulk modes. Moreover, we show that these flat bands are
perfectly immune to random variations in the on-site potential, regardless of
how large the perturbations are, thus ensuring complete robustness of the edge
modes to this type of disorder. We realized Floquet-Lieb insulators using 2D
microring resonator lattices with perfect nearest-neighbor couplings.
Transmission measurements and direct imaging of the scattered light
distributions showed an edge mode supercontinuum spanning more than three
microring free spectral ranges. These broad edge mode transmission bands
persisted even in the presence of lattice disorder caused by fabrication
imperfection. The proposed Floquet-Lieb insulator can potentially be used to
realize topological photonic devices with ultrawide bandwidths and super
robustness for applications in integrated quantum photonics and programmable
photonic circuits.
