  The recent realization of photonic topological insulators has brought the
discovery of fundamentally new states of light and revolutionary applications
such as non-reciprocal devices for photonic diodes and robust waveguides for
light routing. The spatially distinguished layer pseudospin has attracted
attention in two-dimensional electronic materials. Here we report layered
photonic topological insulators based on all-dielectric bilayer photonic
crystal slabs. The introduction of layer pseudospin offers more dispersion
engineering capability, leading to the layer-polarized and layer-mixed photonic
topological insulators. Their phase transition is demonstrated with a model
Hamiltonian by considering the nonzero interlayer coupling. Layer-direction
locking behavior is found in layer-polarized photonic topological insulators.
High transmission is preserved in the bilayer domain wall between two
layer-mixed photonic topological insulators, even when a large defect is
introduced. Layered photonic topological insulators not only offer a route
towards the observation of richer nontrivial phases, but also open a way for
device applications in integrated photonics and information processing by using
the additional layer pseudospin.
