  Topological photonic crystals (TPhCs) provide robust manipulation of light
with built-in immunity to fabrication tolerances and disorder. Recently, it was
shown that TPhCs based on weak topology with a dislocation inherit this
robustness and further host topologically protected lower-dimensional localized
modes. However, TPhCs with weak topology at optical frequencies have not been
demonstrated so far. Here, we use scattering-type scanning near field optical
microscopy to verify mid-bandgap zero-dimensional light localization close to
100 THz in a TPhC with nontrivial Zak phase and an edge dislocation. We show
that due to the weak topology, differently extended dislocation centers induce
similarly strong light localization. The experimental results are supported by
full-field simulations. Along with the underlying fundamental physics, our
results lay a foundation for the application of TPhCs based on weak topology in
active topological nanophotonics, and nonlinear and quantum optic integrated
devices due to their strong and robust light localization.
