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We present an experimental investigation of multiple scattering inphotonic-crystal-based topological edge states with and without engineeredrandom disorder. We map the spatial distribution of light as it propagatesalong a so-called bearded interface between two valley photonic crystals whichsupports both trivial and non-trivial edge states. As the light slows downand/or the disorder increases, we observe the photonic manifestation ofAnderson localization, illustrated by the appearance of localizedhigh-intensity field distributions. We extract the backscattering mean freepath (BMFP) as a function of frequency, and thereby group velocity, for a rangeof geometrically engineered random disorders of different types. For relativelyhigh group velocities (with $n_g < 15$), we observe that the BMFP is an orderof magnitude higher for the non-trivial edge state than for the trivial.However, the BMFP for the non-trivial mode decreases rapidly with increasingdisorder. As the light slows down the BMFP for the trivial state decreases asexpected, but the BMFP for the topological state exhibits a non-conventionaldependence on the group velocity. Due to the particular dispersion of thetopologically non-trivial mode, a range of frequencies exist where two distinctstates can have the same group index but exhibit a different BMFP. While thetopological mode is not immune to backscattering at disorder that breaks theprotecting crystalline symmetry, it displays a larger robustness than thetrivial mode for a specific range of parameters in the same structure.Intriguingly, the topologically non-trivial edge state appears to break theconventional relationship between slowdown and the amount of backscattering.

Multiple backscattering in trivial and non-trivial topological photonic crystal edge states with controlled disorder