  We report the experimental observation of nonlinear light localization and
edge soliton formation at the edges of fs-laser written trimer waveguide
arrays, where transition from non-topological to topological phases is
controlled by the spacing between neighboring trimers. We found that, in the
former regime, edge solitons occur only above a considerable power threshold,
whereas in the latter one they bifurcate from linear states. Edge solitons are
observed in a broad power range where their propagation constant falls into one
of the topological gaps of the system, while partial delocalization is observed
when considerable nonlinearity drives the propagation constant into an allowed
band, causing coupling with bulk modes. Our results provide direct experimental
evidence of the coexistence and selective excitation in the same or in
different topological gaps of two types of topological edge solitons with
different internal structures, which can rarely be observed even in
nontopological systems. This also constitutes the first experimental evidence
of formation of topological solitons in a nonlinear system with more than one
topological gap.
