  Topological insulators are a class of electronic materials exhibiting robust
edge states immune to perturbations and disorder. This concept has been
successfully adapted in photonics, where topologically nontrivial waveguides
and topological lasers were developed. However, the exploration of topological
properties in a given photonic system is limited to a fabricated sample,
without the flexibility to reconfigure the structure in-situ. Here, we
demonstrate an all-optical realization of the orbital Su-Schrieffer-Heeger
(SSH) model in a microcavity exciton-polariton system, whereby a cavity photon
is hybridized with an exciton in a GaAs quantum well. We induce a zigzag
potential for exciton polaritons all-optically, by shaping the nonresonant
laser excitation, and measure directly the eigenspectrum and topological edge
states of a polariton lattice in a nonlinear regime of bosonic condensation.
Furthermore, taking advantage of the tunability of the optically induced
lattice we modify the intersite tunneling to realize a topological phase
transition to a trivial state. Our results open the way to study topological
phase transitions on-demand in fully reconfigurable hybrid photonic systems
that do not require sophisticated sample engineering.
