  Photonic topological insulators (PTIs) have been proposed as an analogy to
topological insulators in electronic systems. In particular, two-dimensional
PTIs have gained attention for the integrated circuit applications. However,
controlling the topological phase after fabrication is difficult because the
photonic topology requires the built-in specific structures. This study
experimentally demonstrates the band inversion in two-dimensional PTI induced
by the phase transition of deliberately-designed nanopatterns of a phase-change
material, Ge2Sb2Te5 (GST), which indicates the first observation of the
photonic topological phase transition with changes in the Chern number. This
approach allows us to directly alter the topological invariants, which is
achieved by symmetry-breaking perturbation through GST nanopatterns with
different symmetry from original PTI. The success of our scheme is attributed
to the ultrafine lithographic alignment technologies of GST nanopatterns. These
results demonstrate to control photonic topological properties in a
reconfigurable manner, providing an insight into new possibilities for
reconfigurable photonic processing circuits.
