  Topological materials are of great interest because they can support metallic
states that are robust against perturbations, with the potential for
technological applications. Here we experimentally explore the light-induced
non-equilibrium properties of two distinct topological phases in NaCd4As3: a
topological crystalline insulator (TCI) and a topological insulator (TI) phase.
This material has surface states that are protected by mirror symmetry in the
TCI phase at room temperature, while it undergoes a structural phase transition
to a TI phase below 200 K. After exciting the TI phase by an ultrafast laser
pulse, we observe a chemical potential shift >150 meV, that slowly builds up
and maximizes after ~0.6 ps, and that persists for ~8 ps. The slow rise time of
the excited electron population and electron temperature suggests that the
electronic and structural orders are strongly coupled in this TI phase. It also
suggests that the excited electronic states immediately and ~0.6 ps after
excitation belong to different orbitals, likely due to partial relaxation of
the lattice distortion that is known to be associated with the TI phase. In
contrast, no distinct excited state is observed in the TCI phase after
photoexcitation, which we attribute to the low density of states and phase
space available near the Fermi level. Our results show how ultrafast excitation
can probe the excited states and interactions within phase rich topological
materials.
