  The spinel-structure CuIr$_{2}$S$_{4}$ compound displays a rather unusual
orbitally-driven three-dimensional Peierls-like insulator-metal transition. The
low-T symmetry-broken insulating state is especially interesting due to the
existence of a metastable irradiation-induced disordered weakly conducting
state. Here we study intense femtosecond optical pulse irradiation effects by
means of the all-optical ultrafast multi-pulse time-resolved spectroscopy. We
show that the structural coherence of the low-T broken symmetry state is
strongly suppressed on a sub-picosecond timescale above a threshold excitation
fluence resulting in a structurally inhomogeneous transient state which
persists for several-tens of picoseconds before reverting to the low-T
disordered weakly conducting state. The electronic order shows a transient gap
filling at a significantly lower fluence threshold. The data suggest that the
photoinduced-transition dynamics to the high-T metallic phase is governed by
first-order-transition nucleation kinetics that prevents the complete ultrafast
structural transition even when the absorbed energy significantly exceeds the
equilibrium enthalpy difference to the high-T metallic phase. In contrast, the
dynamically-decoupled electronic order is transiently suppressed on a
sub-picosecond timescale rather independently due to a photoinduced Mott
transition.
