  Phase transitions are ubiquitous, appearing at every length scale from atoms
to galaxies. In condensed matter, ultrafast laser pulses drive materials to
highly non-equilibrium conditions allowing transitions to new phases of matter
not attainable under thermal excitation. Despite the intense scrutiny these
hidden phases have received, the details of the dynamics of transition and
reestablishment of the ground state remain largely unexplored. Here, we show
the transition to a hidden phase of 1T-TaS2 driven by the screening of
Coulombic repulsive interaction by photoexcited electrons. The temporal
evolution of the coherent lattice dynamics highlights the existence of a novel
phase with a laser fluence-dependent lifetime. The modeling of the dynamics
reveals that the transition is caused by photo-excited carriers and it
disappears at the rate of electron-phonon scattering. Our results demonstrate
how femtosecond laser absorption leads to a decoupling of the electronic and
lattice sub-systems, opening the way to novel states of matter, which can be
controlled with light. We expect our investigation to be a starting point
towards the development of novel ultrafast photonics devices, such as switches
and modulators, taking advantage of fast and tunable phase transitions.

Materials Science;Strongly Correlated Electrons