  The ultrafast electronic structures of the charge density wave material
1T-TiSe$_2$ were investigated by high-resolution time- and angle-resolved
photoemission spectroscopy. We found that the quasiparticle populations drove
ultrafast electronic phase transitions in 1T-TiSe$_2$ within 100 fs after
photoexcitation, and a metastable metallic state, which was significantly
different from the equilibrium normal phase, was evidenced far below the charge
density wave transition temperature. Detailed time- and pump-fluence-dependent
experiments revealed that the photoinduced metastable metallic state was a
result of the halted motion of the atoms through the coherent electron-phonon
coupling process, and the lifetime of this state was prolonged to picoseconds
with the highest pump fluence used in this study. Ultrafast electronic dynamics
were well captured by the time-dependent Ginzburg-Landau model. Our work
demonstrates a mechanism for realizing novel electronic states by photoinducing
coherent motion of atoms in the lattice.
