  Utrafast control of material physical properties represents a rapid
developing field in condensed matter physics. Yet, accessing to the long-lived
photoinduced electronic states is still in its early stage, especially with
respect to an insulator to metal phase transition. Here, by combing transport
measurement with ultrashort photoexcitation and coherent phonon spectroscopy,
we report on photoinduced multistage phase transitions in Ta2NiSe5. Upon
excitation by weak pulse intensity, the system is triggered to a short-lived
state accompanied by a structural change. Further increasing the excitation
intensity beyond a threshold, a photoinduced steady new state is achieved where
the resistivity drops by more than four orders at temperature 50 K. This new
state is thermally stable up to at least 350 K and exhibits the lattice
structure different from any of the thermally accessible equilibrium states.
Transmission electron microscopy reveals an in-chain Ta atom displacement in
the photoinduced new structure phase. We also found that nano-sheet samples
with the thickness less than the optical penetration depth are required for
attaining a complete transition.
