Nonthermal melting of charge density wave order via nucleation in VTe2

Ultrafast optical control of phase change materials is of great importance from fundamental and practical points of view. Transition-metal dichalcogenides are of significant interest in this research field because of the photoinduced phase transition originating from the nonthermal melting of their charge density wave (CDW) orders. In this work, we investigated the ultrafast optical response of VTe2 by using broadband coherent optical phonon spectroscopy with subpicosecond time resolution. With an increase of laser-excitation fluence, a characteristic oscillation around 4.4 THz appeared in the transient reflectivity change. This suggests photoinduction of the high-temperature phase, i.e., a CDW-melt state, of VTe2 within a picosecond. Such an ultrafast transition would be induced by a purely electronic effect, but as with thermodynamic melting, it is strongly suggested that the phase transition dynamics induced spatially by laser irradiation proceeds inhomogeneously via a nucleation mode.