Ultrafast Photo-induced Phase Transition in 2D MoTe$_2$.

Photo-induced phase transition (PIPT) provides an ultrafast, energy-efficient way for precisely manipulating the topological properties in transition-metal ditellurides, and can be used to stabilize a topological phase in an otherwise semiconducting phase. Using two-dimensional (2D) monolayer MoTe$_2$ as an example, we demonstrate that the PIPT from semiconducting 2H to topological 1T$'$ phase is induced purely by electronic excitations from first-principles calculations. Such photo-induced electronic excitation changes electron density, modifying the chemical bonding and hence softening the lattice vibrational modes. These pronounced softenings lead to structural symmetry breaking within sub-picosecond, which is far shorter than the timescale of thermally driven phase transition. The transition can be triggered by photons with energy over 2.34 eV, corresponding to a critical excited carrier density of 10.1 $\times 10^{14}$ cm$^{-2}$, which enables controllable phase transformation by varying laser wavelength. Our results show an ultrafast and controllable phase transition mechanism for topological switching applications of 2D systems, paving the way for future 2D material-based science and technology.