  Photo-induced phase transitions (PIPTs) provide an ultrafast,
energy-efficient way for precisely manipulating the topological properties of
transition-metal ditellurides, and can be used to stabilize a topological phase
in an otherwise semiconducting material. Using first-principles calculations,
we demonstrate that the PIPT in monolayer MoTe$_2$ from the semiconducting 2H
phase to the topological 1T$'$ phase can be triggered purely by electronic
excitations that soften multiple lattice vibrational modes. These softenings,
driven by a Peierls-like mechanism within the conduction bands, lead to
structural symmetry breaking within sub-picosecond timescales, which is shorter
than the timescale of a thermally driven phase transition. The transition is
predicted to be triggered by photons with energies over $1.96$\,eV, with an
associated excited carrier density of $3.4\times10^{14}$\,cm$^{-2}$, which
enables a controllable phase transformation by varying the laser wavelength.
Our results provide insight into the underlying physics of the phase transition
in 2D transition-metal ditellurides, and show an ultrafast phase transition
mechanism for manipulation of the topological properties of 2D systems.
