  Polymorphic transitions in layered transition metal dichalcogenides provide
an excellent platform for discovering exotic phenomena associated with
metastable states, ranging from topological phase transitions to enhanced
superconductivity. In particular, the transition from 2H to 1T'-MoTe$_2$, which
was thought to be induced by high-energy photon irradiation among many other
means, has been intensely studied for its technological relevance in nanoscale
transistors. Despite the remarkable electrical performance arising from this
2H-to-1T' transition, it remains controversial whether a crystalline 1T' phase
is produced because optical signatures of this putative transition are found to
be associated with the formation of elemental Te clusters instead. Here, we
demonstrate the creation of an intrinsic 1T' lattice after irradiating a mono-
or few-layer 2H-MoTe$_2$ with a single field-enhanced terahertz pulse, whose
low photon energy limits possible structural damage by optical pulses. To
visualize the temporal evolution of this irreversible transition, we further
develop a single-shot terahertz pump-second harmonic probe technique, and we
find that the transition out of the 2H phase occurs within 10 ns after
photoexcitation. Our results not only resolve the long-standing debate over the
light-induced polymorphic transition in MoTe$_2$, they also highlight the
unique capability of strong-field terahertz pulses in manipulating the
structure of quantum materials.
