  Transition metal dichalcogenide (TMD) materials have attracted substantial
interest due to their remarkable excitonic, optical, electrical, and mechanical
properties, which are highly dependent on their crystal structure. Controlling
the crystal structure of these materials is essential for fine-tuning their
performance, $\textit{e.g.}$, linear and nonlinear optical, as well as charge
transport properties. While various phase-switching TMD materials, like
molybdenum telluride (MoTe$_2$), are available, their transitions are often
irreversible. Here, we investigate the mechanism of a light-induced reversible
phase transition in mono- and bilayer flakes of rhenium disulfide (ReS$_2$).
Our observations, based on scanning transmission electron microscopy, nonlinear
spectroscopy, and density functional theory calculations, reveal a transition
from the ground T$''$ (double distorted T) to the metastable H$'$ (distorted H)
phase under femtosecond laser irradiation or influence of highly-energetic
electrons. We show that the formation of sulfur vacancies facilitates this
phenomenon. Our findings pave the way towards actively manipulating the crystal
structure of ReS$_2$ and possibly its heterostructures.
