  Nonlinear optical properties, such as bulk photovoltaic effects, possess
great potential in energy harvesting, photodetection, rectification, etc. To
enable efficient light-current conversion, materials with strong
photo-responsivity are highly desirable. In this work, we predict that
monolayer Janus transition metal dichalcogenides (JTMDs) in the 1T' phase
possess colossal nonlinear photoconductivity owing to their topological band
mixing, strong inversion symmetry breaking, and small electronic bandgap. 1T'
JTMDs have inverted bandgaps on the order of 10 meV and are exceptionally
responsive to light in the terahertz (THz) range. By first-principles
calculations, we reveal that 1T' JTMDs possess shift current (SC) conductivity
as large as $2300 ~\rm nm \cdot \mu A / V^2$, equivalent to a
photo-responsivity of $2800 ~\rm mA/W$. The circular current (CC) conductivity
of 1T' JTMDs is as large as $10^4~ \rm nm \cdot \mu A / V^2$. These remarkable
photo-responsivities indicate that the 1T' JTMDs can serve as efficient
photodetectors in the THz range. We also find that external stimuli such as the
in-plane strain and out-of-plane electric field can induce topological phase
transitions in 1T' JTMDs and that the SC can abruptly flip their directions.
The abrupt change of the nonlinear photocurrent can be used to characterize the
topological transition and has potential applications in 2D optomechanics and
nonlinear optoelectronics.
