  Transition metal dichalcogenides (TMDCs) monolayers, as two-dimensional (2D)
direct bandgap semiconductors, hold promise for advanced optoelectronic and
photocatalytic devices. Interaction with three-dimensional (3D) metals, like
Au, profoundly affects their optical properties, posing challenges in
characterizing the monolayer's optical responses within the semiconductor-metal
junction. In this study, using precise polarization-controlled final-state sum
frequency generation (FS-SFG), we successfully isolated the optical responses
of a MoS$_2$ monolayer from a MoS$_2$/Au junction. The resulting SFG spectra
exhibit a linear lineshape, devoid of A or B exciton features, attributed to
the strong dielectric screening and substrate induced doping. The linear
lineshape illustrates the expected constant density of states (DOS) at the band
edge of the 2D semiconductor, a feature often obscured by excitonic
interactions in week-screening conditions such as in a free-standing monolayer.
Extrapolation yields the onset of a direct quasiparticle bandgap of about
$1.65\pm0.20$ eV, indicating a strong bandgap renormalization. This study not
only enriches our understanding of the optical responses of a 2D semiconductor
in extreme screening conditions but also provides a critical reference for
advancing 2D semiconductor-based photocatalytic applications.
