Strain-Tunable Giant Raman Enhancement and Polarization-Dependent Optical Modulation in Janus Monolayers.

Raman spectroscopy is widely used to characterize two-dimensional (2D) materials, thanks to its ultrahigh sensitivity to subtle strains even at the fractional level. Although strain is commonly employed to modulate the physical properties of transition metal dichalcogenides (TMDs), the strain-dependent evolution of Raman fingerprints is rarely explored. In this work, we systematically track the evolution of the characteristic Raman modes under varying strains in WSSe and MoSSe Janus monolayers. Owing to the heavy atomic masses of W and Mo, spin-orbit coupling (SOC) plays a crucial role in determining their physical properties and is thus systematically investigated. Raman intensity mapping reveals a dramatic enhancement driven by SOC, rendering the intensity enhancement induced by ordinary mechanical strain comparatively negligible. Furthermore, we demonstrate how the combined effects of strain and SOC effects modify the allowed optical transitions by shifting the absorption edge and how these materials can be tuned to exhibit a plasma-like optical response at specific wavelengths and strain conditions. This distinct behavior arises from the conductive properties of the materials, which are governed by the effective mass and carrier mobility, both of which are strongly influenced by SOC. These findings provide robust theoretical support for the experimental realization of strained Janus systems and offer valuable insights for the development of advanced optical characterization tools.