Tuning excitonic absorption of monolayer WS₂ by proximity screening effects

In two-dimensional (2D) semiconductors, the Coulomb interaction between charge carriers is significantly enhanced due to reduced dielectric screening, resulting in strong many-body effects and large exciton binding energies. The extension of the Coulomb field beyond the material in this unique 2D structure also allows for effective control of electronic and optical properties by manipulating its surrounding environment. Here, we show that by photoexciting the graphene monolayer in graphene/h-BN/WS₂ heterostructures, the excitonic absorption coefficient of WS₂ can be manipulated. Samples are fabricated using mechanical exfoliation and dry transfer techniques. Photocarriers in graphene are excited by an ultrashort infrared laser pulse, and its effect on the absorption coefficient of WS₂ is studied by measuring the differential reflectance of a probe pulse tuned near the WS₂ A-exciton resonance. The observed carrier-induced transient absorption is attributed to the strong screening effect of mobile photocarriers in graphene on the electron-hole Coulomb interaction in excitons formed within WS₂. By varying the thickness of the middle h-BN layer, we investigate the effect as a function of distance. This effect could have important implications for the application of 2D semiconductors. The ultrafast excitation and short lifetime of photocarriers in graphene could be utilized for high-speed modulation of light absorption in 2D semiconductors at THz frequencies.