Strong Dielectric Response at Exciton Resonance in Monolayer Tungsten Disulfide

Understanding the low-frequency dielectric response of excitons in two-dimensional (2D) materials is of critical importance for novel optoelectronic applications. Combining optical stimulation by a supercontinuum laser and electric detection by microwave impedance microscopy (MIM), we observed the excitonic contribution to the permittivity of monolayer tungsten disulfide (WS2) in quasi-equilibrium. For samples with a high defect density, the MIM response can be explained by photoconductivity from free carriers. On the other hand, for samples with low defect density and encapsulated by hexagonal boron nitride (hBN), the MIM spectrum shows a strong dielectric response at the A and B exciton resonances of monolayer WS2. The large diffusion length and power dependence of the MIM data suggest that the permittivity response may be dominated by dark excitons. Our work contributes to the fundamental understanding of excitons in 2D semiconductors in the low-frequency regime.