Spectrally Resolved Electrical Probing of Optical Excitations in 2D Materials

Understanding light-matter interaction in two-dimensional (2D) materials is of critical importance for novel optoelectronic applications. Traditional studies of optical excitations are usually performed in different spectral regimes, e.g., the probing of bound excitons by optical methods and the detection of mobile carriers by electrical transport. Combining illumination in the optical regime and detection in the low-frequency microwave regime, we have performed the nanoscale electrical probing of optically excited quasiparticles in monolayer tungsten disulfide (WS₂) by microwave impedance microscopy (MIM). The light source in this experiment is a supercontinuum laser with a monochromator, which allows us to obtain spectrally resolved information. For samples with a high defect density, the MIM response is dominated by the photoconductivity from free carriers. On the other hand, for samples with a low defect density and encapsulated by hexagonal boron nitride (hBN), the MIM spectrum shows strong dielectric response at the exciton resonance. The results can be quantified by assuming an effective permittivity of the excitons. Our work contributes to the fundamental understanding of low-frequency response of optical excitations in atomically thin 2D materials.