Valley-selective exciton bistability in a suspended monolayer semiconductor

Monolayer transition metal dichalcogenide (TMD) semiconductors such as WSe₂, are direct band gap materials, which exhibit over 80% reflectance contrast at the fundamental exciton resonance. This extremely strong light-matter interaction can lead to optical nonlinearity or even bistability at high optical pump intensity. Furthermore, the valley degree of freedom (DOF) carries valley contrasting orbital magnetic moments, which enables K and K’ valleys of the Brillouin zone exclusively coupled to the incident light with opposite helicity. Valley-selective exciton bistability can be achieved, in principle, by combining the above two properties of monolayer TMD semiconductors. In this work, we demonstrate robust bistable exciton resonance by continuous-wave (cw) optical excitation in a suspended WSe₂ sample. The detailed excitation wavelength and power dependence studies of the sample reflectance, as well as numerical simulation, quantitatively support a photothermal mechanism with internal feedback contributing to the observed excitonic bistability. The presence of external magnetic field further allows control of the sample reflectivity purely by varying the polarization of incident light.