  We demonstrate robust power- and wavelength-dependent optical bistability in
fully suspended monolayers of WSe2 near the exciton resonance. Bistability has
been achieved under continuous-wave optical excitation at an intensity level of
10^3 W/cm^2. The observed bistability is originated from a photo-thermal
mechanism, which provides both optical nonlinearity and passive feedback, two
essential elements for optical bistability. Under a finite magnetic field, the
exciton bistability becomes helicity dependent, which enables repeatable
switching of light purely by its polarization.
