  We investigate the phenomenon of optical bistability in a driven ensemble of
Rydberg atoms. By performing two experiments with thermal vapors of rubidium
and cesium, we are able to shed light onto the underlying interaction
mechanisms causing such a non-linear behavior. Due to the different properties
of these two atomic species, we conclude that the large polarizability of
Rydberg states in combination with electric fields of spontaneously ionized
Rydberg atoms is the relevant interaction mechanism. In the case of rubidium,
we directly measure the electric field in a bistable situation via two-species
spectroscopy. In cesium, we make use of the different sign of the
polarizability for different l-states and the possibility of applying electric
fields. Both these experiments allow us to rule out dipole-dipole interactions,
and support our hypothesis of a charge-induced bistability.
