  We report a thermo-optical bistability observed in silicon micro-cantilevers
irradiated by a laser beam with mW powers: reflectivity, transmissivity,
absorption, and temperature can change by a factor of two between two stable
states for the same input power. The temperature dependency of the absorption
at the origin of the bistability results from interferences between internal
reflections in the cantilever thickness, acting as a lossy Fabry-P\'erot
cavity. A theoretical model describing the thermo-optical coupling is
presented. The experimental results obtained for silicon cantilevers irradiated
in vacuum at two different visible wavelengths are in quantitative agreement
with the predictions of this model.
