  We study the ultrastrong driving of Rydberg excitons in Cu$_2$O by a
microwave field. The effect of the field is studied using optical absorption
spectroscopy, and through the observation of sidebands on the transmitted laser
light. A model based on Floquet theory is constructed to study the system
beyond the rotating wave approximation. We obtain near quantitative agreement
between theory and experiment across a 16-fold range of microwave field
strengths spanning from the perturbative to the ultrastrong driving regime.
Compared to Rydberg atoms, the large non-radiative widths of Rydberg excitons
leads to new behaviour such as the emergence of an absorption continuum without
ionization.
