Fluorescence and fluorescence quenching in a two-level system

Usually, fluorescence of a two-level system increases with the coupling strength to an electromagnetic field. However, we show numerically that the fluorescence is quenched in the ultra-strong coupling limit of a two-level system interacting with an electromagnetic field. This behavior is attributed to the onset of multi-photon processes which occur for the ultra-strong coupling between the two-level system and the electromagnetic field.