Floquet states and intersubband absorption in strongly driven double quantum wells

The effects of an intense driving field on the linear intersubband-absorption spectrum of a symmetric double quantum well are nonperturbatively calculated within the Floquet theory. The dependence of the absorption spectrum on the intensity of the driving field is qualitatively different for photon energies larger and smaller than the splitting of the excited-state doublet: In the former case the splitting of the two absorption lines originally present in the spectrum of the undriven double quantum well is altered by the intensity of the driving field. For certain intensities, a collapse of the tunnel splitting is predicted. In the latter case photon replicas of the original lines appear in the spectrum of the driven quantum well. The absorption strength of these additional lines depends nonmonotonically on the intensity of the driving field. It is shown that these effects can be interpreted as generalizations of the Rabi or ac Stark splitting, which results from Floquet theory in the limit of the driving frequency being in resonance with the splitting of the excited-state doublet.