Non-equilibrium response in strongly driven systems

Ben Hu and K. Johnsen are studying the response of strongly periodically driven time-dependent systems, such as the optical properties of semiconductors driven by free-electron lasers fields. We are attempting to derive relatively straight-forward methods to calculate the statistical distribution of the occupation of the Floquet states and the linear-response properties of these strongly driven systems.
In a joint project with experimental researchers from U.S.C.B and Japan K. Johnsen and Antti-Pekka Jauho have analyzed optical absorption in a strongly driven multiple quantum well system. The characteristic features of the measured absorption spectrum, as a function of photon energy or flux intensity, can be understand in terms of an interplay of (excitonic) dynamic Franz-Keldysh effect (introduced by A.-P-J and K.J in 1996), and ac Stark Effect. Work is in progress to analyze non-linear mixing signals, and superlattice effects.