Deformation of dynamical stark ladders and CW optical absorption in DC-AC biased semiconductor superlattices

We have investigated the importance of the relative phase (phi) between the THz field and the optical field, which is used to monitor the interband absorption, and cw optical absorption spectrum in biased semiconductor superlattices. All of the absorption spectra have Stark ladder structures and the peaks of the Stark ladders are changed for the different phases. Especially, it is demonstrated that the changes of the Stark ladders front those phi = 0 occur periodically in four ladders for phi = pi and in eight ladders for phi = pi/2 in the absence of dynamical localization. The cw optical absorption spectrum, which is obtained as an average over the phases by using short pulse technique, have no gain peaks. Because this procedure leads to a cancellation of regions of negative absorption which may occur in the coherent THz-driven system.