Dependence of optical absorption spectrum on the phase in THz field-driven semiconductors

We have explored the Stark ladder structures of the absorption spectrum for various strengths and phases of external fields in biased semiconductor superlattices. Gain peaks are thought to be due to the result of the nonlinear sum and difference mixing of the applied combined electric fields which lead to a phase difference between the nonlinear polarization and the applied fields. In the cw absorption spectra, which were obtained by the short pulse technique (OSA Technical Digest, Optical Society of America, Washington DC, 2001, pp. 44–48) the gains disappear, since the nonlinear effects of the combined electric fields are completely cancelled for different phases.