Absorption and wave packets in optically excited semiconductor superlattices driven by direct-current-alternating-current (dc-ac) fields

Within the one-dimensional tight-binding minibands and on-site Coulombic in teraction approximation, the absorption spectrum and coherent wave-packet t ime evolution in an optically excited semiconductor superlattice driven by dc-ac electric fields are investigated using the semiconductor Bloch equati ons. The dominating roles of the ratios of de-field Stark to external ac fr equency, as well as ac-field Stark to external ac frequency, are emphasized . If the former is an integer ni, then also N harmonics are present within one Stark frequency, while the fractional case leads to the formation of ex citonic fractional ladders. The latter ratio determines the size and profil e of the wave packet. In the absence of excitonic interaction it controls t he maximum size that wave packets reach within one cycle, while the interac tion produces a strong anisotropy and tends to palliate the dynamic wave-pa cket localization.