TERAHERTZ ABSORPTION IN A PERIODICALLY MODULATED 2-DIMENSIONAL ELECTRON-GAS UNDER A MAGNETIC-FIELD

We present a theoretical calculation of the THz electromagnetic (EM) a bsorption in a periodically modulated two-dimensional system. It is fo und that: (i) The electron-hole pair excitation of finite momentum is the dominant absorption mechanism below and around the cyclotron frequ ency. Such processes are absent in a uniform electron gas under a magn etic field. (ii) The contribution from the plasmon excitation to the E M absorption is no longer singular due to the angular distribution of the spectral weight. The effect of electron-electron interaction can b e included to any order while the electron-impurity interaction is inc luded to the lowest order.