Electromagnetic absorption in two-dimensional systems under a magnetic field and a unidirectional periodic potential

The absorption of electromagnetic waves by a high mobility two-dimensional electron gas subjected to a magnetic field and a weak periodic potential is investigated. We show that the periodic modulation on the Laudau states has a profound erect on the absorption of electromagnetic waves. We develop a formalism which treats the electron-electron interaction beyond the random-phase-approximation (RPA) and includes the electron-impurity scattering in the lowest order. A RPA dielectric function was employed to study the electromagnetic absorption in modulated systems. Simultaneous excitation of an electron-hole pair with finite momentum contributes significantly to the absorption around and below the cyclotron frequency. Such a process is absent for a uniform electron gas under a magnetic field.