Free-carrier absorption in quantizing magnetic fields

The theory of free-carrier absorption in nondegenerate semiconductors is extended to take into account quantizing magnetic fields. In the presence of a magnetic field, the behavior of the free-carrier absorption depends not only upon the magnitude of the magnetic field but also upon the polarization of the electromagnetic field relative to the magnetic field direction. For electromagnetic radiation polarized transverse to the magnetic field, the phenomenon of free-carrier absorption merges into that of cyclotron resonance, which has been extensively reported in the literature. However, when the radiation is polarized parallel to the magnetic field, the absorption depends critically on the mechanisms by which the carriers are scattered in the semiconductor. When acoustic phonon scattering is dominant, we find that the free-carrier absorption is an oscillatory function of the magnetic field which increases in magnitude with the magnetic field. The oscillations only occur when the photon frequency $\Omega$ is greater than the cyclotron frequency of the carriers. When $\Omega <{\omega }_c$ the free-carrier absorption is predicted to increase linearly with the magnetic field. The magnetic-field dependence of the free-carrier absorption is explained in terms of phonon-assisted transitions between the various Landau levels of the carriers.