Free-carrier absorption from electrons in confined systems

We present a calculation of the free-carrier absorption for ultrathin wires fabricated from III-V compound semiconductor materials, where the carriers are scattered either by polar optical phonons or acoustic phonons. The free-carrier absorption coefficient is found to be an oscillatory function of the photon frequency when the carriers are confined in quasi-one-dimensional structures. We also found that the absorption in quasi-one-dimensional structures is enhanced by going to wires of smaller transverse dimensions over its value in the bulk III-V compound semiconducting materials. In addition, it was found that in quasi-one-dimensional structures made from these materials the electron–polar-optical-phonon interaction gives greater contribution to the absorption than the electron–acoustic-phonon interaction. Moreover, the absorption is found to depend upon the polarization of the electromagnetic radiation relative to the direction of the confinement of the carriers. The oscillatory behavior of the free-carrier absorption is explained in terms of phonon-assisted transitions between the quantized subbands which arise from the confinement of the carriers in quasi-low-dimensional structures.