Free carrier absorption

Free carrier absorption occurs when a material absorbs a photon, and a carrier (electron or hole) is excited from an already-excited state to another, unoccupied state in the same band (but possibly a different subband). This is different from interband absorption because the excited carrier is already in an excited band, such as an electron in the conduction band or a hole in the valence band, where it is free to move. In interband absorption, the carrier starts in a fixed, nonconducting band and is excited to a conducting one.