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Free carrier absorption occurs when a material absorbs a photon, and a carrier is excited from an already-excited state to another, unoccupied state in the same band. This intraband absorption 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.
In the simplest approximation, the Drude model, free carrier absorption is proportional to the square of the wavelength.
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