What is the difference between the Franz-Keldysh effect and free carrier absorption? My first guess to this is that the Franz-Keldysh effect involves a bandgap potential of non-zero volts D.C. while the free carrier absorption effect involves a bandgap potential of zero volts. Obviously, this distinction is very simplistic and I was hoping someone could discuss about this topic.
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When you say "free carrier absorption" I think intra-band transitions. These transitions do not conserve crystal momentum, so they must occur somewhere where translational symmetry is broken. An example of such a place is the region near near the surface of a semiconductor where the bands are bent, or when an electric field is present.
The Franz-Keldysh effect involves inter-band transitions, also occurring where the bands are tilted (possibly due to an electric field), but momentum in this case is conserved. The transitions occur between the tails of the electron wavefunction that tunnel into the (tilted) forbidden gap. The F-K effect is sometimes called tunneling-assisted absorption.
See also an answer I gave to a related question.
