Free-carrier absorption is theoretically studied for semiconductor lasers where the photon energy is as large as the energy gap. A quantum-mechanical investigation shows that intraband absorption is very weak and interband absorption totally negligible, in contrast to the usual classical estimations. Therefore free-carrier absorption in semiconductor lasers is insignificant. It follows that free-carrier absorption is not the reason for the impossibility of laser action in indirect-bandgap semiconductors. Obviously the scattering and mirror losses are responsible for this effect.
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Albert Haug (1992) studied this question.
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