Semiconductor optical gain
Optical Gain is the most important requirement for the realization of a semiconductor laser because it describes the optical amplification in the semiconductor material. This optical gain is due to stimulated emission associated with light emission created by recombination of electrons and holes. While in other laser materials like in gas lasers or solid state lasers, the processes associated with optical gain are rather simple, in semiconductors this is a complex many-body problem of interacting photons, electrons, and holes. Accordingly, understanding these processes is a major objective as being a basic requirement for device optimization. This task can be solved by development of appropriate theoretical models to describe the semiconductor optical gain and by comparison of the predictions of these models with experimental results.
Since defining semiconductor's optical gain is an ambitious undertaking, it is useful to build the understanding by steps. The basic requirements can be defined without the major complications induced by the Coulomb interaction among electrons and holes. To explain the actual operation of semiconductor lasers, one must refine this analysis by systematically including the Coulomb-interaction effects.
For a simple, qualitative understanding of optical gain and its spectral dependence, often so-called free-carrier models are used which is discussed considering the example of a bulk laser here. The term free carrier means that any interactions between the carriers are neglected. A free-carrier model provides the following expression for the spectral dependence
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