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Concept
Semiconductor laser theory
Résumé
Semiconductor lasers or laser diodes play an important part in our everyday lives by providing cheap and compact-size lasers. They consist of complex multi-layer structures requiring nanometer scale accuracy and an elaborate design. Their theoretical description is important not only from a fundamental point of view, but also in order to generate new and improved designs. It is common to all systems that the laser is an inverted carrier density system. The carrier inversion results in an electromagnetic polarization which drives an electric field . In most cases, the electric field is confined in a resonator, the properties of which are also important factors for laser performance.
In semiconductor laser theory, the optical gain is produced in a semiconductor material. The choice of material depends on the desired wavelength and properties such as modulation speed. It may be a bulk semiconductor, but more often a quantum heterostructure. Pumping may be electrically or optically (disk laser). All these structures can be described in a common framework and in differing levels of complexity and accuracy.
Light is generated in a semiconductor laser by radiative recombination of electrons and holes. In order to generate more light
by stimulated emission than is lost by absorption, the system's population density has to be inverted, see the article on lasers. A laser is, thus, always a high carrier density system that entails many-body interactions. These cannot be taken into account exactly because of the high number of particles involved. Various approximations can be made:
Free carrier model: In simple models, many-particle interactions are often neglected. The carrier plasma is then simply seen as a reservoir which relaxes the carrier distributions. However, the many body interaction is necessary to produce the correct linewidth. Therefore, at the free carrier level a scattering time has to be introduced phenomenologically, usually extracted from experiment, but will change with carrier density and temperature.
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