Editing Photoluminescence (section)

==== Photoexcitation ====

In general, three different excitation conditions are distinguished: resonant, quasi-resonant, and non-resonant. For the resonant excitation, the central energy of the laser corresponds to the lowest [[exciton]] resonance of the [[quantum well]]. No, or only a negligible amount of the excess, energy is injected to the carrier system. For these conditions, coherent processes contribute significantly to the spontaneous emission.<ref name="Kira1999" /><ref name="KiraJahnke1999">Kira, M.; Jahnke, F.; Hoyer, W.; Koch, S. W. (1999). "Quantum theory of spontaneous emission and coherent effects in semiconductor microstructures". ''Progress in Quantum Electronics'' '''23''' (6): 189–279. [https://dx.doi.org/10.1016%2FS0079-6727%2899%2900008-7 doi:10.1016/S0079-6727(99)00008-7.]</ref> The decay of polarization creates excitons directly. The detection of  PL is challenging for resonant excitation as it is difficult to discriminate contributions from the excitation, i.e., stray-light and diffuse scattering from surface roughness. Thus, [[speckle pattern|speckle]] and resonant [[Rayleigh scattering|Rayleigh-scattering]] are always superimposed to the [[Coherence (physics)|incoherent]] emission.

In case of the non-resonant excitation, the structure is excited with some excess energy. This is the typical situation used in most PL experiments as the excitation energy can be discriminated using a [[spectrometer]] or an [[optical filter]]. One has to distinguish between quasi-resonant excitation and barrier excitation.

For quasi-resonant conditions, the energy of the excitation is tuned above the ground state but still below the [[Potential barrier|barrier]] [[absorption edge]], for example, into the continuum of the first subband. The polarization decay for these conditions is much faster than for resonant excitation and coherent contributions to the quantum well emission are negligible. The initial temperature of the carrier system is significantly higher than the lattice temperature due to the surplus energy of the injected carriers. Finally, only the electron-hole plasma is initially created. It is then followed by the formation of excitons.<ref name="KaindlCarnahan2003">Kaindl, R. A.; Carnahan, M. A.; Hägele, D.; Lövenich, R.; Chemla, D. S. (2003). "Ultrafast terahertz probes of transient conducting and insulating phases in an electron–hole gas". ''Nature'' '''423''' (6941): 734–738. [https://dx.doi.org/10.1038%2Fnature01676 doi:10.1038/nature01676.]</ref><ref name="ChatterjeeEll2004">Chatterjee, S.; Ell, C.; Mosor, S.; [[Galina Khitrova|Khitrova, G.]]; Gibbs, H.; Hoyer, W.; Kira, M.; Koch, S. W.; Prineas, J.; Stolz, H. (2004). "Excitonic Photoluminescence in Semiconductor Quantum Wells: Plasma versus Excitons". ''Physical Review Letters'' '''92''' (6). [https://dx.doi.org/10.1103%2FPhysRevLett.92.067402 doi:10.1103/PhysRevLett.92.067402.]</ref>

In case of barrier excitation, the initial carrier distribution in the quantum well strongly depends on the carrier scattering between barrier and the well.