Editing Photoluminescence (section)

== Forms ==
[[File:Photoluminescence animation.gif|thumb|Schematic for the excitation-relaxation processes of photoluminescence]]

Photoluminescence processes can be classified by various parameters such as the energy of the exciting photon with respect to the emission.
Resonant excitation describes a situation in which photons of a particular wavelength are absorbed and equivalent photons are very rapidly re-emitted. This is often referred to as [[resonance fluorescence]]. For materials in solution or in the gas [[Phase (matter)|phase]], this process involves electrons but no significant internal energy transitions involving molecular features of the chemical substance between absorption and emission. In crystalline inorganic semiconductors where an electronic [[band structure]] is formed, secondary emission can be more complicated as events may contain both [[Coherence (physics)|coherent]] contributions such as resonant [[Rayleigh scattering]] where a fixed phase relation with the driving light field is maintained (i.e. energetically elastic processes where no losses are involved), and [[Coherence (physics)|incoherent]] contributions (or inelastic modes where some energy channels into an auxiliary loss mode),<ref name="Kira1999">Kira, M.; Jahnke, F.; Koch, S. W. (1999). "Quantum Theory of Secondary Emission in Optically Excited Semiconductor Quantum Wells". ''Physical Review Letters'' '''82''' (17): 3544–3547. [https://dx.doi.org/10.1103%2FPhysRevLett.82.3544 doi:10.1103/PhysRevLett.82.3544]</ref>

The latter originate, e.g., from the radiative recombination of [[excitons]], [[Coulomb interaction|Coulomb]]-bound electron-hole pair states in solids. Resonance fluorescence may also show significant [[Quantum optics|quantum optical]] correlations.<ref name="Kira1999" /><ref name="Kimble1977">Kimble, H. J.; Dagenais, M.; Mandel, L. (1977). "Photon Antibunching in Resonance Fluorescence". ''Physical Review Letters'' '''39''' (11): 691–695. [https://dx.doi.org/10.1103%2FPhysRevLett.39.691 doi:10.1103/PhysRevLett.39.691]</ref><ref name="Carmichael1976">Carmichael, H. J.; Walls, D. F. (1976). "Proposal for the measurement of the resonant Stark effect by photon correlation techniques". ''Journal of Physics B: Atomic and Molecular Physics'' '''9''' (4): L43. [https://dx.doi.org/10.1088%2F0022-3700%2F9%2F4%2F001 doi:10.1088/0022-3700/9/4/001]</ref>

More processes may occur when a substance undergoes internal energy transitions before re-emitting the energy from the absorption event. Electrons change energy states by either resonantly gaining energy from absorption of a photon or losing energy by emitting photons. In [[chemistry]]-related disciplines, one often distinguishes  between  [[fluorescence]] and [[phosphorescence]]. The former is  typically a fast process, yet some amount of the original energy is dissipated so that re-emitted light photons will have lower energy than did the absorbed excitation photons. The re-emitted photon in this case is said to be red shifted, referring to the reduced energy it carries following this loss (as the [[Jablonski diagram]] shows). For phosphorescence, electrons which absorbed photons, undergo [[intersystem crossing]] where they enter into a state with altered [[Spin (physics)|spin]] multiplicity (see [[term symbol]]), usually a [[triplet state]].  Once the excited electron is transferred into this triplet state, electron transition (relaxation) back to the lower singlet state energies is quantum mechanically forbidden, meaning that it happens much more slowly than other transitions.  The result is a slow process of radiative transition back to the singlet state, sometimes lasting minutes or hours. This is the basis for "glow in the dark" substances.

Photoluminescence is an important technique for measuring the purity and crystalline quality of semiconductors such as [[GaN]] and [[InP]] and for quantification of the amount of disorder present in a system.<ref name="entropyalfaraj2017">Alfaraj, N.; Mitra, S.; Wu, F.; Ajia, A. A.; Janjua, B.; Prabaswara, A.; Aljefri, R. A.; Sun, H.; Ng, T. K.; Ooi, B. S.; Roqan, I. S.; Li, X. (2017). "Photoinduced entropy of InGaN/GaN p-i-n double-heterostructure nanowires". ''Applied Physics Letters'' '''110''' (16): 161110. [https://dx.doi.org/10.1063/1.4981252]</ref>

Time-resolved photoluminescence (TRPL) is a method where the sample is excited with a light pulse and then the decay in photoluminescence with respect to time is measured.  This technique is useful for measuring the [[minority carrier lifetime]] of III-V semiconductors like [[gallium arsenide]] ([[GaAs]]).