Photoluminescence is light emission from matter after absorbing photons. Following photon absorption, various relaxation processes occur where photons are re-emitted. Photoluminescence can be classified by excitation energy relative to emission energy. Resonant excitation involves equivalent absorption and emission photon energies, while fluorescence involves energy loss so emitted photons have lower energy. Phosphorescence also involves energy loss but through a spin-forbidden transition, making it a slower process. Photoluminescence is used to measure semiconductor purity and disorder.

1. Photoluminescence(abbreviated as PL) is light emission from any
form of matter after the absorption of photons (electromagnetic
radiation). It is one of many forms of luminescence (light emission) and
is initiated by photoexcitation (excitation by photons), hence the prefix
photo-.[1] Following excitation various relaxation processes typically
occur in which other photons are re-radiated. Time periods between
absorption and emission may vary: ranging from short femtosecond-
regime for emission involving free-carrier plasma in inorganic
semiconductors[2] up to milliseconds for phosphorescent processes in
molecular systems; and under special circumstances delay of emission
may even span to minutes or hours.
Observation of photoluminescence at a certain energy can be viewed as
indication that excitation populated an excited state associated with
this transition energy.
While this is generally true in atoms and similar systems, correlations
and other more complex phenomena also act as sources for
photoluminescence in many-body systems such as semiconductors. A
theoretical approach to handle this is given by the semiconductor
luminescence equations.
Forms 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.

2. For materials in solution or in the gas 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 coherent 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 incoherent contributions (or inelastic modes where some
energy channels into an auxiliary loss mode),[3]
The latter originate, e.g., from the radiative recombination of excitons,
Coulomb-bound electron-hole pair states in solids. Resonance
fluorescence may also show significant quantum optical
correlations.[3][4][5]
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 prior 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, absorbed photons
undergo intersystem crossing where they enter into a state with altered
spin multiplicity (see term symbol), usually a triplet state. Once energy
from this absorbed electron is transferred in this triplet state, electron

3. transition back to the lower singlet energy states 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 GaAs and InP and for
quantification of the amount of disorder present in a system. Several
variations of photoluminescence exist, including photoluminescence
excitation (PLE) spectroscopy.
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).
Ideal quantum-wellstructures
An ideal, defect-free semiconductor quantum well structure is a useful
model system to illustrate the fundamental processes in typical PL
experiments. The discussion is based on results published in Klingshirn
(2012)[8] and Balkan (1998).[9]
The fictive model structure for this discussion has two confined
quantized electronic and two hole subbands, e1, e2 and h1,h2,
respectively. The linear absorption spectrum of such a structure shows
the exciton resonances of the first (e1h1) and the second quantum well
subbands (e2h2), as well as the absorption from the corresponding
continuum states and from the barrier.

4. 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.[3][10] 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 and resonant Rayleigh-scattering are always
superimposed to the 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 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.[11][12]

5. In case of barrier excitation, the initial carrier distribution in the
quantum well strongly depends on the carrier scattering between
barrier and the well.
Relaxation
Initially, the laser light induces coherent polarization in the sample, i.e.,
the transitions between electron and hole states oscillate with the laser
frequency and a fixed phase. The polarization dephases typically on a
sub-100 fs time-scale in case of nonresonant excitation due to ultra-fast
Coulomb- and phonon-scattering
The dephasing of the polarization leads to creation of populations of
electrons and holes in the conduction and the valence bands,
respectively. The lifetime of the carrier populations is rather long,
limited by radiative and non-radiative recombination such as Auger
recombination. During this lifetime a fraction of electrons and holes
may form excitons, this topic is still controversially discussed in the
literature. The formation rate depends on the experimental conditions
such as lattice temperature, excitation density, as well as on the
general material parameters, e.g., the strength of the Coulomb-
interaction or the exciton binding energy.