Photoluminescence
Encyclopedia
Photoluminescence is a process in which a substance absorbs photons (electromagnetic radiation) and then re-radiates photons. Quantum mechanically
, this can be described as an excitation
to a higher energy state
and then a return to a lower energy state accompanied by the emission of a photon. This is one of many forms of luminescence
(light emission) and is distinguished by photoexcitation (excitation by photons), hence the prefix photo-. The period between absorption and emission is typically extremely short, in the order of 10 nanosecond
s. Under special circumstances, however, this period can be extended into minutes or hours.
More interesting processes occur when the chemical substrate undergoes internal energy transitions before re-emitting the energy from the absorption event. The most familiar such effect is fluorescence
, which is also typically a fast process, but in which some of the original energy is dissipated so that the emitted light photons are of lower energy than those absorbed. The generated photon in this case is said to be red shifted, referring to the loss of energy as Jablonski diagram
shows.
Photoluminescence is an important technique for measuring the purity and crystalline quality of semiconductors such as GaAs
and InP
. Several variations of photoluminescence exist, including photoluminescence excitation
(PLE).
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 in measuring the minority carrier lifetime of III-V semiconductors like Gallium arsenide (GaAs
.)
An even more specialized form of photoluminescence is phosphorescence
, in which the energy from absorbed photons undergoes intersystem crossing
into a state of higher spin multiplicity (see term symbol
), usually a triplet state
. Once the energy is trapped in the triplet state, 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.
, the temperature dependence of the photoluminescence process is exploited to measure temperature.
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...
, this can be described as an excitation
Quantum Jump
Quantum Jump was a 1970s British band, consisting of keyboard player and singer Rupert Hine, guitarist Mark Warner, bass player John G. Perry and drummer Trevor Morais .-Career:...
to a higher energy state
Energy level
A quantum mechanical system or particle that is bound -- that is, confined spatially—can only take on certain discrete values of energy. This contrasts with classical particles, which can have any energy. These discrete values are called energy levels...
and then a return to a lower energy state accompanied by the emission of a photon. This is one of many forms of luminescence
Luminescence
Luminescence is emission of light by a substance not resulting from heat; it is thus a form of cold body radiation. It can be caused by chemical reactions, electrical energy, subatomic motions, or stress on a crystal. This distinguishes luminescence from incandescence, which is light emitted by a...
(light emission) and is distinguished by photoexcitation (excitation by photons), hence the prefix photo-. The period between absorption and emission is typically extremely short, in the order of 10 nanosecond
Nanosecond
A nanosecond is one billionth of a second . One nanosecond is to one second as one second is to 31.7 years.The word nanosecond is formed by the prefix nano and the unit second. Its symbol is ns....
s. Under special circumstances, however, this period can be extended into minutes or hours.
Forms of photoluminescence
The simplest photoluminescent processes are resonant radiations, in which a photon of a particular wavelength is absorbed and an equivalent photon is immediately emitted. This process involves no significant internal energy transitions of the chemical substrate between absorption and emission and is extremely fast, of the order of 10 nanoseconds.More interesting processes occur when the chemical substrate undergoes internal energy transitions before re-emitting the energy from the absorption event. The most familiar such effect is fluorescence
Fluorescence
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...
, which is also typically a fast process, but in which some of the original energy is dissipated so that the emitted light photons are of lower energy than those absorbed. The generated photon in this case is said to be red shifted, referring to the loss of energy as Jablonski diagram
Jablonski diagram
A Jablonski diagram, named after the Polish physicist Aleksander Jabłoński, is a diagram that illustrates the electronic states of a molecule and the transitions between them. The states are arranged vertically by energy and grouped horizontally by spin multiplicity. Nonradiative transitions are...
shows.
Photoluminescence is an important technique for measuring the purity and crystalline quality of semiconductors such as GaAs
Gaas
Gaas is a commune in the Landes department in Aquitaine in south-western France....
and InP
INP
INP may stand for:*INP , a Korean chemical company*INP , an early database system developed at the University of California, Berkeley*Integrated National Police, a forerunner of the Philippine National Police...
. Several variations of photoluminescence exist, including photoluminescence excitation
Photoluminescence excitation
Photoluminescence excitation is a specific type of photoluminescence and concerns the interaction between electromagnetic radiation and matter....
(PLE).
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 in measuring the minority carrier lifetime of III-V semiconductors like Gallium arsenide (GaAs
Gaas
Gaas is a commune in the Landes department in Aquitaine in south-western France....
.)
An even more specialized form of photoluminescence is phosphorescence
Phosphorescence
Phosphorescence is a specific type of photoluminescence related to fluorescence. Unlike fluorescence, a phosphorescent material does not immediately re-emit the radiation it absorbs. The slower time scales of the re-emission are associated with "forbidden" energy state transitions in quantum...
, in which the energy from absorbed photons undergoes intersystem crossing
Intersystem crossing
Intersystem crossing is a radiationless process involving a transition between two electronic states with different spin multiplicity.-Singlet and triplet states:...
into a state of higher spin multiplicity (see term symbol
Term symbol
In quantum mechanics, the Russell-Saunders term symbol is an abbreviated description of the angular momentum quantum numbers in a multi-electron atom. It is related with the energy level of a given electron configuration. LS coupling is assumed...
), usually a triplet state
Triplet state
A spin triplet is a set of three quantum states of a system, each with total spin S = 1 . The system could consist of a single elementary massive spin 1 particle such as a W or Z boson, or be some multiparticle state with total spin angular momentum of one.In physics, spin is the angular momentum...
. Once the energy is trapped in the triplet state, 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.
Photoluminescent material in safety applications
One of the major uses of photoluminescent material is for safety and egress marking. It is most commonly seen in the form of "fire exit" signage. The industry is governed by a number of international standards and guidelines that stipulate performance criteria under certain conditions of excitement. A guide to these standards can be found at http://www.photoluminescent.co.uk/standardsPhotoluminescent material for temperature detection
In phosphor thermometryPhosphor thermometry
Phosphor thermometry is an optical method for surface temperature measurement. The method exploits luminescence emitted by phosphor material. Phosphors are fine white or pastel-colored inorganic powders which may be stimulated by any of a variety of means to luminesce, i.e. emit light...
, the temperature dependence of the photoluminescence process is exploited to measure temperature.