Photo-luminescence

[PL]

INTODUCTIONPhotoluminescence(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 photo excitation (excitation by photons).

Time periods between absorption and emission may vary: ranging from short femtosecond-regime for emission involving free-carrier plasma in inorganic semiconductors up to milliseconds for phosphorescent processes in molecular systems; and under special circumstances delay of emission may even span to minutes or hours.

Resonance fluorescence• Resonant excitation describes a situation in which

photons of a particular wavelength are absorbed and equivalent photons are very rapidly re-emitted.

• 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

Semi-conductors• 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).

Forms of Photo Luminescence

FluorescenceThe emission of light by a substance

that has absorbed light or other electromagnetic radiation. It is a form of luminescence.

The emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation. The most striking example of fluorescence occurs when the absorbed radiation is in the ultraviolet region of the spectrum, and thus invisible to the human eye, while the emitted light is in the visible region, which gives the fluorescent substance a distinct color that can only be seen when exposed to UV light.

Fluorescent minerals emit visible light when exposed to ultraviolet light

Bio fluorescent marine organisms

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 mechanics. As these transitions occur very slowly in certain materials, absorbed radiation may be re-emitted at a lower intensity for up to several hours after the original excitation.

Commonly seen examples of phosphorescent materials are the glow-in-the-dark toys, paint, and clock dials that glow for some time after being charged with a bright light such as in any normal reading or room light. Typically the glowing then slowly fades out within minutes to up to a few hours in a dark room. phosphoresc

ent pigment blue in the dark

phosphorescent pigment blue (alkaline earth metal silicate )

Phosphorescent bird figure

Phosphor thermometryPhosphor 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 luminance, i.e. emit light. Certain characteristics of the emitted light change with temperature, including brightness, color, and afterglow duration. The latter is most commonly used for temperature measurement.

PHOTOLUMINESCENT MATERIALS

ZNS:Cu

Rare Earth doped Alumina silicates. Ex.SrAlO4:Eu,Dy CaAlO4:Eu,Dy

Fullerene-doped polymers.

Fullerene-doped inorganic materials.