Fluorescence | previous | next | feedback |
Definition: luminescence arising from irradiation with light
Fluorescence is a kind of luminescence, excited by irradiation of a substance with light. The light hitting a sample puts atoms, ions or molecules in the sample into excited states (by absorption of photons), from where they decay into lower-lying states (e.g. their ground states) through spontaneous emission of fluorescence photons. This phenomenon occurs in various kinds of optically pumped lasers and amplifiers, e.g. in solid-state doped-insulator lasers and amplifiers (including fiber lasers and fiber amplifiers), in optically pumped semiconductor lasers, and in dye lasers. The resulting radiation is called fluorescent light.
Figure 1: Fluorescence spectrum of thulium (Tm3+) ions in ZBLAN fiber under excitation with a laser operating at 1140 nm. Upconversion processes lead to the population of relatively high-lying energy levels, from where blue and even ultraviolet light is emitted. Such fibers are suitable for lasing in the blue spectral region near 480 nm. (The weak peaks around 700 nm are artifacts, caused by second-order diffraction of ultraviolet light in the spectrometer. Data taken by R. Paschotta.)
Emission Spectrum
The optical spectrum of fluorescence light (see e.g. Figure 1) usually differs from that of the light which initially caused the excitation of the medium. A significant Stokes shift (difference in photon energies of absorbed and emitted light) can occur because a part of the excitation energy is converted to heat in the medium. For example, the excited atoms or ions may first undergo an optical or a non-radiative transition to some intermediate level, before emitting fluorescence light in a transition to the ground state, or to some higher-lying energy level. It is also possible that a cascade of emission processes occurs, i.e., a cascade of transitions to lower-lying energy levels; in that case, more than one fluorescence photon can be emitted for one absorbed photon.
In solid-state laser gain media, phonons lead to very fast thermalization within Stark level manifolds. As the emission occurs on a much longer time scale, the spectral shape of the emission spectrum does not depend on the exact wavelength of the exciting light.
Fluorescence Decay
After excitation with a short pulse, the fluorescence decay is often of exponential nature with a decay constant which is called the fluorescence lifetime. For so-called allowed transitions, the fluorescence lifetime is typically of the order of a few nanoseconds. In solid-state laser gain media other than semiconductors, one is often dealing with forbidden transitions, where the fluorescence lifetime can be much longer, e.g. microseconds or even milliseconds. One is particularly often dealing with the upper-state lifetime, which is the lifetime of the upper laser level, and is often limited by fluorescence.
Various processes can lead to pronounced non-exponential fluorescence decay, with a faster decay shortly after excitation and a slower decay later. For example, this can be caused by contributing atoms with different lifetimes, or by upconversion processes which are stronger for high level populations.
Competing Non-radiative Processes
In many cases, there are non-radiative processes which compete with fluorescence and reduce or even fully suppress it (→ quenching). In particular, multi-phonon transitions are very strong for level pairs with an energy distance which is at most a few times the maximum phonon energy of the host material. In other cases, energy transfer processes can deplete the population of a metastable level. The quantum efficiency of fluorescence from some level is the average number of fluorescence photons obtained per ion which is put into the upper level. Solid-state gain media (e.g. laser crystals or rare-earth-doped fibers) often have laser transitions with a quantum efficiency very close to unity, whereas some levels (e.g. the lower laser level) exhibit virtually no fluorescence due to strong multi-phonon transitions.
Fluorescence in Lasers and Amplifiers
In the context of lasers, fluorescence in the laser crystal (or other gain medium) by spontaneous emission is lost for the laser operation, because only a tiny fraction of it goes into the laser resonator mode. The fluorescence lifetime for fluorescence from the upper laser level is usually called the upper-state lifetime. Fluorescence is the most fundamental reason why a certain laser threshold has to be overcome to achieve lasing (exception: thresholdless lasers). Note also that when the pump source of a laser is turned on, laser action normally starts from a tiny amount of fluorescence in the laser resonator mode, amplified to high levels in many cavity round trips.
In optical amplifiers, fluorescence can be even more important. In the form of amplified spontaneous emission (ASE), it may extract significant power and thus limit the gain achievable. It also determines much of the quantum noise contributions to laser noise and amplifier noise.
Fluorescence can be very important for the characterization of laser gain media:
- It indicates any radiative transitions from metastable levels (not only the upper laser level) and can be used for the measurement of the emission bandwidth, and thus also the gain bandwidth. Note, however, that reabsorption losses in quasi-three-level gain media and also excited-state absorption can cause the gain bandwidth to be smaller than that obtained from fluorescence spectra.
- When the quantum efficiency can be assumed to be near unity, the Fuchtbauer-Ladenburg equation and similar tools can be used to calculate the magnitude of emission cross sections. Additional data concerning absorption can be obtained via McCumber theory.
- The temporal decay of the fluorescence after excitation (e.g. with a short pulse) can be measured in order to quantify the level lifetimes, in particular the upper-state lifetime.
- Fluorescence may reveal the presence of impurities, e.g. unwanted rare-earth ions, and also parasitic processes such as cooperative luminescence.
- In fiber lasers and amplifiers, monitoring fluorescence intensities along the fiber allows the determination of parameters such as pump absorption.
Besides, fluorescence light is sometimes used for aligning a laser.
Application in Lighting
Fluorescence is widely used in fluorescent tubes for illumination purposes. The most common fluorescent lamps contain mercury vapor inside a glass tube, where an electric discharge excites the mercury atoms to emit mostly ultraviolet light. A fluorescent material (called the phosphor) on the inner surface of the tube absorbs the ultraviolet light and converts it into fluorescence light, mostly in the visible spectral region. The phosphor contains several substances mixed in such a way that the overall emission spectrum corresponds to white light (with the color tone adjusted according to the envisaged application). Although part of the energy of the ultraviolet light is lost in the phosphor, fluorescence lights are still several times more energy-efficient than incandescent lamps.
There are also light-emitting diodes (LEDs) which generate white light in a process that involves the excitation of a phosphor. In that case, however, the actual LED produces blue rather than ultraviolet light, and this blue light is partly converted to red and green fluorescence light in the phosphor, and partly emitted by the device.
The emission spectrum of any fluorescent lamp can easily be distinguished from that of an incandescent lamp: it is much more structured, with a high power spectral density in some spectral regions and much lower values at other wavelengths. Nevertheless, the visual impression for the human eye can be similar to day light. This is essentially because the eye has only three different kinds of photoreceptors for distinguishing colors; only the relative excitation levels for these kinds of photoreceptors matter for the color impression. In addition, the human visual sense quickly adapts its color calibration according the spectrum of the ambient light; for that reason, the strong changes in the spectrum of day light, e.g. between noon and late afternoon, are hardly perceived.
Using Fluorescence in Other Ways
Fluorescence light can be useful, e.g. for direct use for optical measurements, such as for measuring the transmission spectra of optical devices. It is also the basis of fluorescence microscopy and of optical refrigeration.
Interactions via Fluorescence
Fluorescence of the single atoms, ions or molecules of a sample usually occurs in an uncoordinated manner, i.e., uniformly in all spatial directions and without temporal correlations between the emitted photons. However, certain conditions lead to amplified spontaneous emission (superluminescence) or superfluorescence, where this is no longer the case.
Parametric Fluorescence
A special kind of fluorescence, which is not related to the excitation of atoms or ions in a substance, is parametric fluorescence in nonlinear crystal materials. This effect does not involve the excitation of electrons in the media, but rather a nonlinear interaction. Such fluorescence occurs only as long as some pump light propagates in the medium.
See also: fluorescence microscopy, fluorescent lamps, luminescence, spontaneous emission, amplified spontaneous emission, superluminescence, superfluorescence, upper-state lifetime, cross sections, optical refrigeration, McCumber theory, Fuchtbauer-Ladenburg equation, parametric fluorescence
Category: physical foundations
