Definition: the reduction or limitation of an excited-state population, mostly by unwanted effects
Category: physical foundations
Author: Dr. Rüdiger Paschotta
How to cite the article; suggest additional literature
The term quenching has many different meanings; this article discusses only the context of quenching fluorescence (e.g. in phosphors) or laser transitions, particularly solid-state lasers. In rare-earth-doped or transition-metal-doped laser gain media (laser crystals or glasses), the lifetime of electronic levels of laser-active ions is sometimes strongly reduced. This quenching can have different origins, such as:
- multi-phonon transitions to lower electronic levels
- energy transfer processes between laser-active ions (for high concentrations, and particularly when clustering of ions occurs; this is called concentration quenching or pair-induced quenching) (often associated with upconversion)
- energy transfers to other ions, e.g. present as unwanted impurities, which can be introduced by the raw materials but also during crystal growth e.g. by contamination from the crucible
- energy transfers to color centers, i.e. to defects in the crystal structure
One may in such cases consider the upper electronic level to be quenched, or the radiative transition to a lower level, or the light emission as such.
Quenching becomes apparent as a reduction in the decay time and the overall intensity of the fluorescence from quenched electronic states. The intensity reduction is associated with a reduction of the quantum efficiency; only part of the ions put into the upper level can emit fluorescence.
If the upper laser level for some laser transition is quenched, i.e. the upper-state lifetime is reduced, this can raise the threshold pump power of a laser or reduce the gain of an amplifier. However, quenching processes can also be very helpful or even essential for laser operation:
- They may populate the upper laser level via pumping into a higher-lying level.
- They may help to reduce the lower-state lifetime and thus the population in the lower laser level.
Processes responsible for lifetime quenching may create additional fluorescence at other wavelengths. For example, fluorescence may reveal the population in higher-lying levels (→ upconversion fluorescence) or the excitation of other ion species (sometimes present as impurities) via energy transfers. However, it can be difficult to determine whether the observed fluorescence really results from the same process which is responsible for quenching. For example, weak upconversion fluorescence could either be some additional weak effect with negligible influence on the level lifetime, or could indicate strong quenching via upconversion (and be weak only because the quantum efficiency of the upconversion fluorescence is small).
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See also: upper-state lifetime, lower-state lifetime, energy transfer, upconversion, fluorescence, clustering, laser gain media, rare-earth-doped laser gain media, doping concentration, spotlight 2006-09-06
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You stated that quenching will decrease the lifetime and therefore increase the threshold pump power. But shouldn't it be the other way around? The saturation fluence is a material constant which states how many fluence you need to clear the population inversion. It is inversely proportional to the emission cross-section, which is proportional to the Einstein coefficient, which is inversely proportional to the lifetime. So as the lifetime decreases, the pump threshold should decrease too.
The author's answer:
No, that's not correct. Quenching does neither affect the saturation fluence nor the emission cross-section; it just provides an additional mechanism for excited ions to get to a lower state. Therefore, you have to pump harder in order to get and maintain the required upper-state population.