Quenching

Definition: the reduction or limitation of an excited-state population, mostly by unwanted effects

The term quenching has many different meanings; this article discusses only the context of fluorescence quenching in lasers, particularly solid-state lasers. In rare-earth-doped or transition-metal-doped 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) (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

Quenching becomes apparent as a reduction in the decay time and the overall intensity of the fluorescence from quenched electronic states. 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 helpful e.g. when they help to reduce the lower-state lifetime and thus the population of the lower laser level, or to populate the upper laser level via pumping into a higher-lying level. There are lasers for which such quenching processes are essential.

Processes responsible for lifetime quenching often (but not always) create additional fluorescence at other wavelengths. For example, fluorescence may reveal the population of 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).

Bibliography

See also: upper-state lifetime, lower-state lifetime, energy transfer, upconversion, fluorescence, clustering, gain media, rare-earth-doped gain media, doping concentration, Spotlight article 2006-09-06

Category: physical foundations