RP Photonics

Encyclopedia … combined with a great Buyer's Guide!

Sponsorship opportunity: support this popular resource, which serves the whole photonics community, and get recognition!

Quantum Efficiency

Definition: percentage of input photons which contribute to a desired effect

Alternative term: quantum yield

German: Quanteneffizienz

Categories: light detection and characterization, physical foundations

Formula symbol: ηq

Units: %

How to cite the article; suggest additional literature

The quantum efficiency (or quantum yield) is often of interest for processes which convert light in some way. It is defined as the percentage of the input photons which contribute to the desired effect. Examples are:

2-micron emission with thulium with more than 100% quantum efficiency
Figure 1: 1.9-μm emission in a thulium-doped fiber laser with > 100% quantum efficiency.

In some special cases, the quantum efficiency of a laser or laser amplifier can be larger than unity. This is due to certain energy transfer processes between laser-active ions, which lead to a kind of cross-relaxation: starting with one ion in some excited state, a part of its energy is transferred to some other ion, which was originally in the electronic ground state, and both ions are finally in the upper laser level. This can, of course, only happen when the photon energy of the laser transition is lower than half that of the pump light. An example, illustrated in Figure 1, is that of thulium-doped 1.9-μm fiber lasers, where ions are pumped into the level 3F2−4, and a cross-relaxation process (gray arrows) populates the upper laser level 3H4. This could in principle lead to a quantum efficiency of up to 200%. Values well above 100% can be reached in practice.

The quantum efficiency should not be confused with the quantum defect.

See also: quantum defect, fluorescence, multi-phonon transitions, energy transfer, photodiodes
and other articles in the categories light detection and characterization, physical foundations

How do you rate this article?

Click here to send us your feedback!

Your general impression: don't know poor satisfactory good excellent
Technical quality: don't know poor satisfactory good excellent
Usefulness: don't know poor satisfactory good excellent
Readability: don't know poor satisfactory good excellent

Found any errors? Suggestions for improvements? Do you know a better web page on this topic?

Spam protection: (enter the value of 5 + 8 in this field!)

If you want a response, you may leave your e-mail address in the comments field, or directly send an e-mail.

If you enter any personal data, this implies that you agree with storing it; we will use it only for the purpose of improving our website and possibly giving you a response; see also our declaration of data privacy.

If you like our website, you may also want to get our newsletters!

If you like this article, share it with your friends and colleagues, e.g. via social media: