Encyclopedia … combined with a great Buyer's Guide!

Sponsors:     and others

Lasing Without Inversion

Definition: the quantum-optical phenomenon of laser emission occurring without the presence of a population inversion

German: Lasen ohne Inversion

Categories: laser devices and laser physics, quantum optics


How to cite the article; suggest additional literature

URL: https://www.rp-photonics.com/lasing_without_inversion.html

Usually, the gain medium of a laser works on the basis of a population inversion. (Essentially, that means (for a simple system) that the upper laser level has a higher population than the lower laser level, so that stimulated emission can provide a positive net gain.) In the 1990s, however, it was shown that optical amplification and consequently lasing without inversion are possible by using an additional optical or microwave field which induces a quantum coherence in the atoms of the gain medium. The basic idea is to provide two different pathways for atoms to get from the ground state to the excited state – a direct one and another one via a third energy level –, and to induce a quantum coherence, so that the quantum-mechanical probability amplitudes for both processes cancel. In effect, that suppresses the reabsorption and thus makes it possible to obtain a positive net gain even with a small population in the upper state.

In principle, lasing without inversion could help, e.g., in realizing lasers operating on very short wavelengths, where a population inversion is difficult to achieve. However, to date it is unclear whether this theoretical potential can really be turned to practical advantages. So far, lasing without inversion has to be considered as a definitely interesting quantum-mechanical effect, but not as a practically important technique.


[1]S. E. Harris, “Lasers with inversion: Interference of lifetime-broadened resonances”, Phys. Rev. Lett. 62 (9), 1033 (1989), DOI:10.1103/PhysRevLett.62.1033
[2]M. O. Scully and M. Fleischhauer, “Lasers without inversion”, Science 263, 337 (1994), DOI:10.1126/science.263.5145.337

See also: population inversion, lasers, stimulated emission, coherence, thresholdless lasers

Questions and Comments from Users

Here you can submit questions and comments. As far as they get accepted by the author, they will appear above this paragraph together with the author’s answer. The author will decide on acceptance based on certain criteria. Essentially, the issue must be of sufficiently broad interest.

Please do not enter personal data here; we would otherwise delete it soon. (See also our privacy declaration.) If you wish to receive personal feedback or consultancy from the author, please contact him, e.g. via e-mail.

Your question or comment:

Spam check:

  (Please enter the sum of thirteen and three in the form of digits!)

By submitting the information, you give your consent to the potential publication of your inputs on our website according to our rules. (If you later retract your consent, we will delete those inputs.) As your inputs are first reviewed by the author, they may be published with some delay.


Share this with your friends and colleagues, e.g. via social media:

These sharing buttons are implemented in a privacy-friendly way!

Code for Links on Other Websites

If you want to place a link to this article in some other resource (e.g. your website, social media, a discussion forum, Wikipedia), you can get the required code here.

HTML link on this article:

<a href="https://www.rp-photonics.com/lasing_without_inversion.html">
Article on Lasing without inversion</a>
in the <a href="https://www.rp-photonics.com/encyclopedia.html">
RP Photonics Encyclopedia</a>

With preview image (see the box just above):

<a href="https://www.rp-photonics.com/lasing_without_inversion.html">
<img src="https://www.rp-photonics.com/previews/lasing_without_inversion.png"
alt="article" style="width:400px"></a>

For Wikipedia, e.g. in the section "==External links==":

* [https://www.rp-photonics.com/lasing_without_inversion.html
article on 'Lasing without inversion' in the RP Photonics Encyclopedia]