Lasing Without Inversion in Mercury at 253.7 nm


The principle of LWI is to cancel the absorption at the laser transition by coherent excitation of adjacent transitions so that only a few excited atoms are sufficient for lasing. Whilst there is no more absorption for coherent light, the laser transition can still be pumped by an incoherent light source like a gas discharge lamp.

Until now there has been no LWI scheme where the laser transition  had a significantly shorter wavelength than the lasers used to build up the coherence.

In our LWI experiment, we use a 4-level-scheme in mercury with a laser transition at 253.7 nm as described in [1]. For this scheme a strong driving laser at 435.6 nm, a weak driving laser at 546.1 nm and an incoherent repump at 404.7 nm are required.

4-level scheme in Hg


[1] E. S. Fry, M. D. Lukin, T. Walther, and G. R. Welch, “Four-level atomic coherence and cw vuv lasers,” Optics Communications 179, 499 – 504 (2000).


Prof. Dr. Thomas Walther

Laser und Quantenoptik
Institut für Angewandte Physik
Fachbereich 05 - Physik
Technische Universität Darmstadt
Schlossgartenstr. 7
D-64289 Darmstadt

+49 6151 16-20831 (Sekretariat)

+49 6151 16-20834 (Fax)




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