Line “narrowing” in strong optical fields

S. P. Andreev, Valerii S. Lisitsa

Andrew J. Berglund, James L. Hanssen, Jabez J. McClelland

Laser cooling on weak transitions is a useful technique for reaching ultracold temperatures in atoms with multiple valence electrons. However, for strongly magnetic atoms a conventional narrow-line magneto-optical trap (MOT) is destabilized by competition between optical and magnetic forces. We overcome this difficulty in Er by developing an unusual narrow-line MOT that balances optical and magnetic forces using laser light tuned to the blue side of a narrow (8 kHz) transition. The trap population is spin polarized with temperatures reaching below 2 muK. Our results constitute an alternative method for laser cooling on weak transitions, applicable to rare-earth-metal and metastable alkaline earth elements.

William Smyth, S. Swain

We consider a single two-level atom driven by the output from a degenerate optical parametric oscillator. For squeezing bandwidths of the order of the natural atomic width and smaller, we report remarkable spectral profiles for the scattered light, including the existence of a narrow hole at line-centre. Under appropriate conditions, the spectrum may be both subnatural and narrower than the spectrum of the incident field, a phenomenon which does not depend upon strong coupling. To explain the role of the classical or non-classical nature of the driving field plays, we consider an alternative system which enables the atom to be driven by an arbitrary bandwidth light field with arbitrary two-photon correlations. Both numerical and analytic solutions are employed to provide physical insight. Surprisingly, spectral narrowing does not require squeezing: narrowed spectra appear for driving with a blackbody field. Then the atomic fluorescence can be expressed as the product of the atomic response with the incident spectrum. The resulting narrowing is maximized for incident bandwidths of the order of the natural width. However, the narrow hole at line-centre does require a non-classical driving field. For ideal squeezing, the fluorescence at line-centre vanishes. We test how robust the effects are to changes in experimentally pertinent factors such as the solid angle of coupling.

A.L. Schawlow

James Keaveney, Armen D. Sargsyan, David G. Sarkisyan +2 more

Electromagnetically induced transparency (EIT) resonances using the 5S 1/2 → 5P 3/2 → 5D 5/2 ladder-system in optically thick Rb atomic vapour are studied. We observe a strong line narrowing effect and gain at the 5S 1/2 → 5P 3/2 transition wavelength due to an energy-pooling assisted frequency conversion with characteristics similar to 4-wave mixing. As a result it is possible to observe tunable and switchable transparency resonances with amplitude close to 100% and a linewidth of 15 MHz. In addition, the large line narrowing effect allows resolution of 85 Rb 5D 5/2 hyperfine structure even in the presence of strong power broadening.

Wolfgang Demtröder