Laser Cooling and Trapping Group

Precision Spectroscopy

"Spin Shelving" Two-Photon Spectroscopy Scheme in ⁸⁵Rb

We demonstrated the possibility to achieve strong confinement together with low interaction between the atoms and the trapping light concepts in our rotating beam dark optical trap, by performing a two-photon spectroscopy measurement of the 5S_1/2 -> 5D_5/2 in Rb, using extremely weak laser probe power. In this experiment, we used a new in of spectroscopic technique, which we called "spin-shelving" in an analogy to electron shelving spectroscopy.

In this scheme, atoms are first loaded into the trap and optically pumped to a specific hyperfine level of the ground state (F=2 in our case). Then, the probe two-photon laser is applied for a certain amount of time, exciting atoms to the excited state (5D_5/2) at some low rate. The excited atoms decay spontaneously back to the ground state, either to the initial hyperfine level, from which they can be excited again, or to the other hyperfine level (F=3). In this level, the atoms can be stored or “shelved” for a long time, since they do not interact neither with the probe laser, nor with the trapping laser (dark trap, large detuning).

The fraction of atoms transferred to this hyperfine ground level is the signal of the measurement. It can be accumulated for about a second, and then detected using a detection beam in resonance with a cycling transition. This scheme yields a quantum amplification of ~ 10⁷ in photon rate as compared to measuring the spontaneous emission directly. In our experiment, we could measure the above transition with only 25mW laser power, about 100 times lower (10⁴ times lower transition rate) then in previous experiments. This demonstrates the usefulness of our trap and spectroscopy scheme to measure very weak (forbidden) transitions.

· "Ultrasensitive two-photon spectroscopy based on long spin-relaxation time in a dark optical trap", L, Khaykovich, N. Friedman, S. Baluschev, D. Fathi, and N. Davidson, Europhys. Lett., 50 (4), 454-459 (2000). (Abstract, PDF)

In a different experiment, we present a novel method for reducing the inhomogeneous frequency broadening in the hyperfine splitting of the ground state of optically trapped atoms. This reduction is achieved by the addition of a weak light field, spatially mode-matched with the trapping field and whose frequency is tuned in-between the two hyperfine levels. We experimentally demonstrate the new scheme with ⁸⁵Rb atoms, and report a 50-fold narrowing of the rf spectrum.

· “Suppression of inhomogeneous broadening in rf spectroscopy of optically trapped atoms”, A. Kaplan, M. F. Andersen, and N. Davidson, arXiv: physics/ 0204082 (2002) (Abstract, PDF)