On the use of effective Rabi frequency as a global MOT parameter depending on the trapping beam power M. Głódź* a , K. Kowalski a , J. Szonert a , E. Paul-Kwiek b , S. Gateva c and K. Vaseva c a Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland b Institute of Physics, Pomeranian Univ. in Słupsk, ul. Arciszewskiego 22b, 76-200 Słupsk, Poland c Institute of Electronics, Bulg. Acad. Sc., 72 Tzarigradsko Chaussee, 1784 Sofia, Bulgaria ABSTRACT An analysis is presented of the high resolution Autler-Townes spectra in a pump-probe cascade 5S 1/2 (F=3) → 5P 3/2 (F’=4) → 5D 5/2 (F”) experiment in a working 85 Rb-MOT. It is shown that despite the complex nature of the spatially varying interactions of individual atoms with MOT fields, the probed region of the atomic cloud in MOT can be characterized by effective Rabi frequency, at least for the customary used range of detuning and intensity of the trapping field. This effective value which relates to the averaged atom interactions, is directly dependent on the trapping laser power and practically does not depend of detuning. The applied procedure is based on predictions of a three-level model. The procedure can be used as a method for determination of the effective Rabi frequency experienced by atoms in MOT, and it also indicates applicability limits of the approach for a given MOT implementation. Keywords: magneto-optical trap, three-level cascade, pump-probe spectroscopy, rubidium, Autler-Townes effect, effective Rabi frequency 1. INTRODUCTION Invented over two decades ago, the magneto-optical trap (MOT) 1 has been gradually becoming a standard laboratory tool, providing nearly Doppler-free, collisionless and relatively dense atomic environment. Traps of this kind have been used for many pump-probe experiments. Here we will refer to some experiments performed in cold alkali metal atoms in MOT in a cascade-scheme 2-8 , because such a scheme is used in the presented work. For instance, applicability of MOT was examined for high resolution hyperfine structure (hfs) spectroscopy aiming at new frequency standards, or diagnostics of the trapped atoms themselves was the subject of the experiments 2-5 . The lowest alkali atom transition nS- nP (D2 line) driven by the strong field served for trapping-cooling and as a first step of the studied pump-probe systems. The strong field induced Autler-Townes (AT) effect was probed by the weak field in the second (upper) transition, coupling an intermediate state to another higher excited n’S or n’D state, for example: 133 Cs(9S) 2,3 , 133 Cs(8S, 6D) 5 , 87 Rb(4D) 2 and 85 Rb(4D) 4 . AT splitting was observed of the resonances in the probe absorption spectra. More recently, probing to high Rydberg states (with n≥40) was reported 6,7 . The pump-probe cascade scheme S-P-D was also used for high resolution spectroscopy to determine fine and hyperfine structure of the 7D J states of francium, the short-lived radioactive alkali, trapped in MOT. The authors applied the AT splitting as a frequency scale 8 . MOT allows a wealth of high resolution studies. For the theoretical description of the processes in MOT, the (effective) Rabi frequency due to MOT beams interacting with MOT atoms may be one of the basic parameters. However atoms cooled and trapped in a working MOT are subject to spatially varying interactions due to the inhomogeneous distributions of both the magnetic and the optical field. The most obvious is the (macroscopic) space–dependent detuning due to the quadrupole magnetic field. Also the optical field due to the six crossing laser beams exhibits (microscopic) variations at the scale of a laser wavelength, because of spatial pattern of the beam interference 3,9 . This picture can be supplemented with further details, e.g., with beams misaligned and/or imbalanced in power, the MOT center may be displaced from the position of zero magnetic-field, and thus the MOT cloud may change or lose its symmetry 3,10 . Thus for displaced atoms experiencing a higher magnetic field, and therefore larger Zeeman shifts of m levels, the effective detuning from the resonance decreases. The problem of the three-dimensional nature of the laser (E) field intensity distribution, as well as variations of the magnetic field over the trapping volume, and the consequences thereof were *glodz@ifpan.edu.pl; phone +48 22 843 66 01-3331; fax +48 22 843 09 26 16th International School on Quantum Electronics: Laser Physics and Applications, edited by Tanja Dreischuh, Dimitar Slavov, Proc. of SPIE Vol. 7747, 77470J · © 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.883022 Proc. of SPIE Vol. 7747 77470J-1