Abstract

We have studied the absorption and fluorescence of the D₂ line [6S_1/2(F=3,4)- 6P_3/2 (F'-2,3,4,5)] in dilute Cs atomic vapour for 3 vapour cells: an extremely thin cell with a column length of 8 µm (cell1), a 16.5 mm-long cell with pure Cs vapour (cell2), and a 10.4mm-long Cs cell containing 40 Torr of Ne (cell3). A laser diode with a 20 MHz-linewidth and the maximum intensity of 40 mW/cm² was used The temperature of the cell2, 3 was ~30°C. In order to have a comparable peak absorption (for the case of low laser intensity), the extremely thin cell was heated to ~100°C by an oven made of nonmagnetic materials. Fig.1 shows the effective absorption cross-sections of these cells versus laser intensity. The open symbols correspond to ν₃ (F=3-F=2,3,4) transitions, and solid symbols to ν₄ (F=4- F=3,4,5). It is important to note that the same behaviour was observed for the resonance fluorescence As demonstrated in [1,2] the effect of optical pumping of Zeeman sublevels, leads to a dramatically different behaviour of absorption for the ν₃ and the ν₄ transitions at a laser intensity ≥ 0 1mW/cm² for cell2 and the absorption of ν₃ is very sensitive to the presence of a weak external magnetic field Meanwhile, as seen in Fig.1, there is no hint for this effect both for the extremely thin and the buffer gas-filled cells. The main reason which leads to the reduction of the Cs atom absorption versus laser intensity is the effect of the optical pumping of hyperfine levels. It is also obvious, that the longer the atom-radiation interaction time t is, the stronger the influence the effect of the optical pumping of hyperfine levels is In the case of cell2, the interaction time (time of flight) t is ~ 10 µs, whereas for cell3 this time is ~ 10 ms The reduction of the absorption in cell3 therefore occurs at lower laser intensities For cell1 the time of flight between the two windows is only ~30ns, which leads to frequent collisions of Cs atoms with the windows and thus provides a relaxation mechanism for the populations of the ground state hyperfine levels. For this reason the reduction of the absorption occurs at much higher laser intensities in cell 1.

© 2000 IEEE