Saturated Absorption and Optical Pumping
Saturated absorption spectroscopy uses two laser beams (one weak "probe" and a stronger "pump") passing through the atomic vapour in opposite directions. The strong pump laser preferentially moves atoms out of specific atomic states, depleting these populations. As a result, the probe laser absorption is reduced.
The change in the absorption of the probe beam can be controlled by manipulating the properties of the pump beam in a variety of ways ranging from changing the crossing angle of the two beams, to varying the strength of the pump beam, to controlling its polarization. (Such a manipulation of atoms is crucial for cooling and trapping atoms in a magneto-optical trap.) Baotai Lou investigated the optical pumping of 87 rubidium atoms between the 5S1/2 - 5P3/2 F = 1 - 0 and 1 - 1 transitions.
The saturated absorption spectra consist of 6 lines: F = 1 - 0, F = 1 -1, and F = 1 - 2 along with three "cross-over" peaks labelled C1, C2, and C3. These cross-over peaks appear owing to moving atoms being Doppler shifted into resonace with both the pump and the probe beams. Of note here is cross-over peak C1, which is a resonant interaction of atoms between the F = 1 - 0 and 1 - 1 transitions. By varying the relative directions of the linear polarizations of the pump and the probe beams the atomic absorption can be enhanced or diminished.
Figures (a) and (b) show the saturated absorption signals as a function of laser frequency. The positive going signals in the figures represent decreasing probe laser absorption. The blue traces are the experimental measurements while the red traces are fits to the data.
In figure (a) above, the pump beam is polarized horizontally while the probe beam is vertically polarized resulting in an increased absorption of the F = 1 - 0 transition and a suppression of the absorption of the "cross-over" peak, C1. By rotating the polarization of the pump beam by 90° so that both laser beams are vertically polarized one can achieve the opposite effect. (Shown in figure (b).) In effect, this is a very rudimentary atom switch based on optical pumping.