Saturated Absorption Spectroscopy

1. Saturated Absorption Spectroscopy

1.1 Introduction Detailed instructions for performing saturated absorption measurements are presented. Atomic transition frequencies can be determined precisely using saturated absorption spectroscopy. There are numerous Saturated Absorption setups, however, this paper will only consider the setup used for the Magneto-Optical Trap in the University of Oregon's Advanced Projects Lab. Detailed configuration instructions for this setup are provided in section 1.3. This paper will not provide instructions for constructing components provided to incoming students such as the laser control box. For Complete schematics and electrical diagrams of components not included, see references.

1.2 Purpose Of Saturated Absorption To achieve the necessary cooling of atoms in a Magneto-Optical Trap one must induce the propagation of three orthogonally oriented pairs of overlapped single-mode laser beams. The three pairs must intersect at a central point, call it c. Each beam must propagate in the direction antiparallel to that of its overlapping pair. The frequency of each beam must be tuned just below the F = 3 -› F' atomic transition frequency of 85Rb. By doing so, the doppler shifts experienced by Rb atoms moving inside the trap can be exploited in order to achieve the desired cooling of the atom's kinetic energy. As an atom moves away from the intersection point c, say to the right, it will experience a blue shift of the beam propagating in the opposite direction of its motion, from right to left, as well as a red shift of the beam propagating in the direction of its motion, from left to right. With the lasers tuned just below this atomic transition frequency the blue shift induced by the atom's motion increases the experienced frequency of the laser propagating to the left. This brings it closer to the 85Rb, F = 3 → F' atomic transition frequency. The red shift of the laser propagating to the right decreases its frequency, as experienced by the atom, shifting it farther away from the 85Rb, F = 3 → F' atomic transition frequency. As a result, the momentum imparted by the scattering of photons from the beam propagating left is much greater than the momentum imparted by the scattering of photons from the beam propagating to the right. Thus the three orthogonally configured pairs of beams will provide velocity dependent reduction of the Rb atoms' kinetic energy. By performing saturated absorption measurements the laser can be tuned to any of the four atomic transition frequencies of the two Rb isotopes in their respective ground states.

1.3 Setup and Procedure

1.3.1 Components and Configuration