Experiments

Stimulated Optical Compton Scattering (SOCS)

We have extended some earlier work on velocimetry of a laser cooled vapor by implementing SOCS on the much sparser sample of atoms in an atomic beam. Since the basic principle is the same as Compton scattering, we call this stimulated optical Compton scattering (SOCS). It differs from Bragg reflection in that the absorbed and emitted light are not necessarily of the same frequency. (Although a reference frame can always be found where the frequencies are equal for a particular atomic velocity, it is not defined for a velocity distribution.)

The basic principle underlying SOCS is identical to that of the Compton effect: when incoming electromagnetic radiation is deflected and frequency shifted by a recoiling scattering center, conservation of both energy and momentum constrain the outgoing field. For stimulated processes, where light is transferred from one traveling wave laser beam to another, the momentum and frequency of the two light beams are fixed experimentally, leaving only the atomic motion, including its recoil, as the remaining experimental variable. Measuring absorption or gain allows determination of the initial atomic velocity to accuracy on the order of the recoil velocity. The symmetry of the transition is broken through a non-uniform population of atomic momentum states. Therefore the dependence on the detuning of the two beams maps out the atomic velocity distribution on a sub-recoil scale.

Usually transverse velocimetry is done by measuring the spatial distribution of the atoms at the end of the beam line, so good velocity resolution requires a long beam line and narrow beam-defining slits. By contrast, SOCS replaces this cumbersome scheme by direct velocimetry, thereby reducing the length of the atomic beam apparatus, eliminating the instrumental broadening caused by the longitudinal velocity spread, and removing the need for narrow slits that are required to maintain the spatial resolution. Furthermore, a small extension of SOCS allows measurement of velocity components in two or three dimensions.