Dipole forces

Unlike optical absorption, the dipole force depends upon the real part of the atomic or molecular susceptibility. The optical field must be detuned from resonance and, depending upon the sign of this detuning, the force acts either towards or away from regions of high intensity. When the detuning is large, the dipole force can act upon a wide range of quantum states and is accompanied by only minimal absorption; this is especially so when the laser lies to the blue, for in such conditions the species are trapped in regions of low intensity.

The low optical absorption allows the high intensities required to be achieved by resonant enhancement of continuous-wave radiation in confocal optical cavities and, by tailoring the cavity mode superposition, quite different trapping field geometries may be obtained. Furthermore, in certain regimes, the radiation field is influenced by the trapped species, reflecting their spatial distribution. Hysteresis in this process offers a mechanism for collective cooling of a trapped ensemble, while evaporative cooling provides a perhaps more direct alternative.

We have explored theoretically some aspects of such trapping schemes, which prove of great didactic interest as well as revealing curious and unexpected properties, such as a novel mechanical amplifier whose operation resembles a Vernier scale. While our theoretical analysis continues, we are about to start experimental studies in collaboration with the University of St Andrews, using computer-controlled spatial light modulators to form the complex optical fields dynamically.

Calculated cavity-enhanced fields for, respectively, an array of coaxial ring traps, a single ring trap, and an optical bottle. In each case, the field is detuned to the blue of the radiative transitions, so that the atoms or molecules collect in the dark regions.

"I have visited the Soton uni site and found your pages ... gripping. If I were an untrapped molecule or indeed a cavity, I would be looking to my laurels right now... Any molecule still at liberty should know that his/her days are numbered."

D. Kenny, Buckingham Gate, London