Dipole Trapping

The optical dipole force arises from the coherent interaction of an inhomogeneous electro-magnetic (EM) field with the induced electric dipole moment of an atom. The force is conservative as it is the result of coherent scattering from and to the driving field. The atomic energy levels are shifted by this process (a phenomenon known as the light shift or ac-Stark shift) and so the atoms experience a potential related to the atomic polarisability, α and the EM field strength, E.

The dependence of this potential (and hence maximum trap depth, U0) upon the experimental parameters of laser intensity, I, and detuning, can be approximated as

Here Δ = ω - ω0, where ω is the laser frequency and ω0 is the atomic resonance frequency. The incoherent scattering rate, which leads to heating in the trap, scales as

Since the force an atom experiences is proportional to the gradient of the potential, an atom will be attracted to regions of high or low intensity depending upon the sign of the atomic dipole. Below resonance (red detuned) atom are attracted to regions of high intensity and can be trapped in the focus of a single laser beam as proposed by Ashkin. For blue detuned traps, atoms are attracted to regions of low intensity.
Content © K. J. Weatherill , Durham University 2005