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
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.