Atoms in a laser field undergo an AC Stark shift, proportional to the laser field strength and inversely proportional to the detuning from resonance D (for a sufficiently large detuning). The energy shift changes sign with the detuning. For a positive ("blue") detuning, the energy of the atoms goes up, and atoms are thus repelled from a region of high field strength, and the reverse is true for negative ("red") detuning. The physical mechanism for this energy shift can be found in many text books, for instance "Photons and atoms" by Claude Cohen-Tannoudji (one of last year's Nobel laureates). The mechanism is based on frequency pulling of a driven oscillator, and is entirely classical.
This energy shift is not large; it can be expressed as:
DE=h s G2/(8D)
where s is the saturation parameter I/I0, with I0 the saturation intensity of the transition. G is the inverse natural lifetime of the excited state, h is Planck's constant and again D is the detuning from resonance. The saturation parameter is in the order 10-2-104. Altogether, the shift is less than a Kelvin for reasonable laser beam parameters.
For laser cooled atoms this can be a serious barrier, and this mechanism can be used to confine, guide or focus cold atoms. An application of this energy barrier is in guiding the atoms down the core of a hollow optical fiber. Such a fiber is shown schematically in the figure. The laser light is guided by the glass part of the fiber, but it extends partly into the core: there is an evanescent field. The decay length of this field is of the order of a wavelength. The laser light is blue detuned, and hence repels the atoms and keeps them away from the glass. The principle has been demonstrated elsewhere using Rubidium atoms and multi-mode hollow optical fibres.
Rubidium however is a metal, and if it hits the glass wall it may stick there for a while and come off later, or not at all and clog the fiber. This poses serious problems for applications using these atoms. We use laser cooled metastable helium atoms, generated by the bright beam machine, to greatly improve on these experiments. Metastable helium de-excites on hitting the glass wall, so one can be assured of the coherence of any atom that gets through. Also helium is a gas that can easily be pumped away. We will use a single mode optical fiber, with a small hole in the center for future experiments. As we use colder and colder atoms, the fiber could grow to be a single mode atom wave guide!
Here are results of guiding atoms in a 50 mm long, square cross-section capillary tube. We display the number of atoms coming through as a function of the laser detuning (green line) as well as the signal from a saturated absorption setup (red line). The peak in the latter at zero detuning is the resonance. Above resonance, atoms are being guided through the fiber. The background (about 100 counts) is caused by ballistic metastable atoms.
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