Creating and probing subwavelength atomic gratings using spatially separated fields

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Abstract
The interaction of an atomic beam with three spatially separated linearly polarized standing-wave fields is considered, taking into account the magnetic degeneracy and hyperfine splitting of the atomic levels. A spatial modulation of the ground-state density matrix occurs, and is not washed out as a result of spontaneous decay from the upper states. The interaction with fields separated by a distance L results in a focusing of harmonics at distances L’=(m/n)L, where m and n are integers; the period of the matter gratings is λ/2n, where λ is the wavelength of the radiation field. For a beam having angular divergence θ, one can observe these higher harmonics independently from each other if L≫λ/θ. The gratings can be probed using a third standing-wave laser field in the focal plane of the harmonic under investigation. General expressions that allow one to calculate the probe absorption for atoms having arbitrary fine and hyperfine structure are derived. Explicit calculations for the alkali metals are presented. It is predicted that spatial modulations having periods 30–500 nm and modulation depths 2.5–50 % can be produced in this manner. The focusing of the spatial harmonics is interpreted in terms of a shadow effect.

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