All types of the laser nanofields can produce strong dipole forces which can focus the atomic beams into sports about a few nanometers.

Schematic of atom lens produced by the Fresnel and Bethe holes is shown in Fig. In this scheme, laser light is incident from the left on a conductive screen with a circular aperture of radius a is smaller or about the light wavelength λ. Atomic beam is also incident from the left on the screen and is focused by the gradient force.

(a) Bethe-type atom microlens: the field produced by incident laser light and the light reflected from the aperture (1); the field of the light transmitted through the aperture (2); focused atomic beam (3). (b) Fresnel-type atom microlens: the incident laser light (1); focused atomic beam (2).

1. The Fresnel atom lens

There are some factors which impose a limit on the size of the focal spot. One of them is the velocity spread of the atomic beam. At a degree of velocity monochromatization about 10³—10⁴ the spot size of the Fresnel atom lens can be of the order of 10 nanometers. Anharmonicity of the dipole force also contributes to the spot size at a level of 10 nanometers. Contribution of the velocity diffusion can be estimated to be very small, of the order of tenths of nanometers due to a small time of fight of atoms through the interaction region.[1]

2. The Bethe atom lens

Main factors which limit the spot size of the Bethe atom lens are the same as in a case of the Fresnel lens. It can be seen from the Figure that the atomic microlens focuses atoms to a small spot. At the same time, it can be seen that periphery atoms of the beam are focused at larger distances, thus smearing the focusing region [1,2].

3. Atom lens on photon dot and photon hole

When detuning of the laser frequency relative to the atomic transition frequency is positive, an atom in the laser light configuration is drawn into the weak field region. In the case of a photon hole, the atoms that fly through the apertures in the waveguide walls will be attracted to the axis of the system—and they will be focused.

References:

1. V. I. Balykin and V. G. Minogin, "Comparison of the focusing properties of the Bethe type and Fresnel type atom microlenses", Physical Review A (to be published) (2007)
2. V. I. Balykin, V. G. Minogin, and S. N. Rudnev, "Atomic Beam Focusing by a Near-Field Atomic Microlens", JETP, Vol. 103, No. 5, pp. 679–689, (2006)
3. V. I. Balykin, V. V. Klimov, and V. S. Letokhov, "Atom Nanooptics Based on Photon Dots and Photon Holes", JETP Letters, Vol. 78, 8 (2003).