ATOM OPTICS

Atom optics involves regarding atoms and atomic beams as matter waves. In analogy with light waves we want to construct optical elements for atoms, such as lenses, mirrors, beamsplitters and optical fibers. For coherence, spontaneous emission as is used in laser cooling has to be avoided, as it introduces the random phase of the spontaneously emitted photon.

Lenses

Lenses can be used to focus atomic beams. Due to the very short and variable (deBroglie) wavelength of the atoms, the focus can be made much smaller than using traditional optics with visible light. Atom lenses are constructed using the AC-Stark shift of atoms in an near-resonant laser field. This energy shift constitutes an effective potential for atoms. It is proportional to the intensity of the laser field and inversely proportional to the detuning from the atomic transition. Gradients in the laser field intensity thus excert a force on the atoms. For instance, in a standing laser field there are large intensity gradients, and near the (anti-)nodes the potential is harmonic, and forms a lens. A standing wave is thus an array of cylindrical microlenses for atoms. Recently, such an array has been used to deposit lines of chromium on a substrate, with a period of 213 nm and a width much smaller than that.

Metastable helium has a large internal energy and can as such be used to damage a surface. Using the same principle this damage can be very local and well controlled. A good candidate for such a surface is a so-called self-assembled monolayer, or SAM. These monolayers can protect an underlying layer from etching, whereas the damaged monolayer will allow the layer to be etched. We are presently developping this technique for applications to nanofabrication technology.

Mirrors

 Two succesful principles for the (coherent) reflection of atoms have been demonstrated to date: Reflection off magnetic field gradients and off light field gradients.

Beamsplitters

 Many schemes have been proposed and demonstrated to yield a coherent atomic beamsplitter. In metastable helium, an extremely useful technique to take an initially incoherent beam and create two coherent beams is available. For helium in a laser field consisting of counterpropagating s⁺ and s^- polarized laser beams a "dark state" exists. It is a coherent superposition of atoms in one magnetic substate traveling with minus one photon momentum and atoms in the other magnetic substate traveling with plus one photon momentum. The excitation amplitudes for both laser beams destructively interfer for this state. This means that atoms in this state are not excited by the laser light, and coherence will be preserved. This state thus formes a "well" in phase space, where atoms can diffuse into, but cannot get out of. This "dark state" can be further studied in our and used as a source of two beams of coherent atoms in interferometry experiments.

Hollow Optical fibers

 "Optical" fibers for atoms can be created using a hollow optical fiber. In the glass layer a blue-detuned laser wave propagates, and the evanescant wave in the hollow core creates a repulsive potential preventing the atoms from hitting the glass. The fiber is actually single mode for the laser light, avoiding problems with interference speckles in the fiber. Early experiments will study the use of these fibers as a "hose" for atoms. Metastable helium seems to be a perfect vehicle for these studies, as any atom that actually hits the glass will be de-excited and not be measured. In later experiments we will use the fiber as a storage medium for ultracold atoms. Our most recent experiments have demonstrated the guiding of metastable helium atoms in hollow optical fibres.