Optical Funnel for Focusing Cold Atoms

A new experiment at the Tokyo Institute of Technology uses evanescent light to focus cold atoms and output as a beam. Evanescent light is the faint optical field (a sort of aura of light stuck on a material) that is found on the material surface when a laser beam reflects away from the material via "total internal reflection." In this case, the focusing effect occurs when a hollow laser beam moving upwards splays outward around a funnel-shaped piece of glass. The light, shone downward and covering the inner edge of this funnel, helps to repel and cool a blob of atoms held and chilled in a magneto-optical trap (MOT) and falling slightly under the force of gravity.

Evanescent light has been used before to guide atoms through a hollow optical fiber (see Update 272), but in the Tokyo work there are new features: high flux intensity, low temperature, and small beam diameter. The funnel focuses an atom swarm about 2 mm wide is forced to collimate down to the size of the funnel's exit hole, which in the experiment was 200 microns, for a net focusing factor of 100 (see figure). Furthermore, a micron-sized hole is now being tested, which should result in a focusing factor of a million, and a beam flux intensity of some 10¹⁵ atoms/cm²-s.

Akifumi Takamiazwa (Akifumi.Takamizawa@physik.uni-muenchen.de) says that he and his colleagues hope to make a nanometer-sized funnel as small as atomic de Broglie wavelength and use it eventually for single-atom manipulation, perhaps for processes in which one atom can transfer one bit of information. (Takamizawa et al., Applied Physics Letters, 6 September 2004; also see http://uuu.ae.titech.ac.jp/research-e.html and http://www.coe21-pni.titech.ac.jp/eng/task/index.htm)