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Number 415, February 18, 1999 by Phillip F. Schewe and Ben Stein
LIGHT HAS BEEN SLOWED TO A SPEED OF 17 METERS/SECOND by passing it through a Bose- Einstein condensate (BEC) of sodium atoms at nK temperatures. In general light is slowed in certain materials, a property exploited in making optical lenses. As the index of refraction of these materials gets higher, however, absorption increasingly takes its toll on the light beam. In an experiment at the Rowland Institute of Science, (Lene Vestergaard Hau, hau@rowland.org), physicists have used a BEC as the optical medium, but with the following important modification. They contrived a system of laser beams whose pattern of interference created an effect called electromagnetically induced transparency, allowing light to propagate unabsorbed but at greatly reduced speeds, in this case a factor of twenty million compared to the speed of light in vacuum; greater light-speed slow downs are expected, to as low as cm/sec. The researchers also observed unprecedentedly large intensity-dependent light transmission. Such an extreme nonlinear effect can perhaps be used in a number of opto-electronic components (switches, memory, delay lines) and in converting light from one wavelength to another. (Hau et al., Nature, 18 February 1999.)
TUNABLE X-RAY WAVEGUIDE WITH AN AIR GAP. At synchrotron light sources, electron beams make floods of x rays which must be tamed before they can be used in experiments where typically they probe the structure of some tiny biological sample. One of the ways to focus the beam onto the sample is to compress it and guide the x rays through a thin strip of material sandwiched between reflecting surfaces. Usually the guiding material, often carbon, absorbs a substantial portion of the x rays. Researchers at the University of Amsterdam (Friso van der Veen, vdveen@wins.uva.nl, 011-31-20-525-6330) have now produced a waveguide out of two parallel reflecting plates with only air in between. This not only greatly reduces x ray losses but also, when the gap is filled with liquid, permits the x-ray study of lubricants and colloids. In optics geometry is destiny; the coherent wave pattern in the Amsterdam device can be tuned by prising apart the two flat plates which form the body of the waveguide. For the whole process to work the plates (only about 250 nm apart) must be extremely parallel, the equivalent of suspending one soccer field over another at a height of about 5 mm. (M.J. Zwanenburg et al., Physical Review Letters, 22 Feb; see figure at Physics News Graphics.)
A TWO DIMENSIONAL BOSE EINSTEIN CONDENSATE (BEC) has been observed by a University of Turku (Finland)--Kurchatov Institute (Russia) collaboration. The condensate occurred in hydrogen atoms sitting on top of a layer of liquid helium-4 at a temperature of 120-200 mK (Safonov et al., Physical Review Letters, 23 November 1998). Strong magnetic fields force the nuclei and electron spins of the hydrogen atoms to align. Magnetic fields also help to herd the atoms together into a small portion of the helium surface, achieving the density needed to begin the process of atom overlap at the heart of the BEC process. Explanations for this type of 2D quantum fluid are lacking. (Physics World, Feb 1999.)
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