Number 244 (Story #2), October 11, 1995 by Phillip F. Schewe and Ben Stein BRAGG SCATTERING FROM ATOMS IN OPTICAL LATTICES: In 1912 William and Lawrence Bragg used x rays to elucidate the structure of crystalline solids. The x rays, whose wavelengths were well matched to the lattice spacing of the atoms, scattered from many planes of atoms and produced a characteristic interference pattern. Now artificial crystals consisting of no more than a tenuous swarm of atoms held in midair by a web of intersecting laser beams can be studied using analogous techniques. In such an "optical lattice" the atom spacing is more like a micron rather than an angstrom, and so laser light at optical wavelengths is an appropriate probe for performing a Bragg-like experiment. Scientists at NIST (Gaithersburg, MD) create an optical lattice with a density of about 10**10 cesium atoms per cu.cm. Even at that density only a small fraction of lattice sites is filled with atoms, but the periodic structure of the lattice is still evident; for example, the Bragg-scattered light beam has a Gaussian profile when the atoms are in their optical lattice formation, whereas the profile is flat when the atoms are in a disordered state. The NIST physicists see Bragg scattering as an important new tool for measuring optical lattice properties, such as its density or index of refraction. (G. Birkl et al., Physical Review Letters, 9 October 1995 and Physical Review Letters, 18 December 1995.)