Diffraction of Atoms by Real, Complex and Imaginary Crystals of Light

8:30 AM-9:30 AM

*Room: Ballrooms I, II & III*

*Chair: Steven H. Strogatz, Cornell University*

Quantum atoms interact with light through a spatially-varying
sinusoidal potential. This diffracts the atoms into a series of Bragg
beams (previously studied for the diffraction of light by
ultrasound). For real (nondissipative) potentials, classical motion
is integrable; nevertheless, the semiclassical limit for thick
crystals is ergodic, and caustics give large "twinkling"
fluctuations. The potential can be made arbitrarily complex by
adjusting the detuning. If it is purely imaginary, diffraction
reveals nonhermitian degeneracies of the Bloch waves, and the
distribution of intensities is dominated by complex rays rather than
caustics. For complex potentials proportional to exp(*iKx*),
the intensities can be calculated exactly, giving lopsided
diffraction -- and a dramatic violation of Friedel's law of
crystallography, even a fractal.

**Sir Michael Berry**

*H. H. Wills Physics Laboratory*

*University of Bristol, United Kingdom*

*MMD, 2/9/99*