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We have observed the reflection of a probe laser beam by a three-dimensional optical lattice of rubidium atoms. The reflection spectrum shows different kinds of narrow resonances which we attribute to degenerate (phase conjugation) and non-degenerate multiwave mixing processes. In order to explain the extremely narrow linewidth (300 Hz) of the degenerate four-wave mixing resonance and its dependence on the angle of incidence of the probe, we propose a novel physical mechanism which includes the formation of a long-range atomic-density grating acting as a hologram.
Hemmerich et al. (Sat,) studied this question.