  When atoms scatter photons from a transverse laser into a high-finesse
optical cavity, they form crystalline structures which maximize the intracavity
light field and trap the atoms in the ordered array. Stable organization occurs
when the laser field amplitude exceeds a certain threshold. For planar
single-mode cavities there exist two equivalent possible atomic patterns, which
determine the phase of the intracavity light field. Under these premises, we
show that the effect of an additional laser pumping the cavity makes one
pattern more favorable than the other and that it can dynamically force the
system into a predetermined configuration. This is an instance of pattern
formation and seeding in a nonlinear quantum-optical system.
