  We study the expansion of bosonic Mott insulators in the presence of an
optical lattice after switching off a confining potential. We use the
Gutzwiller mean-field approximation and consider two different setups. In the
first one, the expansion is restricted to one direction. We show that this
leads to the emergence of two condensates with well defined momenta, and argue
that such a construct can be used to create atom lasers in optical lattices. In
the second setup, we study Mott insulators that are allowed to expand in all
directions in the lattice. In this case, a simple condensate is seen to develop
within the mean-field approximation. However, its constituent bosons are found
to populate many nonzero momentum modes. An analytic understanding of both
phenomena in terms of the exact dispersion relation in the hard-core limit is
presented.
