  Searching for three-dimensional spatiotemporal solitons (also known as
light/optical bullets) has recently attracted keen theoretical and experimental
interests in nonlinear physics. Currently, optical lattices of diverse kinds
have been introduced to the stabilization of light bullets, while the
investigation for the light bullets of gap type -- nonlinear localized modes
within the finite gap of the underlying linear Bloch spectrum -- is lacking.
Herein, we address the formation and stabilization properties of such light gap
bullets in periodic media with defocusing nonlinearity, theoretically and in
numerical ways. The periodic media are based on two-dimensional periodic
standing waves created in a coherent three-level atomic system which is driven
to the regime of electromagnetically induced transparency, which in principle
can also be replaced by photonic crystals in optics or optical lattices in
ground-state ultracold atoms system. The temporal dispersion term is tuned to
normal (positive) group velocity dispersion so that to launch the light gap
bullets under self-repulsive nonlinearity; two types of such light gap bullets
constructed as 3D gap solitons and vortices with topological charge m=1 within
the first finite gap are reported and found to be robustly stable in the
existence domains. On account of the light bullets were previously limited to
the semi-infinite gap of periodic media and continuous nonlinear physical
systems, the light gap bullets reported here thus supplement the missing type
of three-dimensional spatiotemporal localized modes in periodic media which
exhibit finite band gaps.
