  We address the concept of three-dimensional light bullet formation in
structures where nonlinearity and dispersion are contributed by different
materials, including metamaterials, which are used at their best to create
suitable conditions where bullets can form. The particular geometry considered
here consists of alternating rings made of highly dispersive but weakly
nonlinear media and strongly nonlinear but weakly dispersive media. We show
that light bullets form for a wide range of parameters.
