  We study the propagation of three-dimensional (3D) bipolar ultrashort
electromagnetic pulses in an inhomogeneous array of semiconductor carbon
nanotubes. The heterogeneity is represented by a planar region with an
increased concentration of conduction electrons. The evolution of the
electromagnetic field and electron concentration in the sample are governed by
the Maxwell's equations and continuity equation. In particular, non-uniformity
of the electromagnetic field along the axis of the nanotubes is taken into
account. We demonstrate that, depending on values of parameters of the
electromagnetic pulse approaching the region with the higher electron
concentration, the pulse is reflected from the region or passes it.
Specifically, our simulations demonstrate that, after interacting with the
higher-concentration area, the pulse can propagate steadily, without
significant spreading. The possibility of such ultrashort electromagnetic
pulses propagating in arrays of carbon nanotubes over distances significantly
exceeding characteristic dimensions of the pulses makes it possible to consider
them as 3D solitons.
