  Gap solitons are localized nonlinear coherent states which have been shown
both theoretically and experimentally to propagate in periodic structures.
Although theory allows for their propagation at any speed $v$, $0\le v\le c$,
they have been observed in experiments at speeds of approximately 50% of $c$.
It is of scientific and technological interest to trap gap solitons. We first
introduce an explicit multiparameter family of periodic structures with
localized defects, which support linear defect modes. These linear defect modes
are shown to persist into the nonlinear regime, as {\it nonlinear defect
modes}. Using mathematical analysis and numerical simulations we then
investigate the capture of an incident gap soliton by these defects. The
mechanism of capture of a gap soliton is resonant transfer of its energy to
nonlinear defect modes. We introduce a useful bifurcation diagram from which
information on the parameter regimes of gap soliton capture, reflection and
transmission can be obtained by simple conservation of energy and resonant
energy transfer principles.
