  We show numerically that ultrashort self-defocusing temporal solitons
colliding with a weak pulsed probe in the near-IR can convert the probe to the
mid-IR. A near-perfect conversion efficiency is possible for a high effective
soliton order. The near-IR self-defocusing soliton can form in a quadratic
nonlinear crystal (beta-barium borate) in the normal dispersion regime due to
cascaded (phase-mismatched) second-harmonic generation, and the mid-IR
converted wave is formed in the anomalous dispersion regime between
$\lambda=2.2-2.4~\mu\rm m$ as a resonant dispersive wave. This process relies
on non-degenerate four-wave mixing mediated by an effective negative
cross-phase modulation term caused by cascaded soliton-probe sum-frequency
generation.
