  The scattering of a linear wave on an optical event horizon, induced by a
cross polarized soliton, is experimentally and numerically investigated in
integrated structures. The experiments are performed in a dispersion-engineered
birefringent silicon nanophotonic waveguide. In stark contrast with
co-polarized waves, the large difference between the group velocity of the two
cross-polarized waves enables a frequency conversion almost independent on the
soliton wavelength. It is shown that the generated idler is only shifted by 10
nm around 1550 nm over a pump tuning range of 350 nm. Simulations using two
coupled full vectorial nonlinear Schr\"odinger equations fully support the
experimental results.
