  We present a pump-probe technique for monitoring ultrafast polarizability
changes. In particular, we use it to measure the plasma density created at the
temporal focus of a self-compressing higher-order pump soliton in gas-filled
hollow-core photonic crystal fiber. This is done by monitoring the wavelength
of the dispersive wave emission from a counter-propagating probe soliton. By
varying the relative delay between pump and probe, the plasma density
distribution along the fiber can be mapped out. Compared to the recently
introduced interferometric side-probing for monitoring the plasma density, our
new technique is relatively immune to instabilities caused by air turbulence
and mechanical vibration. The results of two experiments on argon- and
krypton-filled fiber are presented, and compared to numerical simulations. The
technique provides an important new tool for probing photoionization in many
different gases and gas mixtures as well as ultrafast changes in dispersion in
many other contexts.
