  We report measurements of the attenuation of a beam of
orthoexciton-polaritons by a photoionizing optical probe. Excitons were
prepared in a narrow resonance by two photon absorption of a 1.016 eV, 54 ps
pulsed light source in cuprous oxide (Cu2O) at 1.4 K. A collinear, 1.165 eV, 54
ps probe delayed by 119 ps was used to measure the photoionization cross
section of the excitons. Two photon absorption is quadratic with respect to the
intensity of the pump and leads to polariton formation. Ionization is linear
with respect to the intensity of the probe. Subsequent carrier recombination is
quadratic with respect to the intenisty of the probe, and is distinguished
because it shifts the exciton momentum away from the polariton anticrossing;
the photoionizing probe leads to a rise in phonon-linked luminescence in
addition to the attenuation of polaritons. The evolution of the exciton density
was determined by variably delaying the probe pulse. Using the probe irradiance
and the reduction in the transmitted polariton light, a cross section of
3.9*10^(-22) m^2 was deduced for the probe frequency.
