Probing slow polariton propagation by bipolaritons

Using a Four-Wave-Mixing (FWM) experiment in backward-scattering geometry t he coherent dynamics of the quadrupole polarization associated with exciton ic molecules is studied in bulk CdSe. By analyzing the FWM signal at negati ve delay times and exploiting the large differences in group velocities of the propagating polaritons, a picosecond dephasing time is found for the bi polariton. By comparison of the measured dephasing time with the calculated radiative lifetime of the bipolariton, the validity of the bipolariton rep resentation is verified for excitonic molecules in CdSe at low temperatures .