Femtosecond dynamics of semiconductor microcavity polaritons

Dynamics of semiconductor microcavity modes in ZnS-Ni structure was investigated by femtosecond pump supercontinuum probe spectroscopy in wide spectral region 1.6 - 3.2 eV. The change of reflectivity of the ZnS thin films (of 0.29 mu m, 1.08 mu m and 1.17 mu m) on thick Ni film on quartz substrate was monitored. Two different pumping photon energies (h) over bar omega(pu1) = 2.75 eV and (h) over bar omega(pu2) = 5.5 eV were used for the excitation of the microcavity and produced different photoresponse spectra. The first pumping energy(2.75 eV) is lower than the energy gap of ZnS (3.7 eV) and the powerful laser pulse excites mainly electrons of metal (i.e. boundary of the microcavity) and of ZnS layer (by two-photon absorption). Nonequilibrium carriers of metal penetrate through Schottky electron barrier into the semiconductor. For the second pumping energy (5.5 eV) the pumping pulse is practically totally absorbed in thin surface ZnS layer of 40 nn in thickness and creates nonhomogeneous hot carriers distribution in semiconductor. The differences in the processes of carrier excitation are responsible for the differenses in the changes of dielectric functions of ZnS and Ni and for the differenses in the change of cavity modes.