  Whispering gallery modes in a microwire are characterized by a nearly
equidistant energy spectrum. In the strong exciton-photon coupling regime, this
system represents a bosonic cascade: a ladder of discrete energy levels that
sustains stimulated transitions between neighboring steps. In this work, by
using femtosecond angle-resolved spectroscopic imaging technique, the ultrafast
dynamics of polaritons in a bosonic cascade based on a one-dimensional ZnO
whispering gallery microcavity is explicitly visualized. Clear ladder-form
build-up process from higher to lower energy branches of the polariton
condensates are observed, which are well reproduced by modeling using rate
equations. Moreover, the polariton parametric scattering dynamics are
distinguished on a timescale of hundreds of femtoseconds. Our understanding of
the femtosecond condensation and scattering dynamics paves the way towards
ultrafast coherent control of polaritons at room temperature, which will make
it promising for high-speed all-optical integrated applications.
