Femtosecond coherence dynamics of exciton-polaritons.

Exciton-polaritons perform as ideal carriers of macroscopic quantum coherence, which can be potentially manipulated through precisely shaping the driving laser field. However, the connection of the coherence properties between the pumping laser and the strongly coupled light-and-matter system is studied to a lesser extent. In this paper, we visualize the femtosecond dynamics of coherence transfer from the driving laser field to the resonantly excited exciton-polariton by an interferometric measurement. The resonant polaritons can effectively preserve the coherence of the pumping laser field in femtosecond timescales. At a high excitation strength, non-resonant polaritons appear at higher energies delayed by several picoseconds, without the phase coherence from the pump, which is understood by a coupled oscillator model. Our results offer the possibility of regulating the polariton coherence by finely shaping the external pump laser fields.