Temporal Signature of Bosonic Stimulation Induced by Dark Exciton in a Two-Photon-Pumped Polariton Condensate

Strongly coupled light-matter exciton-polaritons constitute an on-chip solid-state platform where the macroscopic quantum phenomenon of condensation is not only achievable at elevated temperatures, but can also be optically controlled, including through their interaction with inaccessible "dark" states. A recent study has shown that polariton condensation can be established under nonlinear two-photon pumping, paving the way toward dark-state-condensate coherent control and highly efficient terahertz (THz) lasing. In this Letter, we show for the first time a temporal signature of dark exciton-induced bosonic stimulation by investigating one- and two-photon-pumped condensation with time-resolved photoluminescence spectroscopy. Our results show a clear difference in the detuning dependence of the buildup and relaxation rates of the condensate-induced blueshifts under one- and two-photon pumping. This difference is associated with the stronger exciton-fraction dependence expected for one-photon-pumped condensation, where polariton-polariton stimulation dominates, compared with two-photon-pumped condensation, where a 2p-exciton-to-lower-polariton THz transition can provide another stimulation channel, together with various spin-flip, electron-hole exchange, and phonon-based mechanisms. These observations indicate the presence of a new dark-state-originated stimulation channel that could facilitate highly-efficient THz lasing in semiconductor microcavities.