Pure dephasing induced single-photon parametric down-conversion in a strongly coupled plasmon-exciton system

We propose highly efficient photon pair generation via down-conversion in a polaritonic cyclic three-level system formed via coupling an exciton and a localized plasmon polariton. Pure dephasing of the exciton enables optical transitions from the upper to lower polariton state via symmetry breaking and thus opens a pathway for the cascaded emission of two photons. The strength of this down-conversion pathway is determined by the coupling strength between exciton and plasmon polariton and the exciton dephasing rate. In the case of strong up to ultrastrong coupling and strong dephasing, this results in a nanoscopic chi (2)-system capable of few-photon down-conversion. A semiconductor quantum dot embedded in a gap plasmon, which supports strong coupling between plasmon and exciton, in combination with fast pure dephasing in the quantum dot exciton, allows one to reach conversion efficiencies down to the single-photon limit under experimentally achievable parameters at ambient conditions.