Realizing zero-threshold population inversion via plasmonic doping

Lowering the population inversion threshold is key to leveraging quantum dots (QDs) for nanoscale lasing and laser miniaturization. However, optical realization of population inversion in QDs has an inherent limitation: the number of excited electrons per QD is bound by the absorbed photons. Here we show that one can break this population limit and realize near-zero threshold inversion via plasmonic doping. Specifically, we integrate QDs into a grating-like plasmonic resonator, which, upon optical excitation, can transiently dope the QDs with numerous highly energetic electrons and make excited electrons in the QDs outnumber absorbed photons. This high population under low excitation blocks QD absorption and reduces the population-inversion threshold over 100 times compared to neutrally populated QDs. Our findings not only reveal new understanding of cavity-emitter interactions but also provide practical avenues for zero-threshold lasing, nanolasing and amplification devices.