Zero-threshold and Broadband Optical Gain Induced by Plasmonic Doping

Population inversion in emitters is a prerequisite of lasing. This is typically enabled by strongly pumping the emitters, which requires relatively intense pump fluence because the sub-wavelength scale sizes of emitters make them inherently difficult to interact with pump photons. This can lead to small optical gain and increases the lasing thresholds. Here, we report an approach to achieve nearly zero-threshold and broadband optical gain induced by the doping of plasmonic hot electrons. Specifically, we integrate CdSe/CdS core/shell QDs with a silver nano-line array resonator, in which the excitation of plasmon resonances (i.e. collective oscillation of conduction band electrons) in the silver resonator produces highly energetic electrons that can dope the QDs, filling unoccupied states in QDs’ conduction band. As a result, the plasmonic doping blocks the QD’s optical absorption, establishing population inversion with a very low pump power. Transient absorption spectroscopy demonstrates that we can achieve a near-zero threshold optical gain ( ~10^-4 ) at 660 nm and a broadband (550-700 nm) and ultralow-threshold gain (~ 0.17) in the QDs. These results initiate a practical opportunity for zero-threshold lasing and nanolaser devices.