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Plasmonic nanolasers have ultrahigh lasing thresholds, especially those devices for which all three dimensions are truly subwavelength. Because of a momentum mismatch between the propagating light and localized optical field of the subwavelength nanocavity, poor optical pumping efficiency is another important reason for the ultrahigh threshold but is normally always ignored. Based on a cavity-embedded nanoantenna array design, we demonstrate a room-temperature low-threshold plasmonic nanolaser that is robust, reproducible, and easy-to-fabricate using chemical-template lithography. The mode volume of the device is~0.22(/2n) 3 (here,  is resonant wavelength and n is the refractive index), and the experimental lasing threshold produced is ~2.70MW/mm 2 . The lasing polarization and the function of nanoantenna array are investigated in detail. Our work provides a new strategy to achieve room-temperature low-threshold plasmonic nanolasers of interest in applications to biological sensoring and information technology. Regarding conventional lasers, the experimental lasing threshold is known to be determined by not only the intrinsic loss of the cavity but also the pumping efficiency, which has drawn little attention in previous reports on spasers. Although electric pumping is considered to offer better prospects for spasers 22,23 Here, we report a low-threshold, room-temperature plasmonic laser and demonstrate that an optical antenna array can efficiently lower the lasing threshold of a spaser. The new device is constructed by embedding a fluorescence polystyrene sphere into a silver nanoparticle (nanoantenna) array, which combines the nanocavity and optical antennas together to promote pumping efficiency. The lasing threshold is 2.70 MW/mm 2 , more than 20 times lower than that of a room-temperature arrayed nanocavity spaser 18 . Unlike earlier struggles to decrease lasing thresholds by eliminating spaser intrinsic metal loss, we propose to increase the pumping efficiency with the optical antenna array. This array will resonantly absorb light from the pump beam and concentrate the energy into the cavity of the embedded fluorescence polystyrene sphere. Moreover, the cavity mode, which is mainly localized within the 4 polystyrene sphere, will also reduce the intrinsic metal loss. The lasing threshold and other properties of the spaser are reported and their dependences on geometric parameters of the devices are described. KEYWORDSThe lasing system, sketched in Fig.1a, is constructed by embedding a fluorescence polystyrene ball into a two-dimensional silver nanoantenna array. The 150-nm-diameter ball functions not only as a gain medium but also as a nanocavity together with the surrounding silver nanoantennas. A scanning electron microscopy (SEM) micrograph (Fig.1b), shows a periodicity of about 65 nm for the silver nanoantenna array and a diameter of about 100 nm for the cavity. The height of the nanoantenna is around 15 nm as indicated in the AFM image (Fig.1c). All these parameters are optimized to match ...

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