Low-threshold surface plasmon amplification from a gain-assisted core–shell nanoparticle with broken symmetry

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We report that the gain threshold of a core–shell nanoparticle-based spaser can be reduced significantly by offsetting the gain-doped dielectric core within the metallic shell. By investigating the optical cross sections of the reduced symmetry core–shell nanoparticle with different levels of gain, we determined the gain threshold of the asymmetric nanoparticle-based spaser fueled by different plasmon modes. The calculation results indicate that when multipolar plasmon oscillations excited in asymmetric core–shell nanostructures are used as lasing modes, the gain threshold of an asymmetric spaser particle can drop by 30% as compared to the case of a perfect or symmetric particle. The underlying physics of low-threshold surface plasmon amplification is explained by investigating the Q factor and the optical field confinement and enhancement associated with the lasing mode.