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Surface plasmon amplification by stimulated emission in nanolenses

We propose surface plasmon amplification by stimulated emission of radiation (spaser) in nanolenses: Linear center-symmetric chains of metal nanospheres, embedded in an active medium of quantum dots. Predominantly amplified are the dark, odd-parity eigenmodes, which do not suffer dipole-radiative losses and produce coherent local optical fields comparable in strength to atomic fields, with minimal far-field radiation. There are many prospective applications for such spasers.