Nonlinear and Active Optics in Nanolenses*

We consider surface plasmon amplification by stimulated emission of radiation (SPASER) and second harmonic generation (SHG) in an effective nanolens. Such a nanolens is an aggregate of several nanospheres with progressively decreasing radii and separations .....[1]. It has a ``hottest spot'' of highly enhanced local fields between the smallest spheres. We show that such a system surrounded by semiconductor quantum dots is also an efficient SPASER that generates dark eigenmodes with gigantic, temporarily coherent local fields .()..()[2]. We also consider SHG in the nanolenses where we show that highly enhanced SHG local fields are generated at the nanofocus. Numerical data are presented for silver spheres .[1] K. Li, M. I. Stockman, and D. J. Bergman, \textit{Self-Similar Chain of Metal Nanospheres as an Efficient Nanolens}, Phys. Rev. Lett. \textbf{91}, 227402-1-4 (2003). [2] K. Li, X. Li, M. I. Stockman, and D. J. Bergman, \textit{Surface Plasmon Amplification by Stimulated Emission in Nanolenses}, Phys. Rev. B (2005 (In Print)).