Enhanced second harmonic generation in a self-similar chain of metal nanospheres

We show theoretically that an efficient nanolens (a self-similar chain of metal nanospheres) creates a nanofocus of highly enhanced (up to six orders of magnitude with respect to the flat surface) second harmonic local fields. This nanofocus is situated in the gap between the nanospheres at the surface of the smallest nanosphere on the axis of symmetry. The spectral maximum for silver nanolens in vacuum is in near-Uv at 337 nm. The generation of the second harmonic radiation in the far zone is also predicted due to the absence of the center symmetry of the nanolens considered. Perspective applications in chemical and biological sensing are discussed.