Introduction

The nanoantenna concept refers to electromagnetic phenomena related to field amplification and confinement at visible or near-IR light by nanometer-sized objects [29, 206]. Nanoantennas rely on electric field enhancement by the LSPR, which takes place in metallic NPs embedded in dielectric media. There is a profuse literature about this topic and several reviews can be found elsewhere [202, 507, 508].

The simplest model for understanding LSPR is to consider the electrostatic problem of a sphere in a dielectric medium under a homogeneous applied field [151, 234, 509]. The solution is a homogeneous internal field modified by the effect of depolarization generated by surface charges. Contrary to this, the external field presents an evanescent character, decaying as r-3 outside the NP. However, the most interesting fact is that internal and surface fields diverge when the medium єd and NP єm dielectric functions are such that 2єd = -єm. From an experimental point of view, this condition can be approximately fulfilled for several metals (mainly Ag, Au and Cu) at some specific frequencies. The electric field at the NP surface can increase up to 1000 times. The resonance condition can be modified by changing the matrix or the shape of the NPs. Therefore, for either oblate or prolate NPs, the resonance condition is given by (1 - L)/єd = -Lєm, where L is the so-called depolarization factor [510], which only depends on the NP geometry. For an irregular shape, the NP is described by several depolarization factors Lk, each with its corresponding LSPR associated with it.