We investigate the second-order nonlinear response of dipole nanoantennas with narrow gaps. Second-harmonic light originates mainly from weak nonlinear forces acting on free-electrons, such as Lorentz and Coulomb forces, as well as convective and quantum pressure forces [1]. However, dipole nanoantennas provide several mechanisms to magnify such nonlinearities both in the near- and far-field regions. The excitation of localized surface plasmon resonances associated with antenna modes produces a field enhancement effect that involves the whole antenna surface. Additionally, the large charge accumulation at the metallic edges facing the gap for input light polarized along the antenna’s long axis results in a local field enhancement effect that involves only the gap region. Moreover, coupling of second harmonic light to antenna modes is an efficient mechanism to radiate second-harmonic light into the far-field region. The roles of field enhancement and antenna modes are evaluated in symmetric configurations [inset of Fig. 1(a)], i.e., with the gap located at the antenna’s center, and in asymmetric configurations [inset of Fig. 1(b)], i.e., when the gap is displaced with respect to the antenna’s center. In Fig. 1 we show second-harmonic conversion efficiency spectra as a function of the gap size for (a) the symmetric case and (b) as a function of the gap displacement for the asymmetric case. The total antenna length is L = 300 nm and the antenna width is w = 10 nm. We clarify why either enhancement or inhibition of second- harmonic light can be observed at a specific input frequency as the gap size is reduced [2-3].

© 2015 IEEE

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