Abstract
We study optical second harmonic generation from metallic dipole antennas with narrow gaps. Enhancement of the fundamental-frequency field in the gap region plays a marginal role on conversion efficiency. In the symmetric configuration, i.e., with the gap located at the center of the antenna axis, reducing gap size induces a significant red-shift of the maximum conversion efficiency peak. Either enhancement or inhibition of second-harmonic emission may be observed as gap size is decreased, depending on the antenna mode excited at the harmonic frequency. The second-harmonic signal is extremely sensitive to the asymmetry introduced by gap's displacements with respect to the antenna center. In this situation, second-harmonic light can couple to all the available antenna modes. We perform a multipolar analysis that allows engineering the far-field SH emission and find that the interaction with quasi-odd-symmetry modes generates radiation patterns with a strong dipolar component.
Highlights
The manipulation of light with optical nanoantennas [1] may unlock a plethora of new opportunities for microscopy and spectroscopy devices, enhanced photovoltaic cells, and for a large variety of optical and electro-optical applications that are typically limited by diffraction
We have assessed the roles of field enhancement and antenna modes when both the fundamental and harmonic frequency fields are resonant with the antenna
We find that field enhancement in the gap has minimal impact on the amount of radiated SH light
Summary
The manipulation of light with optical nanoantennas [1] may unlock a plethora of new opportunities for microscopy and spectroscopy devices, enhanced photovoltaic cells, and for a large variety of optical and electro-optical applications that are typically limited by diffraction. A pronounced sensitivity to small geometrical perturbations has been predicted by observing modulations in the quadrupolar far-field pattern of SH light scattered by symmetric dipole antennas [29]. This property may be suitable for shape characterization of nanoparticles. We clarify the role of antenna resonances and field enhancement in SH generation from both symmetric and asymmetric dipole antennas, when the full quadratic nonlinear response (both surface and bulk contributions) of metal is considered. We observe that the field enhancement in the gap region has minimal effect on the SH scattering efficiency and far field, which remain inherently quadrupolar in symmetric dipole antennas. We provide a multipolar representation of SH generation in order to explain the peculiar behavior of asymmetric dipole antennas
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