  The enhanced electric field at plasmonic resonances in nanoscale antennas can
lead to efficient harmonic generation, especially when the plasmonic geometry
is asymmetric on either inter-particle or intra-particle levels. The planar
Archimedean nanospiral offers a unique geometrical asymmetry for
second-harmonic generation (SHG) because the SHG results neither from arranging
centrosymmetric nanoparticles in asymmetric groupings, nor from
non-centrosymmetric nanoparticles that retain a local axis of symmetry. Here we
report forward SHG from planar arrays of Archimedean nanospirals using 15 fs
pulse from a Ti:sapphire oscillator tuned to 800 nm wavelength. The measured
harmonic-generation efficiencies are 2.6*10-9, 8*10-9 and 1.3*10-8 for
left-handed circular, linear, and right-handed circular polarizations,
respectively. The uncoated nanospirals are stable under average power loading
of as much as 300 uW per nanoparticle. The nanospirals also exhibit a selective
conversion between polarization states. These experiments show that the
intrinsic asymmetry of the nanospirals results in a highly efficient,
two-dimensional harmonic generator that can be incorporated into metasurface
optics.
