Broadband Light Beaming from a Nanoaperture Coupled to a Nanohelix

Directional light scattering from a subwavelength aperture in a metallic film has been enabled by coupling the nanoaperture to surface plasmon modes supported by a surrounding concentric grating. However, such plasmonic platforms, typically larger than the wavelength, operate over a narrow spectral bandwidth. Here, we demonstrate nearly wavelength-invariant light beaming from an individual nanoaperture coupled to a subwavelength helical nanoantenna operating in the nonresonant traveling wave regime. We interpret this broadband beam steering behavior by modeling the nanohelix as a subwavelength plasmonic waveguide whose modal dispersion approaches the air light line over a wide frequency range. As a result, broadband on-axis beaming enables efficient light collection with a low-numerical-aperture microscope objective.