Direct laser writing of symmetry-broken spiral tapers for polarization-insensitive three-dimensional plasmonic focusing

Plasmonic focusing with metallic probes has attracted extensive studies due to its successful applications in advanced technologies such as near-field scanning optical microscopy and tip-enhanced Raman spectroscopy. Here the fabrication and characterization of a unique spiral metallic taper with polarization-insensitive three-dimensional (3D) plasmonic focusing properties are reported. Metallic probes with spiral corrugations are readily fabricated along the surfaces of the conical structures with a 3D direct laser writing method followed by a metal deposition process. With the broken structural symmetry induced by the spiral corrugations, plasmonic focusing is demonstrated under excitation of linearly polarized light with different polarization directions. Moreover, apertures with various sizes can be flexibly introduced at the apex of the conical probe structures with direct fabrication, which enables the observation of scattered light from waveguide modes, cutoff of waveguide modes and scattering from surfaces plasmons, respectively. The studies provide a novel methodology of design, realization, and application of 3D plasmonic focusing structures.