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Split-ball resonator

We introduce a new concept of split-ball resonator and demonstrate a strong magnetic dipole response for both gold and silver spherical plasmonic nanoparticles with nanometer-scale cuts. Tunability of the magnetic dipole resonance throughout the visible spectral range is demonstrated by changing the depth and width of the nanoscale cut. We realize this novel concept experimentally by employing the laser-induced transfer method to produce near-perfect spheres and helium ion beam milling to make cuts with the nanometer resolution. Due to high quality of the spherical particle shape governed by strong surface tension forces during the laser transfer process and clean, straight side walls of the cut made by helium ion milling, the magnetic resonance is observed at 600 nm in gold and at 565 nm in silver nanoparticles. Structuring arbitrary features on the surface of nanoscale spherical resonators provides new ways for engineering hybrid resonant modes and ultra-high near-field enhancement.