Magnetic Excitations in Silver Nanocrescents at Visible and Ultraviolet Frequencies

We study magnetic excitations at optical frequencies in disordered crescent-form split-ring resonators made of silver. The resonators that are less than 100 nm in diameter are fabricated on a 4 in. quartz wafer by using a simple, fast, and inexpensive fabrication technique. The measured transmission and polarization-rotation spectra of the resonators reveal the excitation of circulating electric currents that give rise to magnetic dipole moments in the structures at visible and ultraviolet light frequencies. These frequencies have higher values than the limiting magnetic-resonance frequency predicted by the conventional LC resonance model presumably due to the plasmonic nature of the excitations.