Pure magnetic resonances controlled by the relative azimuth angle between meta-atoms.

A plasmonic molecule showing strong magnetic resonance modes and flexible tunability is proposed. The molecule is composed of two elements, a crescent shaped metallic disk and a smaller one embedded in the cavity of the larger one. The cavity and gap formed by these two elements enable the molecule to support magnetic resonances in the visible and near infrared spectral region, while electric resonances are much weaker to be detected. We show that by changing the relative orientation angle of these two meta-atoms, the resonance wavelength can be changed from the visible to near infrared without modification of the size of the molecule. Anti-crossings and crossings of resonance energy levels, which stem from the coupling effect, are analyzed. When resonating magnetically, the local electric field enhancement at the crescent tip can reach up to hundreds of times with high spatial confinement, which renders the molecule promising applications in many fields.