Concave plasmonic particles: broad-band geometrical tunability in the near-infrared.

Optical resonances spanning the near-infrared spectrum (1-2 microm) were exhibited experimentally by arrays of plasmonic nanoparticles with concave cross-section. The concavity of the particle was shown to be the key component for enabling the broad band tunability of the resonance frequency, even for particles with dimensional aspect ratios of order unity. The atypical flexibility of setting the resonance wavelength is shown to stem from a unique interplay of local geometry with surface charge distributions.