Emergent Optical Phenomena in Disordered Nanoparticles: Quadrupole Enhanced Raman Scattering (QERS) and Strong Optical Magnetic Plasmons

The rule of thumb in the design of plasmonic nano-antennae and waveguides is to fabricate, or to synthesize nanostructures with perfect shapes and uniform size distribution. The properties of such nanostructures can then be precisely predicted using readily available modeling methods such as finite difference time-domain (FDTD) simulations. Here we present a new paradigm in nanoparticle design. Disordered and randomly packed nanostructured synthesized using templated surfactant-assisted seed growth method show highly tunable properties, and exceptional optical response. I present two examples of such emergent phenomena.

1. Spiky nanoshells, composed of randomly oriented sharp cones of gold, decorated on a polystyrene core, have designable quadrupole resonances that efficiently enhance Raman scattering with unprecedented reproducibility on the single particle level. The efficiency and reproducibility of Quadrupole Enhanced Raman Scattering (QERS) is due to their heterogeneous structure which broadens the quadrupole resonance both spatially and spectrally. This spectral breadth allows for simultaneous enhancement of both the excitation and Stokes frequencies. The quadrupole resonance can be tuned by simple modifications of the nanoshell geometry. The combination of tunability, high efficiency and reproducibility makes these nanoshells an excellent candidate for applications such as biosensing, nano-antennaes and photovoltaics.
   
2. Magnetic effects in optical materials are much weaker than electric effects. Artificial magnetic resonances can be produced by oscillating charges on a circular pattern, similar to inductors in electronics. This phenomenon has been theoretically predicted and observed in properly tailored plasmonic structures based on top-up lithographic methods, but in these structures the magnetic plasmon is always oriented normal to the plane of the nanostructure. Raspberry-like magnetic metamolecules, randomly closed-Packed clusters of gold nanoparticles with strong optical magnetic plasmon resonances consist of many individual gold nano-beads randomly closed-packed on the surface of a polystyrene core. The nanoparticles are protected from touching each other by a coating layer of surfactant. These nanoclusters exhibit isotropic plasmon resonances in the vis-near-IR region that can be attributed to multiple resonances, including electric dipole, electric quadrupole and magnetic dipole resonances. Furthermore, when the overall size of the nanoparticles exceeds a certain level, a strong magnetic quadrupole is also observed that is visible in the far-field. These nanoparticles are isotropic and present strong scattering peaks that are omni-directional. These properties make these nanoparticles ideal candidates for negative index materials and disordered media waveguides.