Spiral Enhancement

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Science  19 Nov 2010:
Vol. 330, Issue 6007, pp. 1022
DOI: 10.1126/science.330.6007.1022-b

When light strikes tiny metal particles, it can excite electrons at their surfaces along a pathway termed a plasmon resonance. Recently this phenomenon has been put to use to enhance Raman scattering by molecules in the vicinity of the particles, often by a factor of a billion or more. Hendry et al. explore a distinct application of plasmon excitation, in which they assemble mirror-image arrays of gold particles that possess a spiral sort of two-dimensional chirality, and as a result interact differently with left and right circularly polarized light. They then deposit chiral proteins onto the surfaces and examine the associated perturbations to circularly polarized plasmon excitation of left- versus right-handed particles (validated by control experiments with achiral particles of similar size and shape). They find that proteins with β-sheet motifs induce significantly greater shifts to one array's resonances over its counterpart's, whereas those with α helices induce little dissymmetry. Moreover, the effective refractive index shifts extracted from the β-sheet measurements are roughly a millionfold higher than those associated with differential circularly polarized light scattering by the biomolecules in solution. Though the detailed mechanism underlying the effect remains under study, the authors posit that quadrupolar interactions between the proteins and chiral optical fields at the surface play a key role.

Nat. Nanotechnol. 5, 10.1038/NNANO.2010.209 (2010).

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