Abstract
Plasmonic nanojunctions have been extensively used as antennas to couple far-field radiation to enhanced local fields at the junction.1,2 The confined local fields enable surface enhanced Raman scattering (SERS) with single molecule detection sensitivity.3 A molecule at the hot spot of a nantenna is subject to not only to large fields but also large field gradients, which lead to multipolar Raman scattering.4 Through polarization resolved electronic Raman scattering in the background of SERS,5 we characterize the multipolar response of the nantenna. The observed polarization patterns can be uniquely reproduced by expanding the light-matter interaction to second order in spatial dispersion, which consists of four response functions illustrated by the Feynman diagrams in Fig. 1. The response that involves the product of magnetic and dipolar transitions, <md>, implies the excitation of chiral plasmons. Both left-handed and right-handed chirality is observed on nantennas distinguished only by asymmetries in structure typical of real nanospheres, and this is illustrated in Figure 2.
© 2014 Optical Society of America
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