Abstract

Surface plasmons are collective oscillations of electrons that can confine light within extremely tiny nanoscale spaces, acting to enhance the spectral signals (such as Raman and circular dichroism) of molecules. Borrowed from circular dichroism in chemistry, plasmonic local circular dichroism (CD) refers to the difference in electric field enhancement excited by left-handed (LCP) and right-handed (RCP) circularly polarized light in a nanogap. Nanosphere trimers have been employed to realize local CD. However, spherical nanoparticles exhibit a small gap area and poor structural stability. In this article, we propose asymmetric nanocylinder trimers on SiO2/Si substrate to realize a strong local CD signal. In contrast to nanospheres, nanocylinder sidewalls increase effective gap areas and can be obtained by nanofabrication. We find that the asymmetric nanocylinder trimer achieves a local CD parameter 0.94 under the excitation of 782 nm. This strong local CD within the gaps mainly originates from the near-field interference between different modes generated by circularly polarized light. Furthermore, the gap distance and symmetry have a significant influence on the local CD. A giant local CD can be obtained for small gaps (1 nm) and right-angle trimers. These results provide a physical basis for the development of plasmonic CD and chirality, and they are of interest for the field of nanosensors, nanoantennas, solar energy conversion, and polarization-dependent photochemistry.

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