Giant circular dichroism of a molecule in a region of strong plasmon resonances between two neighboring gold nanocrystals

We report on giant circular dichroism (CD) of a molecule inserted into a plasmonic hot spot. Naturally occurring molecules and biomolecules typically have CD signals in the UV range, whereas plasmonic nanocrystals exhibit strong plasmon resonances in the visible spectral interval. Therefore, excitations of chiral molecules and plasmon resonances are typically off-resonant. Nevertheless, we demonstrate theoretically that it is possible to create strongly enhanced molecular CD utilizing the plasmons. This task is doubly challenging since it requires both creation and enhancement of the molecular CD in the visible region. We demonstrate this effect within the model which incorporates a chiral molecule and a plasmonic dimer. The associated mechanism of plasmonic CD comes from the Coulomb interaction, which is greatly amplified in a plasmonic hot spot. An important feature of the system is anisotropy that results in giant enhancement for the molecular dipoles parallel to the dimer axis. The proposed effect offers very interesting possibilities for enhanced sensing of chiral molecules using visible light. DOI: 10.1103/PhysRevB.87.075410