Optical properties of hybrid nanostructures: exciton-plasmon interaction, Fano effect, and plasmon-induced chirality

Coulomb and electromagnetic interactions between excitons and plasmons in nanocrystals cause several prominent effects: Energy transfer between nanoparticles, plasmon enhancement, exciton energy shifts, Fano interference and non-linear effects, and new mechanisms of optical chirality. An interaction between a discrete state of exciton and a continuum of plasmonic states gives rise to interference effects (Fano-like asymmetric resonances and anti-resonances). These interference effects greatly enhance a visibility of relatively weak exciton signals and can be used for spectroscopy of single nanoparticles and molecules. If a system includes chiral elements (chiral molecules or nanocrystals), the exciton-plasmon interaction is able to dramatically change the circular dichroism (CD) of chiral elements. In particular, the exciton-plasmon interaction may create new chiral plasmonic lines in the CD spectra of a molecule-nanocrystal complex. Two mechanisms, in which a plasmonic nanocrystal can influence the CD effect of a chiral molecule, have been identified. The first mechanism is the plasmon-induced change in the electromagnetic field inside the chiral molecule (or chiral semiconductor nanocrystal). The second is the optical absorption of the exciton-plasmon system due to the chiral dissipative currents inside the metal nanocrystal. Strong CD signals may also appear in purely plasmonic systems with a chiral geometry and a strong particle-particle interaction. Recent experiments on the protein-nanocrystal complexes demonstrated an appearance of new chiral plasmonic lines in the CD spectra.