Microscopic Origin of Surface-Enhanced Circular Dichroism

Circular dichroism (CD), the difference in absorption of two opposite circularly polarized light sources by chiral molecules, can be significantly enhanced when molecules are adsorbed on the surface of nanostructures. We present a theory based on Poynting's theorem adapted for chiral media to analyze the surface-enhanced CD of a chiral molecule/nanostructure coupled system. Our theory clarifies the microscopic origin of surface-enhanced CD signals by showing that the enhanced CD has two forms, inherent and induced. The inherent CD is the direct molecular CD that becomes enhanced due to the strongly localized optical helicity density near the nanostructure. The induced CD, previously ignored, derives from asymmetric excitation and absorption of electromagnetic fields inside the nanostructures surrounded by chiral molecules upon the injection of two oppositely circularly polarized light sources. Moreover, it is demonstrated that the induced CD can contribute significantly to the CD signals measured by surface-enhanced chiroptical spectroscopy.