Circular Dichroism of Single Particles

Circular dichroism (CD) occurs in chiral molecules and nanostructures. It is manifested over a wide range of length scales, from molecules to crystals and large nanostructures. Being a weak phenomenon (typically a small fraction of absorption), CD is routinely measured on macroscopic quantities of matter in solutions, crystals, or arrays of fabricated meta-particles. These ensemble-averaged measurements mask the sensitivity of CD to small structural variations between individual nano-objects or to the possible coexistence of opposite enantiomers in the ensemble. We have developed far field extinction microcopy, which enables sensitive CD measurement on individual nano-objects, encompassing dramatic suppression of linear optical activity artifacts. Here, we report on CD of both chiral shaped plasmonic nanostructures (gold Gammadions) and achiral shaped inorganic nanocrystals having a chiral crystal lattice (HgS). CD spectra and spatial mapping were measured. Simulations on Gammadions, mimicking the experimental conditions, showed good correlation to the measurements. We have also demonstrated in simulations and experiments the high sensitivity of the measurements to minor misalignments of the experimental setup. This work extends the single-particle-spectroscopy toolbox to include CD spectroscopy and enables a unique diffraction limited imaging technique sensitive to chirality at the nanoscale.