Photochemical Imaging of Near-Field and Dissymmetry Factor in Chiral Nanostructures

Chiral nanostructures interact differently with right- or left-handed circularly polarized light. Due to the lack of experimental approaches able to unveil the optical near-field around chiral nanostructures, their optical characterization generally relies either on far-field techniques or on numerical simulations. Here, a subwavelength imaging approach based on the interaction between the enhanced optical near-field around chiral nanoparticles and an azobenzene molecular probe is reported. Under the action of light, the azobenzene molecules undergo isomerization cycles resulting in a measurable displacement of matter. The resulting imprint of the optical near-field can then be imaged using atomic force microscopy. Using two shifted gold nanorods separated by a nanogap as a canonical chiral structure, a dissymmetry in the near-field of the chiral structures under left and right circular polarizations is experimentally evidenced. Notably, it is observed that the gap mode is excited only when the handedness of the exciting light matches the handedness of the nanostructure, evidencing a chiral hot spot inside the gap. Moreover, it is shown that it is possible to experimentally map the near-field dissymmetry factor, a quantity related to the local circular dichroism of the near-field intensity.