Chiral Sum Frequency Generation by Chiral Gold Nanoparticles Amplifies Weak Sum Frequency Signals and Enables Monitoring of Enantioselective Reactions

Chiral nanostructures or chiral arrangements of plasmonic nanoparticles can strongly interact with circularly polarized light, giving rise to chiroptical effects such as circular dichroism and optical rotation. In parallel, enhanced optical fields around metallic nanostructures have been shown to be capable of enhancement of both linear and nonlinear optical processes. Sum frequency generation (SFG) spectroscopy, a nonlinear process, has shown chiral transitions where s- and p-polarizations mix to generate so-called chiral SFG. Here we combine chiral plasmonic nanostructures with chiral SFG to explore chiral nonlinear optical responses. We use chiral gold hooks (chiral), which are compared to disks (achiral) in their ability to generate chiral SFG signals and enable chiral SFG transitions. The plasmonic characteristics of the nanostructures measured by SFG and calculated by finite-difference time-domain (FDTD) reveal multiple polarization-dependent plasmon modes. Here we show, for the first time, that chiral gold nanoparticles can generate chiral SF electronic resonances. This optical metasurface with chiral cells and its enhancement mechanism are expected to be applied in sensing and detection to chiral macromolecules or the signal processing in optical communications.