Octupolar Plasmonic Meta-Molecules for Nonlinear Chiral Watermarking at Subwavelength Scale

Nonlinear chiroptical effects of precisely designed chiral plasmonic nanomaterials can be much stronger than such effects observed in the linear regime. We take advantage of this property to demonstrate the use of circularly polarized second-harmonic generation microscopy toward the efficient read-out of a microscopic pattern encoded by an array of triangular gold nanoprisms forming arrangements of adequate chiral symmetry. Strong chiroptical effects are observed in the backscattered second-harmonic generation intensity, enabling clear distinction of the laterally arranged enantiomers, down to nearly 1 pm resolution, with an overall intensity contrast of about 40% (second-harmonic generation circular dichroism of 20%). Numerical simulations show a noticeable change in the spatial distribution of plasmonic hot spots within the individual nanostructures under excitation by circularly polarized light of different handedness. This leads to rather weak chiroptical effect in the linear backscattering (theoretical circular dichroism not exceeding 3%), in contrast with the much more significant change of the second-harmonic generation in the far-field (second-harmonic generation circular dichroism from 16 up to 37%). These results open the possibility of designing deeply subwavelength chiral nanostructures for encoding microscopic "watermarks", which cannot be easily accessed by linear optical methods, moreover requiring a nonlinear microscopy setup for reading out the encoded pattern.