Nonlinear Born-Kuhn Analog for Chiral Plasmonics

Chiral plasmonic metasurfaces are intriguing platforms to achieve large nonlinear chiroptical effects, which could be desirable for sensitive chiral molecular analysis. The combination of a superlinear response with high plasmonic near-field enhancement leads to large nonlinear optical activity, which can be much stronger than its linear counterpart. A complex mixture of multiresonances, structural symmetry, and anisotropy in 3D chiral plasmonic nanostructures as well as experimental configurations has up to now hampered understanding of the underlying physics and quantitative modeling of the relevant nonlinear chiroptical effects. Here we study third-harmonic circular dichroism of archetypical Born-Kuhn type bilayer chiral metasurfaces made of corner-stacked orthogonal gold nanorods arranged in C-4 symmetry, which avoids most of the aforementioned issues. With a coupled anharmonic oscillator model built upon chirally coupled electric dipole moments, we are able to retrieve the nonlinear chiroptical response of such chiral nanostructures. The comparison of nonlinear spectroscopic measurements with our model as well as nonlinear simulations enables interpretation of the large nonlinear circular dichroism. Our research might pave the way toward flexible control over nonlinear susceptibility tensors of artificial chiral meta-molecules at will and could allow for highly sensitive nonlinear chiral sensing.