Abstract
Based on the facts that chiral molecules response differently to left- and right-handed circular polarized light, chiroptical effects are widely employed for determining structure chirality, detecting enantiomeric excess, or controlling chemical reactions of molecules. Compared to those in natural materials, chiroptical behaviors can be significantly amplified in chiral plasmonic metamaterials due to the concentrated local fields in the structure. The on-going research towards giant chiroptical effects in metamaterial generally focus on optimizing the field-enhancement effects. However, the observed chiroptical effects in metamaterials rely on more complicated factors and various possibilities towards giant chiroptical effects remains unexplored. Here we study the optical-active second harmonic generation (SHG) behaviors in a pair of planar sawtooth gratings with mirror-imaged patterns. Significant multipolar effects were observed in the polarization-dependent SHG curves. We show that the chirality of the nanostructure not only give rise to nonzero chiral susceptibility tensor components within the electric-dipole approximation, but also lead to different levels of multipolar interactions for the two orthogonal circular polarizations that further enhance the nonlinear optical activity of the material. Our results thus indicate novel ways to optimize nonlinear plasmonic structures and achieve giant chiroptical response via multipolar interactions.
Highlights
Though optical-activity effects tend to be rather weak in all natural materials, it has been demonstrated that giant chiroptical effects can be achieved in chiral plasmonic metamaterials[1]
Centrosymmetry is broken by the presence of chirality, anisotropic second harmonic generation (SHG) and SHG circular dichroism (SHG-CD) can serve as highly sensitive probes for exploring chiral structures
Compared to the linear chiroptical effects, the strong CD-SHG behaviors in metamaterials were usually attributed to the enormous electromagnetic field enhancement in the structure, confirmed by the fact that the SHG microscopy pattern matches very well with the distribution of hotspots at the fundamental frequency[23]
Summary
Though optical-activity effects tend to be rather weak in all natural materials, it has been demonstrated that giant chiroptical effects can be achieved in chiral plasmonic metamaterials[1]. In addition to the linear chiroptical effects, nonlinear optical processes are significantly enhanced in chiral metamaterials as compared to conventional nonlinearity in natural materials. Compared to the linear chiroptical effects, the strong CD-SHG behaviors in metamaterials were usually attributed to the enormous electromagnetic field enhancement in the structure, confirmed by the fact that the SHG microscopy pattern matches very well with the distribution of hotspots at the fundamental frequency[23]. More complicated factors play a role in the SHG activity observed in the far-field, including the chiral symmetry-breaking of the structure, polarization properties of the local field, and the out-coupling efficiency at the second harmonics. The chirality of the nanostructure give rise to nonzero chiral-symmetry-allowed nonlinear susceptibility tensor components, and leads to different levels of effective multipolar interactions that contribute to the CD-SHG effects. It allows for the possibility to further enhance the nonlinear chiroptical response of nanostructures by optimizing their chiral symmetry
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