Abstract
Chiral plasmonic nanostructures, those lacking mirror symmetry, can be designed to manipulate the polarization of incident light resulting in chiroptical (chiral optical) effects such as circular dichroism (CD) and optical rotation (OR). Due to high symmetry sensitivity, corresponding effects in second-harmonic generation (SHG-CD and SHG-OR) are typically much stronger in comparison. These nonlinear effects have long been used for chiral molecular analysis and characterization; however both linear and nonlinear optical rotation can occur even in achiral structures, if the structure is birefringent due to anisotropy. Crucially, chiroptical effects resulting from anisotropy typically exhibit a strong dependence on structural orientation. Here we report a large second-harmonic generation optical rotation of ±45°, due to intrinsic chirality in a highly anisotropic helical metamaterial. The SHG intensity is found to strongly relate to the structural anisotropy; however, the angle of SHG-OR is invariant under sample rotation. We show that by tuning the geometry of anisotropic nanostructures, the interaction between anisotropy, chirality, and experimental geometry can allow even greater control over the chiroptical properties of plasmonic metamaterials.
Highlights
Modern nanofabrication techniques have allowed the development of optical “metamaterials”, whose properties are determined by the choice of materials and by their geometry
Chiral plasmonic nanostructures, those lacking mirror symmetry, can be designed to manipulate the polarization of incident light resulting in chiroptical effects such as circular dichroism (CD) and optical rotation (OR)
We demonstrate clear second-harmonic generation (SHG)-OR in plasmonic metamaterials, which is due to intrinsic chirality; the SHG-OR angle does not depend on the sample rotation angle, and it reverses upon mirroring the geometry
Summary
Modern nanofabrication techniques have allowed the development of optical “metamaterials”, whose properties are determined by the choice of materials and by their geometry. The strong dependence on geometry enables the design of metamaterials exhibiting tailored optical properties.[1−7] Optical metamaterials, consisting of subwavelength metallic nanostructures, can greatly benefit from surface plasmon resonances The latter result from collective excitations of free electrons, at the frequency of incident light. Since SHG is a three-wave mixing process, chiroptical effects can arise from the 3D chiral arrangement of electric dipoles only.[30] SHG chiroptical effects can originate from the interaction between electric and magnetic dipoles, as well as between electric dipoles and quadrupoles This specificity allows SHG to discriminate between the two principal models of chirality:[31] Kuhn’s “chirally coupled dipoles”[32] and Kauzmann’s “one electron on a helix”[33] models. An unambiguously chiral origin of SHG-OR has not been reported in nano/metamaterials
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