Chiral Light-Matter Interactions in the Near and Far Field

Abstract

Chiral matter is inherently handed in its structural arrangement. Its distinct interaction with chiral light holds promise for a variety of applications in all-optical selection and separation processes. The scientific area of nanophotonics has provided tools to uniquely and controllably enhance chiral light–matter interactions at the nanoscale. However, it remains elusive how best to exploit this phenomenon. This thesis addresses this challenge by viewing chiral nanophotonics through the lens of optical antenna theory. With this perspective, we introduce a theoretical and experimental framework to quantify nanoscale chiral light–matter interactions in the near and far field, thus enabling the rational design of chiral light. First, with the objective to optimize the interaction between chiral matter and highly twisted light, we introduce a time-averaged conservation law of optical chirality in lossy, dispersive media. Analogous to Poynting’s theorem, this conservation law represents a balance between the physical mechanisms of optical chirality dissipation and optical chirality flux. We then identify the optical chirality flux as an ideal far-field observable for characterizing chiral near fields and verify this concept analytically and numerically. Bounded by the conservation law, we show that it provides precise information, unavailable from circular dichroism spectroscopy, on the magnitude and handedness of highly twisted fields near nanostructures. With this knowledge, we further investigate how to best utilize and rationally design the highly twisted electromagnetic fields near nanostructures. Drawing inspiration from optical antenna theory, we introduce chiral antenna parameters: the chirality flux efficiency and the chiral antenna aperture. Based on chirality conservation, these parameters quantify the generation and dissipation of chiral light. We then present a label-free experimental technique, chirality flux spectroscopy, which measures the chirality flux efficiency, providing valuable information on chiral near fields in the far field. This principle is verified theoretically and experimentally with two-dimensionally chiral coupled nanorod antennas, for which we show that chiral near and far fields are linearly dependent on the magnetoelectric polarizability. This elementary system confirms our concept to quantify chiral electromagnetic fields and paves the way toward broadly tunable chiral optical applications including ultrasensitive detection of molecular chirality or optical information storage and transfer. Subsequently, we apply chirality flux spectroscopy to develop a molecular sensing scheme for antenna-enhanced enantiomeric recognition. From chirality conservation, we introduce the chiral antenna impedance, a quantity directly proportional to the optical chirality density, thus mirroring the relationship between the optical antenna impedance and the local density of electromagnetic states. Similarly, the chiral antenna impedance demonstrates how the optical chirality dissipation of a molecule depends on the chirality of its electromagnetic environment. In an experimental feasibility study, we demonstrate the introduced antenna-enhanced enantiomeric sensing concept with two-dimensionally chiral coupled nanorod polymers, indicating the potential of this technique for enantioselective enhancement by orders of magnitude. Thus, by directly exploiting the antenna-mediated near fields with a far-field technique, the framework presented in this thesis holds promise to realize optimized chiral optical applications, where highly twisted evanescent fields enhance the transmission of chiral information from a chiral receiver to the free radiation field. Finally, the introduced concepts are further investigated numerically and experimentally for three-dimensionally chiral metallic nanopyramids. Then, on the single-particle level, a colloidal chiral nanopyramid is arrested in a standing-wave optical trap and enantiomeric recognition is performed by polarimetric analysis. In addition, the studied chiral pyramid structure was altered on the micrometer scale and assessed for highly reproducible, polarization-tunable plasmonic tip-focusing. Upon linearly polarized excitation, the chiral metallic micropyramid was applied for tip-enhanced Raman scattering of single carbyne chains with sub-20-nm resolution.