Formation of Chiral Fields Near Symmetric Structures
Geometrical chirality is required to obtain chiroptical far-field responses. We demonstrate in this chapter that chiral near-field responses, on the other hand, can be obtained in systems that are completely achiral. This is shown for a linear rod antenna
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Formation of Chiral Fields Near Symmetric Structures
Abstract Geometrical chirality is required to obtain chiroptical far-field responses. We demonstrate in this chapter that chiral near-field responses, on the other hand, can be obtained in systems that are completely achiral. This is shown for a linear rod antenna illuminated with linearly polarized light parallel to the antenna axis under normal incidence. We explain the origin of the chiral near-fields in such systems with a simple dipole model and compare different illumination conditions. Based on these findings, we discuss a scheme for chiroptical spectroscopy with alternating orthogonal linear polarizations. This scheme is theoretically substantiated by numerical simulations combining the plasmonic near-field source with an effective chiral medium.
We have seen in Chap. 5 that the OC of circularly polarized (and, therefore, chiral) electromagnetic fields can be locally enhanced by geometrically chiral plasmonic nanostructures. However, we could demonstrate that this is not just due to the field enhancement effect, because the polarization of the incident light (and, therefore, its OC) is not preserved in the resulting near-field of the structure. In a different interpretation, chiral near-fields are formed due to an interplay between the incident field and the near-fields of the structure. From this perspective, the analyzed combination of chiral light with chiral structures to obtain chiral nearfields is the most complex scheme that could be considered. We will discuss much simpler schemes in this chapter to obtain better insights how the formation of chiral near-fields in the presence of plasmonic nanostructures works. Chiroptical far-field responses depend on intrinsic or extrinsic geometrical chirality.1 Chiral near-fields, on the other hand, have been found in the vicinity of planar and three-dimensional geometrically achiral structures. Biagioni and collaborators utilized the phase difference between arms with different lengths (and, accordingly, different resonance frequencies) of a cross-shaped antenna, which results in circularly polarized near-fields in the center [2]. Lin and Huang demonstrated OC in a
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rotationally symmetric planar geometrically chiral systems, differential scattering and absorption has been predicted. However, both responses add up such that no differential extinction, which is the only response directly accessible via far-field measurements, occurs [1].
© Springer International Publishing Switzerland 2017 M. Schäferling, Chiral Nanophotonics, Springer Series in Optical Sciences 205, DOI 10.1007/978-3-319-42264-0_7
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7 Formation of Chiral Fields Near Symmetric Structures
slant-gap antenna, where the gap provides an electric field component parallel to the external magnetic field [3]. This work indicates that OC can occur in systems without geometrical chirality. In this section, we analyze an even simpler system that consists of only one plasmonic antenna and discuss, on a fundamental theoretical basis, how
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