Chiral Eigenmodes of Geometrically Chiral Structures
Strongest optical chirality requires electric and magnetic fields that are parallel but out of phase. In this chapter, we show that this requirement is best fulfilled by the fundamental mode of a helix. Because the chiral near-fields are already generated
- PDF / 1,588,288 Bytes
- 22 Pages / 439.37 x 666.142 pts Page_size
- 71 Downloads / 227 Views
Chiral Eigenmodes of Geometrically Chiral Structures
Abstract Strongest optical chirality requires electric and magnetic fields that are parallel but out of phase. In this chapter, we show that this requirement is best fulfilled by the fundamental mode of a helix. Because the chiral near-fields are already generated by this chiral eigenmode, the incident light is only necessary to excite this mode. Based on this finding, we present a structure with multiple helices that provides strong chiral near-fields in an extended volume and discuss their optimization. Additionally, a simplified design with diagonal slits on top of a mirror is analyzed. We identify the distance between the slits and the mirror as sensitive optimization parameter, which controls both the strength as well as the uniformity of the chiral near-fields. The diagonal-slit design, which is optimized for simple fabrication and usage, results in a novel chiroptical spectroscopy technique that works in reflection.
After discussing the enhancement of chiral light by geometrically chiral structures in Chap. 5, we showed in Chap. 7 that chiral near-fields can also occur in geometrically achiral systems illuminated with LPL. For the second case, interference between the incident and the scattered fields is crucial for the OC of the near-fields. In this section, we will return to the case of geometrically chiral structures and demonstrate that the electromagnetic fields of their modes exhibit nonzero OC on their own. This is what we term the chiral eigenmodes of a plasmonic structure. Compared to the examples in Chap. 7, the incident polarization is only necessary to excite the respective modes. Interference (and, therefore, the phase difference) between incident and scattered fields must not necessarily be controlled. Therefore, the resulting designs are expected to be more reliable. Additionally, we can control the background signal of such systems. If we choose the plasmonic near-field source, which generates enantiomorphic fields, such, that no birefringence occurs for the incident polarizations, the detected differential response stems purely from the chiral analyte.
© Springer International Publishing Switzerland 2017 M. Schäferling, Chiral Nanophotonics, Springer Series in Optical Sciences 205, DOI 10.1007/978-3-319-42264-0_8
115
116
8 Chiral Eigenmodes of Geometrically Chiral Structures
8.1 Chiral Eigenmodes As it can be inferred from (4.7), the electric and magnetic fields must have parallel components that are out of phase for non-zero OC. A prototypical design that fulfills these conditions is a helical structure, as shown in Fig. 8.1. Its fundamental mode has a dipolar character, which leads to an electric field vector pointing from one end to the other. A magnetic field in the same direction is generated due to the coiled wire. Furthermore, the phase condition between the electric and the magnetic field is automatically fulfilled because the magnetic field scales with the current, while the electric field is maximal when the carriers are
Data Loading...
