Chiral Light Design and Detection Inspired by Optical Antenna Theory

Abstract

Chiral metallic nanostructures can generate evanescent fields which are more highly twisted than circularly polarized light. However, it remains unclear how best to exploit this phenomenon, hindering the optimal utilization of chiral electromagnetic fields. Here, inspired by optical antenna theory, we address this challenge by introducing chiral antenna parameters: the chirality flux efficiency and the chiral antenna aperture. These quantities, which are based on chirality conservation, 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.

Optical Helicity and Optical Chirality in Free Space and in the Presence of Matter

“…The optical chirality flux generated by lossy, dispersive media has been experimentally observed in the far field for periodic arrays of two-dimensionally chiral metallic nanoantennas [83] and colloidal dispersions of three-dimensionally chiral metallic nanopyramids at the single-particle level [86]. These experimental results demonstrated how the optical chirality flux is a physically relevant far-field observable, with the ability to provide information on chiral light-matter interactions in the near and far field.…”

“…For applications in nanophotonics, we focus on chiral light-matter interactions in artificial nanostructures composed of linear, homogeneous, isotropic media, where material losses can play a significant role in the generation of chiral electromagnetic fields [66,67,83]. We consider time-harmonic electromagnetic fields with notation E(r, t) = Re E (r)e −iωt for the electric field, where E (r) is the complex electric field amplitude of the electric field at spatial coordinate r. For short-hand notation, we write complex field amplitudes as E (r) = E. In linear media, the complex electric permittivity = 0 ( + i ) has its imaginary part of the relative permittivity = σ(r, ω)/( 0 ω), where σ is the conductivity and J cond = σ(r, ω)E (r) is the complex amplitude of the conduction current density.…”

“…Subsequently, the rapidly-developing research area of chiral nanophotonics, summarized in a series of review articles [25,[40][41][42], devoted itself to constructing solutions to Maxwell's equations for whichχ exceeds its corresponding value for CPL, whereχ/|χ CPL | was termed optical chirality enhancement by Schäferling et al [87]. A single electromagnetic plane wave reaches its maximum value ofχ at the circular polarization state, resulting in |χ|/|χ CPL | = 1 [83]. However, the scale discrepancy between chiral molecules and the wavelength of CPL results in inherently weak selectivity [54].…”

“…In free space, theoretical studies have predicted |χ|/|χ CPL | > 1 for the diffraction-limited focusing of a circularly polarized Gaussian beam or the appropriate superposition of two Gaussian beams with radial and azimuthal polarization [90,91]. To better match molecular dimensions, evanescent waves can achieve a theoretically-unlimited spatial concentration of electromagnetic fields at material interfaces [92], enabling optical chirality enhancement beyond the diffraction limit [83]. In particular, artificial nanostructures, with dimensions comparable to the wavelength of light, show great promise for the rational design of concentrated chiral electromagnetic fields.…”

Optical Chirality of Time-Harmonic Wavefields for Classification of Scatterers

“…We wish to remark that in recent experiments, the helicity components of the scattered light were measured in transmission [73] and discussed by dual symmetry [74]. Furthermore, a chirality flux spectroscopy, measuring the third Stokes parameter, was used to analyze the chiroptical response of two-dimensional chiral structures [75]. We note that the helicity of light has been measured mostly in transmission.…”