Investigating the nature of chiral near-field interactions

Abstract

In recent years, there have been reports of enhanced chiroptical interactions in the near-fields of antennas, postulated to be mediated by high spatial gradients in the electromagnetic fields. Here, using gigahertz experimentation, we investigate the nature of the chiral near-field generated by an array of staggered-rod antennas through its interaction with an array of aligned, subwavelength metallic helices. This allows us to eliminate many potential origins of enhancements, such as those associated with plasmon-exciton interactions, and search solely for enhancements due to the high spatial gradients in the chirality of the fields around chiral antennas (so-called 'superchiral fields'). By comparing the strength of the chiral interaction with our helices to that of a homogeneous chiral layer with effective material parameters, we find that the strength of this chiral interaction can be predicted using a completely local effective medium approximation. This suggests no obvious enhancement in the chiral interaction in the near-field and indicates that nonlocal interactions are negligible in this system.

Toward Chiral Sensing and Spectroscopy Enabled by All‐Dielectric Integrated Photonic Waveguides

“…[17] Nevertheless, since the original suggestion [10] and preliminary demonstrations [12,16] of the existence of superchiral fields (i.e., optical fields carrying chirality larger than that of CPL) to enhance the chiroptical interactions, the interest in this field has grown considerably. In particular, metallic nanostructures supporting plasmonic resonances at subwavelength dimensions have been proven to be well-suited platforms for strengthening chiroptical effects, [20][21][22][23] leading to local chiral enhancements by orders of magnitude. [24,25] Further developments have led to improve and simplify the chiroptical schemes to achieve ultrasensitive enantiomeric detection, recognition, and separation, [26] for example, by stacking twisted planar metasurfaces, [27] or in arrays of both chiral [28] and even achiral plasmonic nanostructures, such as spheres, [29] nanoslits, [30] or nanorods.…”

Towards Chiral Sensing and Spectroscopy Enabled by All-Dielectric Integrated Photonic Waveguides

“…17 Nevertheless, since the original suggestion 10 and preliminary demonstrations 12,16 of the existence of superchiral fields (i.e., optical fields carrying chirality larger than that of CPL) to enhance the chiroptical interactions, the interest in this field has grown considerably. In particular, metallic nanostructures supporting plasmonic resonances at subwavelength dimensions have been proven to be well-suited platforms for strengthening the chiroptical effects, [19][20][21] leading to local chiral enhancements by orders of magnitude. 22,23 Further developments have led to improve and simplify the chiroptical schemes to achieve ultrasensitive enantiomeric detection, recognition, and separation, 24 for example, by stacking twisted planar metasurfaces, 25 or in arrays of both chiral 26 and even achiral plasmonic nanostructures, such as spheres, 27 nanoslits, 28 or nanorods.…”

Probing Chirality with Inelastic Electron-Light Scattering

“…Because of the diffraction limit, spatial resolution is tied to the wavelength of light. The chiral contribution to molecular absorption is many orders of magnitude smaller than the nonchiral contribution; limited sensitivity necessitates the use of large ensembles of molecules. , One potential route toward higher sensitivity is to enhance the chiral part of optical absorption with spatially structured fields. − Another option is to measure near fields. Scanning near-field optical microscopy can be employed to measure near-field circular dichroism at surfaces, but resolution and bandwidth are limited by the tip and care must be taken to preserve polarization. , Photoemission electron microscopy can measure optical near fields with high spatial and temporal resolution − and illuminate spin-dependent plasmon dynamics; − the technique is also commonly used in conjunction with X-ray magnetic circular dichroism. , Cathodoluminescence polarimetry is another promising new approach to probe optical interactions with nanometer resolution and access to polarization .…”