Electron Beam Induced Circularly Polarized Light Emission of Chiral Gold Nanohelices

Abstract

Chiral plasmonic nanostructures possess a chiroptical response orders of magnitude stronger than that of natural biomolecular systems, making them highly promising for a wide range of biochemical, medical, and physical applications. Despite extensive efforts to artificially create and tune the chiroptical properties of chiral nanostructures through compositional and geometrical modifications, a fundamental understanding of their underlying mechanisms remains limited. In this study, we present a comprehensive investigation of individual gold nanohelices by using advanced analytical electron microscopy techniques. Our results, as determined by angle-resolved cathodoluminescence polarimetry measurements, reveal a strong correlation between the circular polarization state of the emitted far-field radiation and the handedness of the chiral nanostructure in terms of both its dominant circularity and directional intensity distribution. Further analyses, including electron energy-loss measurements and numerical simulations, demonstrate that this correlation is driven by longitudinal plasmonic modes that oscillate along the helical windings, much like straight nanorods of equal strength and length. However, due to the three-dimensional shape of the structures, these longitudinal modes induce dipolar transverse modes with charge oscillations along the short axis of the helices for certain resonance energies. Their radiative decay leads to observed emission in the visible range. Our findings provide insight into the radiative properties and underlying mechanisms of chiral plasmonic nanostructures and enable their future development and application in a wide range of fields, such as nano-optics, metamaterials, molecular physics, biochemistry, and, most promising, chiral sensing via plasmonically enhanced chiral optical spectroscopy techniques.

Separating the Material and Geometry Contribution to the Circular Dichroism of Chiral Objects Made from Chiral Media

“…An example of such an object is shown in Figure . The object in the shape of a helix is geometrically chiral, which can be characterized by a handedness that describes its twist. − Additionally, the object is made from a material characterized by chiral constitutive relations. In this specific case, the material is supposed to be organic and made from chiral molecules. , For such a system, the question arises as to whether the CD can be dissected regarding the two origins.…”

Dielectric Sphere Oligomers as Optical Nanoantenna for Circularly Polarized Light

“…Encoding information in the polarization of light is swiftly becoming an alternative enabling technology for the transfer of optical information in classical and quantum communication schemes. , In addition, circularly polarized light (CPL) could be useful for reading out material information as well as for robust signal transferring compared to linear polarization. − For such applications, switchable CPL light sources are required, where an emitting material or an emitter-coupled antenna should respond equally to light with right-handed and left-handed circular polarizations (RCP and LCP). Since chiral structures respond differently to CPL of different parity, the use of symmetric, achiral structures such as nanoantennas and waveguides has been extensively explored. − Even the ultimately symmetric structure, a sphere, can work as a CPL nanoantenna whose CPL parity is controllable by the use of an electron beam (e-beam) through the extrinsic chirality (i.e., breaking the symmetry of the object by the detection and excitation geometry − ). Such sphere antennas have omnidirectional responses and, for example, can be placed in liquids, where the orientation of the antennas is random .…”

Theoretical Study of Strong Coupling between Molecular Shells and Chiral Plasmons of Gold Nanoparticles Helices

“…The interaction of these structures with polarized light can produce strong asymmetric local field originating from the so-called surface plasmon response, , which leads to high circular dichroism (CD) value, high asymmetry or g -factor . Asymmetric metal nanostructures manufactured by lithography can be periodically arranged to form meta-surfaces, − and electron beam deposition can produce helical structures like DNA . The plasmon modes can be manipulated by changing the pitch and diameter of the helix structure .…”