Helicity maximization in a planar array of achiral high-density dielectric nanoparticles

Abstract

We investigate how a periodic array composed of achiral isotropic high-refractive index dielectric nanospheres generates nearfield over the array surface reaching helicity density very close to its upper bound. The required condition for an array of nanospheres to generate "optimally chiral" nearfield, which represents the upper bound of helicity density, is derived in terms of array effective electric and magnetic polarizabilities that almost satisfy the effective Kerker condition for arrays. The discussed concepts find applications in improving chirality detection based on circular dichroism (CD) at surface level instead of in the bulk. Importantly the array would not contribute to the generated CD signal when used as a substrate for detecting chirality of a thin layer of chiral molecules. This eliminates the need to separate the CD signal generated by the array from that of the chiral sample.

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

“…Thus, tailoring the magnitude and phase shift of the intrinsic electric and magnetic dipole moments can enhance the chiral electromagnetic fields generated by dielectric nanostructures. This can be controlled by phase-shifted electric and magnetic fields in the excitation source, as is the case for CPL (Figure 3b), or further geometric tuning, as was demonstrated in recent research for achiral silicon nanospheres [29,30], silicon disk and sphere metasurfaces [32,33,35,36], or dielectric dimer structures [31,34,37,38]. These additional degrees of freedom inherent to the mechanism of chiral light-matter interactions in dielectric nanostructures enable the generation of highly enhanced chiral electromagnetic fields in simplified geometric configurations, suitable for high-throughput applications where strong optical chirality enhancement can be rationally designed in the nanostructure near field.…”

“…The interaction between chiral electromagnetic plane waves, such as circularly polarized light (CPL), and matter is inherently limited in sensitivity due to their bounded spatial distribution. Rapidly-evolving research efforts in the field of chiral nanophotonics aim to address this challenge by the tailored design of metallic [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] and dielectric [29][30][31][32][33][34][35][36][37][38] nanostructures, arranged periodically in sub-wavelength metamaterials and metasurfaces, or in colloidal dispersions, achieving highly concentrated electromagnetic chirality in their evanescent field (see also review articles [25,[39][40][41][42]). However, the rational design of enhanced electromagnetic chirality in the presence of matter requires the definition of physical observables by which to quantify the chirality of light.…”

Orientation dependence of optical activity in light scattering by nanoparticle clusters

“…It is the case, however, for a dimer or indeed any arrangement of such scatterers, that the helicity of incident light is preserved in the scattering process 31,71 near the Kerker condition. [72][73][74] This is apparent in the map of C at l = 1175 nm, where only negative values appear (Fig.…”

Orientation dependence of optical activity in light scattering by nanoparticle clusters

“…It is the case, however, for a dimer or indeed any arrangement of such scatterers, that the helicity of incident light is preserved in the scattering process 31,68 near the Kerker condition [69][70][71] . This is apparent in the map of C at λ = 1175 nm, where only negative values appear (Fig.…”