Helicity Density Maximization in a Planar Array of Achiral High-Density Dielectric Nanoparticles

TL;DR

This paper explores how a periodic array of achiral dielectric nanospheres can generate nearfield helicity density close to its maximum, enabling enhanced surface-level chirality detection without interference from the array.

Contribution

It derives conditions for achieving optimal helicity density in nanosphere arrays and discusses applications in improved surface-level chirality detection methods.

Findings

Array of nanospheres reaches near-maximum helicity density.

Derived conditions relate to array effective polarizabilities and Kerker condition.

Array does not contribute to CD signal when used as a substrate.

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.