Plasmonic nanostructures with spatial symmetry breaking have a variety of applications, from enhancing the enantioselective detection of chiral molecules to creating photonics devices such as circular polarizers. Compared to their molecular counterparts, engineered nanostructures exhibit orders of magnitude larger circular dichroism (CD) at optical frequencies. Although 3D nanostructures such as nanohelices have been reported with high CD at mid-IR frequencies, such high CDs have not yet been achieved at visible frequencies with decent efficiencies. Here, we propose a planar array of plasmonic ramp-shaped nanostructures with an azimuthally gradient depth that exhibits a giant CD and dissymmetry factor at visible frequencies. The structure is fabricated on a gold-coated glass slide using focused ion beam (FIB) with gradient intensity to induce the required gradient depth, hence, breaking symmetry. Optical experimental characterization in the reflection spectrum shows a CD up to 64% and a dissymmetry factor up to 1.13 at 678 nm, in a good agreement with numerical simulations. We envision our proposed structure together with the suggested fabrication method to inspire the design of novel optical devices such as nanoscale circular polarizers and a host of chiral molecules to improve enantioselectivity in the pharmaceutical industry.

We introduce the universal upper bound of helicity density, which is the maximum helicity light can have at a given light energy density. Helicity maximization as discussed here is applicable to any structured light, and it defines optimally chiral fields. We further demonstrate that using structured light with maximized helicity density for determining the chirality of a nanoparticle using the dissymmetry factor g eliminates the need for specific knowledge of the values of field energies and helicity densities. We also show that nearfields that satisfy the universal bound discussed in this paper can be generated by high-density dielectric nanoantennas illuminated by structured light that satisfies such a bound, leading also to helicity enhancement. The helicity maximization concept generalizes the use of the dissymmetry factor g for nanoparticle chirality detection to any chiral structured light illumination.

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.

Optimally-chiral electromagnetic fields with maximized helicity density, recently introduced in [1], enable chirality characterization of optically small nanoparticles. Here, we demonstrate a technique to obtain optimally-chiral nearfields that leads to the maximization of helicity density, under the constraint of constant energy density, beyond the diffraction limit. We show how optimally-chiral illumination induces balanced electric and magnetic dipole moments in an achiral dielectric nanoantenna which leads to generating optimally-chiral scattered and total nearfield. In particular, we explore helicity and energy densities in nearfield of a spherical dielectric nanoantenna illuminated by an optimally-chiral combination of azimuthally and radially polarized beams that generates parallel induced electric and magnetic dipole moments that in turn also generate optimally-chiral scattered field with the same handedness of the incident field. The application of helicity maximization to nearfields results in helicity enhancement at nanoscale which is of great advantage in the detection of nanoscale chiral samples, microscopy, and optical manipulation of chiral nanoparticles.

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