Optimally Chiral Light: Upper Bound of Helicity Density of Structured Light for Chirality Detection of Matter at Nanoscale

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.