Exploring chiral nanogap structures using topology optimization

Nanophotonic structures are crucial for controlling light at scales smaller than its wavelength. While designing for linear polarization is straightforward, creating nanostructures for helically structured light, like circularly polarized light and optical vortices, is challenging due to complex near-field chiral interactions with matters in helical electromagnetic fields. In this presentation, we apply topology optimization, an intelligent design approach, to create 3D nanogap antenna structures with outstanding chiroptical functionalities. With these structures, we demonstrate giant chiral dissymmetry (up to g = 1.70), polarization conversion around the Poincaré sphere, and circularly polarized far-field emission from a linear dipole embedded within the gap. Additionally, our in-depth analysis reveals a physical connection between the flow of spin angular momentum of light within the nanostructure and the local density of optical chirality. The insight, combined with our developed structures, offers a fresh perspective for designing chiral nanophotonic structures for better control of chiral molecules using light.