Absorptive chiral metasurfaces designed by topological optimization (Conference Presentation)

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The strong optical chirality arising from certain synthetic metamaterials has important and widespread applications in polarization optics, stereochemistry and spintronics. Recently we have shown that strong intrinsic optical chirality can arise in planar high-index dielectric nanostructures whose thickness is greater than an optical wavelength, due to the excitation of magnetic dipoles that lie in the same plane as, but are orthogonal to, their electric counterparts. However these structures were comprised of a lossless dielectric, and incident light of the undesired helicity was diffracted instead of transmitted in the zeroth order. Here we explore the possibility of designing absorptive, subwavelength chiral metasurfaces with a desired transmission and absorption spectrum. We find that while the usual design process using discrete, well-known geometries can lead to structures with efficient contrast in transmission, it is extremely challenging to simultaneously engineer the reflection or absorption spectrum. We use topological optimization techniques to realize chiral metasurfaces comprised of freeform geometries, and show that they can exhibit a large intensity contrast in both transmission and absorption, depending on the helicity of incident circularly polarized light. These metasurfaces could be useful particularly for display technologies, and potentially overcome the inherent 50% transmission limit set by a regular circular polarization analyzer comprised of an absorptive linear polarizer and half-waveplate.