All-dielectric optical metasurfaces as platforms for sensing different analytes with identical real parts of refractive index

Metasurfaces are currently among the most attractive research fields, in no small measure due to their usability as refractometric sensors, offering unparalleled sensitivity up to sensing single atoms or molecules. This is rooted in their ability to localize electromagnetic fields in volumes orders of magnitude below the diffraction limit. Numerous materials, both conductive (metals, transparent conductive oxides, graphene, MXenes…) and all-dielectric (oxides, low-loss semiconductors) can be used to build them, thus tailoring metasurfaces, imparting multifunctionalities and enhancing design freedom. However, the fundamental sensing mechanism is practically identical across refractometric platforms – spectral dispersion changes of scattering parameters due to a difference between the real parts of refractive index of the analyte and the environment. Here we describe the use of exceptional capabilities of metasurfaces in transforming optical space to recognize analytes with identical real parts of refractive index but different imaginary parts. We consider a metasurface biosensor with cruciform openings array in an ultrathin silicon layer on silica. We simulate variations of the imaginary part of the effective refractive index in the openings while keeping the real part constant. The circular power flow that increases the optical path, field localization and intrinsically low losses in the optical range jointly cause that adding even the minuscule amounts of analyte with slightly increased optical absorption significantly reduces transmission through the structure, despite its exceptionally low thickness. The described principle can be used to distinguish bio-analytes with identical real part of refractive index like RNA, DNA, proteins, lipids, but also non-biological analytes.