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Chiral molecules show differences in their chemical and optical propertiesdue to different spatial arrangements of the atoms in the two enantiomers. Acommon way to optically differentiate them is to detect the disparity in theabsorption of light by the two enantiomers, i.e. the absorption circulardichroism (CD). However, the CD of typical molecules is very small, limitingthe sensitivity of chiroptical analysis based on CD. Cavity ring-downspectroscopy (CRDS) is a well-known ultrasensitive absorption spectroscopicmethod for low-absorbing gas-phase samples because the multiple reflections oflight in the cavity greatly increase the absorption path. By inserting a prisminto the cavity, the optical mode undergoes total internal reflection (TIR) atthe prism surface and the evanescent wave (EW) enables the absorption detectionof condensed-phase samples within a very thin layer near the prism surface,called EW-CRDS. Here, we propose an ultrasensitive chiral absorptionspectroscopy platform using a dielectric metasurface-assisted EW-CRDS. Wetheoretically show that, upon linearly polarized and oblique incidence, themetasurface exhibits minimum scattering and absorption loss, introducesnegligible polarization change, and locally converts the linearly polarizedlight into near fields with finite optical chirality, enabling CD detectionwith EW-CRDS that only works with linearly polarized light. We evaluate thering-down time in the presence of chiral molecules and determine thesensitivity of the cavity as a function of total absorption from the molecules.The findings open the avenue for an ultrasensitive thin film detection of thechiral molecules using the CRDS techniques.

Dielectric metasurface-assisted cavity ring-down spectroscopy for thin-film circular dichroism analysis