  Chiral molecules show differences in their chemical and optical properties
due to different spatial arrangements of the atoms in the two enantiomers. A
common way to optically differentiate them is to detect the disparity in the
absorption of light by the two enantiomers, i.e. the absorption circular
dichroism (CD). However, the CD of typical molecules is very small, limiting
the sensitivity of chiroptical analysis based on CD. Cavity ring-down
spectroscopy (CRDS) is a well-known ultrasensitive absorption spectroscopic
method for low-absorbing gas-phase samples because the multiple reflections of
light in the cavity greatly increase the absorption path. By inserting a prism
into the cavity, the optical mode undergoes total internal reflection (TIR) at
the prism surface and the evanescent wave (EW) enables the absorption detection
of condensed-phase samples within a very thin layer near the prism surface,
called EW-CRDS. Here, we propose an ultrasensitive chiral absorption
spectroscopy platform using a dielectric metasurface-assisted EW-CRDS. We
theoretically show that, upon linearly polarized and oblique incidence, the
metasurface exhibits minimum scattering and absorption loss, introduces
negligible polarization change, and locally converts the linearly polarized
light into near fields with finite optical chirality, enabling CD detection
with EW-CRDS that only works with linearly polarized light. We evaluate the
ring-down time in the presence of chiral molecules and determine the
sensitivity of the cavity as a function of total absorption from the molecules.
The findings open the avenue for an ultrasensitive thin film detection of the
chiral molecules using the CRDS techniques.
