  The differential response of chiral molecules to incident left- and right-
handed circularly polarized light is used for sensing the handedness of
molecules. Currently, significant effort is directed towards enhancing weak
differential signals from the molecules, with the goal of extending the
capabilities of chiral spectrometers to lower molecular concentrations or small
analyte volumes. Previously, optical cavities for enhancing vibrational
circular dichroism have been introduced. Their enhancements are mediated by
helicity-preserving cavity modes which maintain the handedness of light due to
their degenerate TE and TM components. In this article, we simplify the design
of the cavity, and numerically compare it with the previous one using an
improved model for the response of chiral molecules. We use parameters of
molecular resonances to show that the cavities are capable of bringing the
vibrational circular dichroism signal over the detection threshold of typical
spectrometers for concentrations that are one to three orders of magnitude
smaller than those needed without the cavities, for a fixed analyte volume.
Frequency resolutions of current spectrometers result in enhancements of more
than one order (two orders) of magnitude for the new (previous) design. With
improved frequency resolution, the new design achieves enhancements of three
orders of magnitude. We show that the TE/TM degeneracy in perfectly helicity
preserving modes is lifted by factors that are inherent to the cavities. More
surprisingly, this degeneracy is also lifted by the molecules themselves due to
their lack of electromagnetic duality symmetry, that is, due to the partial
change of helicity during the light-molecule interactions.
