On enhanced sensing of chiral molecules in optical cavities

TL;DR

This paper presents a simplified optical cavity design that significantly enhances the detection of chiral molecules by amplifying vibrational circular dichroism signals, enabling lower concentration sensing with improved spectrometer resolution.

Contribution

The authors introduce a simplified cavity design and compare it with previous models, demonstrating substantial improvements in chiral molecule detection sensitivity.

Findings

Cavities can detect chiral molecules at 10-1000 times lower concentrations.

Enhancements of over two orders of magnitude are achieved with current spectrometer resolutions.

The new design's enhancement increases to three orders of magnitude with better frequency resolution.

Abstract

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.