Optical parameter determination in chiral media: Refractive index and Pasteur parameter insights

The detection of molecular chirality is vital in pharmaceuticals and toxicology, as the therapeutic efficacy is frequently attributed to a single enantiomer. Recent advancements in nanophotonics have sparked interest in designing micro-nano structures to enhance chirality detection. However, current designs often rely on estimated optical parameters for chiral analytes, particularly the Pasteur parameters κ∼ = κ′ - iκ″, which complicates the validation of their effectiveness in chirality determination. This study presents an approach to accurately determine the optical parameters of chiral media by integrating ellipsometry with circular dichroism (CD) spectroscopy. We leverage the relationship between CD and κ'' to determine the optical characteristics, while noting that κ∼ has minimal impact on the determination of the refractive index through ellipsometry measurements. Using cysteine films deposited by thermal evaporation as an example, we demonstrate that the effect of chirality can be neglected for refractive index determination via ellipsometry when |κ∼| < 10⁻³. The measured Pasteur parameters for cysteine are κ′ ∼ 3 × 10⁻⁶ and κ″ ∼ 4 × 10⁻⁷ in the range of 400–800 nm. We anticipate that the accurate optical parameters for chiral analytes will enhance the design and optimization of chirality detection methods, which is of great significance for enhancing the detection sensitivity of weak CD signals.