Transfer-Matrix Framework for Modeling Mid-Infrared Vibrational Circular Dichroism Spectra

We present a rigorous electromagnetics-based theoretical model for mid-infrared (mid-IR) absorption and vibrational circular dichroism (VCD) spectra of homogeneous samples. The formalism based on 4 × 4 transfer-matrix methodology enables quantitative interpretation of the VCD signal for samples of varying thicknesses and under different incidence angles. Using an idealized material with distinct absorption bands, we explicitly show how the interference effects and coupling between left- and right-circularly polarized (LCP and RCP) waves contribute to the measured VCD signal. Importantly, contrary to conventional wisdom in mid-IR VCD, we show that tightly focused illumination can suppress the coupling between the LCP and RCP, which leads to a cleaner and more interpretable VCD signal. Our findings provide theoretical guidance for experimentalists to identify regimes where reliable mid-IR VCD measurements can be obtained and to recognize conditions dominated by optical artifacts from interference and coupling between LCP and RCP waves.