Chiral sensing enhancement via subwavelength plasmonic gratings

Chirality is used to describe the asymmetry of a physical system that cannot be superimposed on its mirror images. Despite being chemically identical, chiral molecules can differ drastically in their biological activity, exhibiting therapeutic or toxic effects depending on their handedness. The primary challenge during the measurement of chiral substances is that most conventional methods are often limited by weak chiroptical signals at low concentrations of molecules.

To address this gap, we investigated a system consisting of a 50 nm gold film coated with a chiral analyte on a quartz substrate. By employing the Kretschmann prism-coupling configuration, we excited surface plasmon polaritons with a TM-polarized plane wave to analyze the resulting reflection spectra. It is observed that the presence of a chiral medium leads to a small non-zero response in orthogonal (transverse electric) polarization due to magneto-electric coupling. As a result, an angular difference emerges between the spectral resonances in terms of right- and left-handed circularly polarized waves. The magnitude of this angular shift determines the sensitivity of the sensor—a larger shift corresponds to higher sensitivity.

To overcome the film's limitations, we replaced it with a gold subwavelength grating. This approach amplified the chiral sensitivity by nearly two orders of magnitude. The reason for this enhancement is explained by the Rayleigh anomaly, which occurs when a diffracted wave propagates tangentially to the grating surface, transitioning into an evanescent mode. Consequently, the coupling between the evanescent wave strongly localized at the edges of the grating and the chiral substance leads to the significant amplification of the chiroptical response.

We demonstrated that the structural transition from thin films to subwavelength gratings significantly improves the limits of chiral detection. These findings offers a promising platform for detecting small concentrations of chiral biomolecules in real time for pharmaceutical and biomedical applications.