Toward Optical Detection of Single Chiral Molecules Using Magneto-Optical Hyperbolic Metasurfaces

Chiral molecule detection is normally made with circular dichroism spectroscopy, which often requires high analyte concentrations or large sample volumes. This study proposes an optical metasurface architecture consisting of nanodisks made from magneto-optical hyperbolic metamaterials, which enables detection of single chiral molecules. The nanodisks are engineered from alternating layers of metallic and magneto-optical dielectric materials, forming two-dimensional gratings that eliminate the need for traditional prism coupling. When phase-matching conditions are met, incident light couples into bulk plasmon-polariton modes ─whose resonantly enhanced and localized electromagnetic fields are highly responsive to changes in the surrounding medium. Chiral sensing is enabled by applying a magnetic field in the polar configuration, which induces differences in the reflection of left- and right-handed circularly polarized light, producing a measurable magnetic circular dichroism (MCD) signal. Two complementary MCD-based sensing strategies are demonstrated: refractometric detection of achiral analytes and chiroptical sensing of chiral molecules. The refractometric approach achieves an MCD sensitivity of S = 245 nm·RIU^-1, while the chiroptical method enables detection at ultralow concentrations, with MCD peak values of |MCD_p| = 4.71°, |MCD_p| = 2.41°, and |MCD_p| = 2.36°, corresponding to concentration ratios of four, two, and one molecule per unit cell, respectively. These results highlight the concept here as a platform for label-free chiral biosensing.