Polarization-Encoded Breath Biomarker Fingerprinting via the Dielectric Anisotropy of BeS Thin Films

Abstract

Beryllium sulfide (BeS) is a wide-bandgap II–VI chalcogenide whose permittivity tensor responds anisotropically to molecular adsorption, with the in-plane component εyy and the out-of-plane component εzz perturbed by different amounts depending on the adsorbed species. We model a silicon microring resonator conformally coated with a 10 nm BeS film and show computationally that this anisotropy is sufficient to distinguish five exhaled-breath volatile organic compounds (acetone, isoprene, 4-hydroxyhexenal, 2-propenal, and benzene) from a single unfunctionalized resonator. Density functional theory calculations of the BeS dielectric tensor under each adsorbate provide the optical properties of the material. Three-dimensional finite-difference time-domain simulations of the coated ring show that the transverse-electric (TE) mode couples to εyy and the transverse-magnetic (TM) mode couples to εzz, producing two independent resonance observables per simulation. The TE shift is the same for all five analytes at 0.263 nm and acts as a concentration reference; the TM shift ranges from 0.200 to 0.426 nm and is analyte specific. Benzene is further separated by a sign inversion in TM transmission amplitude that none of the other gases reproduce. TE sensitivities reach 5.1 nm/RIU. TM sensitivities reach 6.5 nm/RIU. Figures of merit reach 14.9 RIU–1, and detection limits reach 1.5 mRIU. Cross-sensitivity simulations for CO2 and H2O show that both interferents shift the TM resonance in the negative direction, opposite to every target biomarker, so a sign comparison on ΔλTM separates interferent events from biomarker events before any further processing. The results show that the directional optical properties of BeS are themselves the selectivity mechanism, removing the need for a sensor array or chemical functionalization.