Chiral Microneedle Arrays With Terahertz Chiroptical Activity With Chiral-Plasmon-Chiral-Phonon Resonance

Microneedle arrays (MNAs) is a rapidly emerging technology with broad biomedical applications in drug delivery and biosensing. With sub-millimeter dimensions and periodicity, MNAs possess geometries nearly ideal for biomedical devices operating within the terahertz (THz) spectral window. Because chirality is crucial to the function of deposited drugs and surrounding tissues, realizing chiroptical resonances within MNAs could impart new capabilities to microneedle-based technologies. However, methodologies for fabricating chiral MNAs are largely unknown and their importance remains largerly unrecognized. Here, we present a pathway to arrays of chiral microneedles (ARCHIMs) that exhibit strong and predictable chiroptical resonances in the THz range. These chiroplasmonic microneedles were prepared by glancing angle deposition of two sequential gold layers. ARCHIMs with thin, non-centrosymmetric caps on each needle exhibit strong chiral plasmonic modes characterized by distinct THz circular dichroism (TCD) bands and polarization rotations as large as 5 degrees. To emulate chiral drugs and biologics, we coated the ARCHIMs with L- and D-cystine crystals. We found that chiral phonons in the biocrystals resonate with chiral plasmons in the microneedles; their coupling induces handedness-dependent shifts in the TCD spectra. This photonic effect was quantitatively described using a modified temporal coupled mode theory that incorporates polarization-dependent resonator parameters. Our findings demonstrate that ARCHIMs provide an effective, tunable, and scalable platform for exploiting chiral light-matter interactions, opening new opportunities in TCD sensing, chiral diagnostics, chiral phonon detection and THz photonics.