Enhanced Enantioselective Optical Trapping enabled by Longitudinal Mie Resonances in Silicon Nanodisks

TL;DR

This paper introduces a novel optical trapping method using silicon nanodisks with longitudinal Mie resonances, achieving high enantioselectivity and thermal stability for nanoscale chiral separation.

Contribution

It demonstrates that longitudinal Mie resonances in silicon nanodisks can decouple enantioselective forces from achiral background, enabling highly selective and thermally stable optical trapping.

Findings

Achieves trapping selectivity ratios above 100 for certain chiral particles.

Maintains selectivity above 2 for weakly chiral analytes.

Proposes a non-invasive platform for chiral analysis and separation.

Abstract

Optical enantioseparation of nanoscale matter is fundamentally limited by the intrinsic weakness of chiroptical forces compared to the dominant achiral gradient forces and thermal fluctuations. Conventional plasmonic approaches typically enhance chirality at the cost of amplifying achiral attraction and heating. Here, we overcome this trade-off by exploiting longitudinal Mie resonances in silicon nanodisks. By employing an Azimuthally-Radially Polarized Beam (ARPB) illumination, we selectively excite longitudinal Mie resonances, with strong optical chirality gradients and comparatively uniform electric field intensities. Specifically, magnetic quadrupole (MQ) resonances, effectively decouple enantioselective forces from the achiral background, providing uniquely favorable conditions for enantioselective optical trapping. Combining full-wave simulations with Kramers' escape-rate theory, we demonstrate a robust trapping system that is both thermally stable and highly selective. We predict trapping selectivity ratios above 100 for particles with Pasteur parameters $|\kappa| \geq$ 0.03 and maintain selectivities above 2 even for weakly chiral analytes ($|\kappa| \approx$ 0.006). These results establish longitudinal Mie resonances in high-index dielectric nanostructures as a promising, non-invasive platform for all-optical chiral analysis and enantiomer separation.