Plasmonic Chiral Switching via Sub-10-nm Gap Engineering in a Fiber-Taper-Silver-Nanowire Hybrid System

The nanoscale generation and manipulation of chiral surface plasmon polaritons (SPPs) are pivotal for advancing near-field optics and chiral biosensing, yet the microscopic coupling mechanisms governing chiral switching in hybrid plasmonic systems remain incompletely understood. Here, we report an ultra-sensitive chiral switch realized in a fiber taper-silver nanowire (Ag NW) hybrid structure, in which the handedness of guided SPPs is completely reversed by sub-10 nm variations of the coupling gap. Three-dimensional finite-difference time-domain (FDTD) simulations demonstrate a clear chirality inversion as the gap increases from 0 to 10 nm. By combining coupled mode theory with a transfer matrix approach, we elucidate the origin of the ultralow-threshold chiral switching. We find that the switching is governed by a gap-induced accumulation of the relative phase between the HE₊₁ and HE_-1 modes exceeding π/2. This effect arises from operating the mode converter in the vicinity of a phase singularity. The system exhibits an exceptional phase-sensitivity figure of merit (FOM) of ~0.12π/nm. Our findings establish nanogap engineering as a powerful and hitherto overlooked control parameter for chiral plasmonics, offering a straightforward route for ultra-sensitive probing and active manipulation of chiral optical fields at deep-subwavelength scales.