Explain the mismatch between near-unity circular polarization and only ~50% coupling directionality in the nanofiber

Determine the exact physical mechanism responsible for the observation that near-perfect circular polarization of light emitted from a gold nanorod coupled to an optical nanofiber (degree of circular polarization exceeding 0.9 at a wavelength near 750 nm) corresponds to only approximately 50% directionality in coupling to the nanofiber’s guided modes; in particular, ascertain whether and how the presence of the gold nanorod modifies the polarization state of the nanofiber’s evanescent fundamental mode to produce this discrepancy.

Background

The paper demonstrates that an achiral gold nanorod can emit chiral light when a point emitter excites surface plasmon modes and the system operates in the Purcell regime. In simulations, increasing the emission wavelength to around 750 nm yields a degree of circular polarization (DCP) of the emitted light exceeding 0.9. However, the observed directionality of coupling into the optical nanofiber’s guided modes reaches only about 50%, indicating a nontrivial relationship between emitted light’s polarization and nanofiber coupling.

The authors suggest that the mismatch between high DCP and moderate directionality likely arises because the gold nanorod alters the polarization of the nanofiber’s evanescent field near the coupling region. A precise understanding of how the nanorod perturbs the nanofiber mode’s polarization would clarify the mapping between circular polarization of emission and directionality D, which is used as a proxy for chirality in their measurements.