Surface plasmon resonance effect in helical core fibers

Using a rigorous modeling method based on transformation optics formalism, we have studied, for the first time to our knowledge, the surface plasmon resonance (SPR) effect in helical core fibers with a cladding covered by a gold layer. The obtained results prove that by twisting the fiber one can tune several parameters of the SPR resonance which may be of importance in sensing applications. In particular, we have shown that circularly polarized fundamental modes propagating in the helical core fiber exhibit almost the same SPR loss. Moreover, the SPR loss can be amplified with a twist rate by more than two orders of magnitude due to twist-induced displacement of the core modes towards a metal layer. The fiber twist modifies the coupling conditions between the fundamental modes and plasmons, which results in the redshift and split of the resonance wavelengths for circularly polarized modes of opposite handedness. Analytical formulas were derived for the SPR peak loss, redshift and split, which are valid for small twist rates, in which the fundamental modes couple only with plasmons. For higher twist rates we observed the coupling between fundamental and cladding modes, which results in significant broadening of the SPR resonance peaks and emergence of additional maxima in the SPR loss curves.