Localization of Light in Multi‐Helical Arrays of Discrete Coupled Waveguides

Abstract

AbstractLocalization of light in disordered or periodically structured dielectric media has been studied for many years. A recent example is coreless (i.e., defect‐free) photonic crystal fiber (PCF), which is observed to confine light when drawn in chiral form from a spinning preform. Although heuristic arguments based on a radially‐dependent photonic bandgap have been advanced, the underlying mechanism is not fully resolved. Here, a novel analytical model for twisted coreless PCF is presented, based on chiral arrays of coupled single‐mode step‐index sub‐cores in which chirality causes the effective axial refractive index of the cores to rise quadratically with radius. It produces results in excellent agreement with numerical solutions of Maxwell's equations, while offering orders of magnitude reductions in computational time. As a result it is possible to treat chiral structures with very large numbers of sub‐cores, to clarify the confinement mechanism, and to establish that exponentially localized helical Bloch modes (HBMs) do indeed form. As an example, localization in hexagonal and honeycomb lattices of chiral coupled sub‐cores is studied, and it is shown that in every case whole families of guided HBMs exist. The results open a new chapter in the 2D localization of light.