Perturbative and phase-transition-type modification of mode field profiles and dispersion of photonic-crystal fibers by arrays of nanosize air-hole defects.

Based on the results of a fully vectorial finite-difference analysis, we identify three important regimes of field-profile and dispersion management of photonic-crystal fibers with a solid core modified by arrays of nanosize air-hole defects. In the first regime, very small air holes act as weak perturbations, slightly modifying the field profiles of fiber modes and red-shifting the wavelength of zero group-velocity dispersion (GVD). In the second regime, larger holes reduce the effective mode area, tightening the confinement of the light field in the fiber core and blue-shifting the zero- GVD wavelength. Finally, in the third regime, the nanosize air-hole defects with diameters above a critical value induce a phase-transition-type behavior of mode field profiles, dramatically reducing the localization of the field in the fiber core and increasing the radiation power in the fiber cladding. This phase transition in mode field profiles qualitatively modifies the wavelength dependence of the effective mode area and dispersion parameters of fiber modes, especially in the long-wavelength range, suggesting an attractive strategy for fiber dispersion and mode area engineering.