Extremely large bandwidth and ultralow-dispersion slow light in photonic crystal waveguides with magnetically controllability

A line-defect waveguide within a two-dimensional magnetic-fluid-based photonic crystal with 45^o-rotated square lattice is presented to have excellent slow light properties. The bandwidth centered at $ lambda_{0} $  = 1,550 nm of our designed W1 waveguide is around 66 nm, which is very large than that of the conventional W1 waveguide as well as the corresponding optimized structures based on photonic crystal with triangular lattice. The obtained group velocity dispersion $ beta_{2} $ within the bandwidth is ultralow and varies from ?1,191 $ a/(2pi c^{2} ) $ to 855 $ a/(2pi c^{2} ) $ (a and c are the period of the lattice and the light speed in vacuum, respectively). Simultaneously, the normalized delay-bandwidth product is relatively large and almost invariant with magnetic field strength. It is indicated that using magnetic fluid as one of the constitutive materials of the photonic crystal structures can enable the magnetically fine tunability of the slow light in online mode. The concept and results of this work may give a guideline for studying and realizing tunable slow light based on the external-stimulus-responsive materials.