Tunable near-zero index of self-assembled photonic crystal using magnetic fluid

Abstract

In a zero index material, the phase velocity of light is much greater than the speed of light in vacuum and can even approach to infinity. Thus, the phase of light throughout a piece of zero-index material is essentially a constant. The zero index material has recently been used in many areas due to its extraordinary optical properties, including beam collimation, cloaking and phase matching in nonlinear optics. However, most of zero index materials usually have narrow operating bandwidths and the operating frequencies are not tunable. In this work, the model of tunable near-zero index photonic crystal is established by using colloidal magnetic fluid. Magnetic fluid, as a kind of easy-made mature nanoscale magnetic material, has proved to be an excellent candidate for fabricating self-assembled photonic crystal, especially the band-tunable photonic crystal with fast and reversible response to external magnetic field. The band structure can be calculated using the plane wave expansion method. For TE mode, it can be seen that a triply-degenerate point (normalized frequency f=0.734) at point under external magnetic field H=147 Oe, forms a Dirac-like point in the band structure, which is called an accidental-degeneracy-induced Dirac-like point. The effective permittivity eff and permeability eff are calculated using an expanded effective medium theory based on the Mie scattering theory. The calculated results show that both eff and eff are equal to zero at Dirac-like point, which means that the effective index neff is zero and the effective impedance Zeff is 1. The lattice structure of such a self-assembled photonic crystal will change with the external magnetic field, leading to the disappearance of Dirac-like point. However, when 143.6 OeH 152.4 Oe (1 Oe=79.5775 A/m), |neff | can keep less than 0.05 under the condition of Zeff = 1. Correspondingly, the operating frequency will change from 0.75 to 0.716. The model is verified by the numerical simulations (COMSOL Multiphysics) and the theoretical results agree well with the numerical ones.