Evidence of Tunable Fano Resonance in a Liquid Crystal-Based Colloidal Metamaterial

A colloidal metamaterial composite, realized by dispersing sub-micrometer-sized high refractive index dielectric resonators (selenium) in a nematic liquid crystal medium, exhibits electrically tunable Mie resonances in the optical regime. Darkfield hyperspectral imaging reveals that when the nematic liquid crystal (NLC) molecules reorient from the pristine planar state on application of an AC electric field, the scattered image from the particle splits into two, owing to the birefringence of the NLC medium. At higher voltages, a doughnut-shaped scattering pattern is obtained, indicating the occurrence of Fano resonance. The analysis of the darkfield scattering spectra based on the "Multi-pole Fano interference model" confirms the presence of Fano resonance arising due to the interference between the broad electric dipolar and narrow electric quadrupolar modes. With increasing voltage, the value of Fano parameter q decreases and approaches unity corresponding to an ideal Fano shape. Thus, the inherent optical anisotropy of the NLC medium and Fano coupling between the Mie resonant modes lead to highly reversible changes in the far-field scattering pattern with potential applications in refractive index sensing, wave-guiding, etc.