Broadband and omnidirectional antireflective coatings with gradient refractive index prepared using chitin nanofibers...

Gradient refractive index coatings have attracted significant attention in the field of optics due to their antireflective performance over a broad range of wavelength and incident angle. Herein, single layer coatings with various refractive indices were prepared using suspensions of zwitterionically charged chitin nanofibers (ZC-ChNFs) containing different amounts of sodium dihydrogen citrate (SDCA) via layer-by-layer self-assembly. Subsequently, the coatings with different gradient refractive indices were fabricated by stacking the single layers together. Effects of the number of the stacked layers and the changes of gradient refractive index on antireflective performance of the coatings were investigated. The results showed that the gradient refractive index coating with five layers and a quadratic change in refractive index with downward opening yielded gains of 6.6 % at 550 nm and 7.6 % at 650 nm in the transmittance (vertical incidence) of a glass slide, and the transmittance gain of a glass slide coated with this coating reached 8 % at light incident angle of 45°. This work not only demonstrates that SDCA can be used as pore generation agent to reduce refractive index of ChNF coatings, but also offers a feasible pathway to construct gradient refractive index coatings possessing excellent broadband and omnidirectional antireflective performance using ChNFs.