Reflective spectrum broadened by nano-particle-network in chiral liquid crystals

Abstract

Based on the analytical expression of the electromagnetic field solution of a helical symmetric dielectric material, the relationship between spectral reflectivity and birefringence of a chiral liquid crystal and the blue shift of the Bragg reflection in the condition of oblique incidence are presented in this paper. The theoretical results indicated that: 1) If birefringence (Dn) of the liquid crystal is greater than 0.2 and the thickness of the liquid crystal layer reaches 3mm, the liquid crystal layer will reflect more than 90% of the incident light; 2) To reflect the whole visible spectrum by Bragg reflection, Dn of the liquid crystals in plane alignment state should exceed 0.6; 3) When the incident beam inclines to 60u from the normal direction, the blue shift of the reflective spectrum will reach to 100 nm. On the other hand, since the Dn of the commercial chiral liquid crystals is not larger than 0.2, to get the entire visible reflective spectrum, it needs to introduce a polymer network into the liquid crystals and make a sagging structure on the surface of substrates. The contribution of the network is to establish random anchorage that makes the pitch varied, hence broadening the Bragg reflection spectra. The random distribution of the sagging structure on the surface substrate is used to induce random screw axes of the chiral liquid crystal, which not only causes a blue shift of Bragg reflection but also further stretches the reflection spectra. Experiments demonstrated that the Bragg reflection spectrum could be broadened from 80 nm to more than 120 nm, and the contrast reaches 4:1 by introducing both polymer network and sagging structure on the substrate surface cell.

Electro-optical properties of polymer stabilized cholesteric liquid crystal film

“…Liquid crystals (LCs) and polymers have been extensively studied over the past decades. The LCs with chiral dopant and a certain amount of polymer (< 10 wt%), i.e., polymer stabilized cholesteric LCs (Ch-LCs) have been developed for various applications such as displays, [1−3] optical switches, [4,5] light shutters, [6] polymer stabilized blue phases, [7] polymer stabilized isotropic displays, [8] broad reflection band materials [9,10] and gratings, [11,12] etc. The interests of fundamental researches and potential applications are still increasing.…”