Cholesteric Liquid Crystals with a Broad Light Reflection Band

Abstract

The cholesteric-liquid-crystalline structure, which concerns the organization of chromatin, collagen, chitin, or cellulose, is omnipresent in living matter. In technology, it is found in temperature and pressure sensors, supertwisted nematic liquid crystal displays, optical filters, reflective devices, or cosmetics. A cholesteric liquid crystal reflects light because of its helical structure. The reflection is selective - the bandwidth is limited to a few tens of nanometers and the reflectance is equal to at most 50% for unpolarized incident light, which is a consequence of the polarization-selectivity rule. These limits must be exceeded for innovative applications like polarizer-free reflective displays, broadband polarizers, optical data storage media, polarization-independent devices, stealth technologies, or smart switchable reflective windows to control solar light and heat. Novel cholesteric-liquid-crystalline architectures with the related fabrication procedures must therefore be developed. This article reviews solutions found in living matter and laboratories to broaden the bandwidth around a central reflection wavelength, do without the polarization-selectivity rule and go beyond the reflectance limit.

Evidence for a dispersion relation of optical modes in the cuticle of the scarab beetle Cotinis mutabilis

“…The broadening of the selective reflection band of ChNLC has been achieved by using structures of multiple pitches (discrete or graded) inspired by ideas from nature [9]. Indeed, structures with a jump in the pitch have been imaged for beetle cuticles [4,5].…”

“…Other characteristics of the reflection band largely depend on the refractive indices. For simplicity, we restrict the discussion to uniaxial systems and for a given  the anisotropy parameter n=n1-n2 determines the spectral width of the reflection band [9,13]. Solutions of type C for the wavector appear at large angles of incidence when locally biaxial slabs are considered as those deduced from regression analysis of Mueller-matrix data of Cetonia aurata [19].…”

“…Thus, the comparison of experimental and simulated data at near-normal incidence using reflectance [4,5], ellipsometric [6], and Mueller-matrix spectra [7,8] have provided a qualitative description of beetle cuticles in terms of their thin-film analogue systems. In fact, a recent review shows how structures in living matter have inspired the improvement of optical performance of liquid crystals for the normal incidence case [9].…”

Liquid Crystal Polymer Networks: Preparation, Properties, and Applications of Films with Patterned Molecular Alignment

“…In that case, special procedures are necessary. 16,17 The ability of the chiral molecules to induce a molecular helix is quantified by the so-called helical twisting power (HTP), which is defined as HTP = (pc) −1 where c is the concentration of chiral dopant and assuming that the dopant consists of one enantiomer. Some HTP values can be found in Figure 4.…”

Bandwidth-variable tunable optical filter unit for illumination and spectral imaging systems using thin-film optical band-pass filters

“…7,8 Cholesteric liquid crystals (CLCs) exhibit reflection properties that depend on the angle of incidence, polarization, electrical or magnetic field, temperature, radiation, or mechanical pressure. [13][14][15][16][17][18] CLCs were used to demonstrate a) Author to whom correspondence should be addressed. Electronic mail: georg.hennig@med.lmu.de.…”