Augmenting Bragg Reflection with Polymer-sustained Conical Helix

Abstract

There has been a recent surge of interest in smart materials and devices with stimuli-responsive properties for optical modulations. Cholesteric liquid crystals (CLCs) are a unique class of light-manipulating materials, and strongly interact with light and other electromagnetic (EM) waves. Because of their intricate helical structure, new properties of CLC have emerged revealing unique optical behavior that has resulted in rewriting Braggs’ law for how light interacts with soft materials. The aim of this work is to push the limits of spectral tuning with a new method of augmenting light-cholesteric interactions using a polymer-sustained conical helix (PSCH) structure. We experimentally explore the reversibility of reflective wavelength modulation and validate the mechanism enhanced by a polymer-sustained helicoidal structure via theoretical analyses. The conical helix structure of a CLC, formed by low-field-induced oblique orientation of cholesteric helices, is comprised of a chiral dopant, a conventional nematic, and bimesogenic and trimesogenic nematics. Polymerizing a small amount of a reactive mesogen in the CLC with an applied electric field produces a templated helical polymer network that enables three switched optical states, including light-scattering and transparent states as well as color reflection in response to an applied increasing or decreasing electric field. An electro-activated PSCH optical film covers a wide color space, which is appropriate for tunable color device applications. We envisage that this PSCH material will lead to new avenues for controlling EM waves in imaging and thermal control, smart windows and electronic papers.

Ultrafast Broadband Transient Absorption and Circular Dichroism Reveal Relaxation of a Chiral Copolymer

“…Organic homo- and copolymers based on polyfluorenes and other conjugated systems have gathered widespread attention because of promising applications in organic photovoltaics, organic light-emitting diodes (OLEDs), and organic lasers. − Characterizing the excited-state dynamics of such systems is not only of general interest from the perspective of photophysics but also bears relevance to the understanding of the function of these polymers in such optoelectronic devices. − Polymers featuring large circular dichroism (CD) response are of considerable interest, as they have particular relevance for applications in polymer photonics, such as LEDs with circularly polarized emission or color-tuning devices. − Cholesteric stacking of rod-like polymer chains has been suggested as the main factor for obtaining large CD values above 1000 mdeg, and careful optimization of the annealing conditions of such films further improves the long-range order. ,,− …”

Photo-driven effects in twist-bend nematic phases: Dynamic and memory response of liquid crystalline dimers

“…It is possible, for example, to control the spectral response of mixtures containing twistbend nematogens by switching between light-scattering, colour reflection and transparent states, as a function of electrical fields. 16,17 The optical properties can be modulated by changing the helicoidal pitch formed by the directors, via fast polar switching 18 under the application of variable electrical fields. 19 Light can also be used as a tool with which to exert spatial control over molecular order in liquid crystals, for example, by utilising the trans-to-cis photoisomerization of an azobenzene unit.…”

P‐181: Enhancing the Bragg‐Reflection by Blending Ag Nanoparticles in Polymer Stabilized Cholesteric Liquid Crystals

“…Doping with nanomaterials has been shown to enhance the reflection strength and sufficiently modulate the reflection wavelength. TiO2 nanoparticles were used to enhance the refractive strength of PSCLCs and MnO2 nanoparticles were used to extend the refractive wavelength [8][9][10].…”