Recently, tunable color filters have been desirable for various applications such as tunable colored glasses, camouflage materials, smart labels, smart windows, and dynamic displays. Herein, we present an optical rotation-based color tuning (ORCT) method, which is a cost-effective and non-destructive way to provide a wide range of color tunability using a controlled cholesteric liquid crystal (CLC) as a chiral photonic crystal and two polarizers. Moreover, the color range of the ORCT can be manipulated by engineered optical rotation of the CLC film, which is verified by numerical calculations and experiments. As a proof of concept, tunable colored glasses are demonstrated to filter out unexpected light sources and provide better visibility. Our ORCT technology can be a cornerstone in potential color variation applications.
We present electrically-driven generation and dynamics of the real-space optical vortices based on the high-dimensional gradient-thickness optical cavity. The structure consists of a metal-dielectric multi-layer that supports non-trivial topological phases, generating optical vortex lines and rings in three-dimensional generalized parameter space. We experimentally demonstrated the high-dimensional gradient-thickness optical cavity by liquid crystal-based multi-layer structure, which bijectively projects a surface slice of generalized parameter space into the real space. By electric control of the alignment of liquid crystal molecules, we successfully spanned the architecture of the high-dimensional optical singularity in real space, which actively generates and manipulates the optical vortex-antivortex pair.
The control of the molecular orientation of liquid crystals (LCs) is important in both understanding phase properties and the continuing development of new LC technologies including displays, organic transistors, and electro-optic devices. Many techniques have been developed for successfully inducing alignment of calamitic LCs, though these techniques typically do not translate to the alignment of bent-core liquid crystals (BCLCs). Some techniques have been utilized to align various phases of BCLCs, but these techniques are often unsuccessful for general alignment of multiple materials and/or multiple phases. Here, we demonstrate that glass cells treated with polydimethylsiloxane (PDMS) thin films induce high quality homeotropic alignment of multiple mesophases of four BCLCs. On cooling to the lowest temperature phase the homeotropic alignment is lost, and spherulitic growth is seen in crystal and crystal-like phases including the dark conglomerate (DC) and helical nanofilament (HNF) phases. Evidence of homeotropic alignment is observed using polarized optical microscopy. We speculate that the methyl groups on the surface of the PDMS films strongly interact with the aliphatic tails of each mesogens, resulting in homeotropic alignment.
The helical nanofilament (HNF) liquid crystal (LC) phase of bent core mesogens (also commonly termed the B4 phase) is among the most unique and exotic LC phases known. The structure involves a spontaneous hierarchical self assembly of molecules leading to a supramolecular twisted rod structure with a square cross section approximately 30 nm on a side, with the length of the filaments unconstrained.
Based upon solid state NMR data, we have proposed that the helical nanofilaments actually represents an unusual assembly of organic crystalline nanoparticles, which form an LC phase, in the case of NOBOW existing in the temperature range between about 150°C and 110°C, which becomes a glass at temperatures below 110°C. That is, the HMF phase is not a typical molecular LC, but an LC of nanocrystals.
Descriptions of the structure of these nanoparticles, their LC phases, and approaches to alignment will be given. Also, the fascinating and potentially useful behavior of the HNF phase as a host for other materials (LC and non-LC) in nanostructured composites will be presented.
The helical nanofilament (HNF) liquid crystal phase is a member of an unusual class of thermotropic phases with lamellar structures dominated by a tendency towards developing negative Gaussian curvature of the layers. Members of this family are sometimes termed “dark conglomerates,” due to their behavior in polarized light microscopy. These include a fluid phases - the high temperature dark conglomerate phase, which is a kind of sponge phase, and the low temperature dark conglomerate phase, also seemingly a sponge phase with structural details currently under investigation. The HNF phase, also a “dark conglomerate,” seems to be unique in the family, since slow conformational dynamics indicate a quasi-crystalline structure within layers, but no long range positional correlations across layers. We have been exploring possible applications of the HNF phase, which is highly porous, as a host for the formation of alignable composites for photovoltaics and other organic semiconductor applications. Recent results regarding the structure of these composites, including data suggesting a remarkably elegant nanostructure for HNF-chiral nematic composites, will be discussed.
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