Cholesteric liquid crystals (CLCs) have a self-assembled helicoidal structure that results in vibrant reflective colors. These chiral nematic materials can be stabilized by in-situ photopolymerization of reactive liquid crystal monomers present in the CLC mixture to form so-called polymer stabilized cholesteric liquid crystals (PS-CLC’s). These PSCLCs exhibit several novel electric field driven optical responses, including color changing and spectral bandwidth changes. This changes are associated with electrical deformations of the polymer network and the surrounding liquid crystals response to the network deformations. Here we present our recent results in this area, including work to elucidate the mechanistic details of the electrically driven response and our application of this mechanism to other liquid crystals.
Nanoparticles of various shapes and sizes can affect the optical properties and stabilization of blue phase liquid crystals (BPLCs) due to the various compatibility with the LC host. Nanoparticles (NPs) that are highly compatible with the LC host can be dispersed in BPLCs of both the double twist cylinder (DTC) and disclination defects. We present a systematic study of the phase behavior of BPLC using CdSe nanoparticles with three different shapes (spheres, tetrapods and nanoplatelets) with identical cores and nearly identical long-chain hydrocarbon ligand materials. The spherical NPs are more compatible with the LC host than the tetrapod shape and platelet shape NPs, showing a wider temperature range of BP and a redshift of the reflection band of BP. While the inclusion of spherical NPs significantly tunes the optical properties of BPLCs, BPLCs with nanoplatelets have limited impact on the optical properties and stability of BPs due to poor compatibility with LC hosts. We report that the optical behavior of BPLC can be tuned by the type and concentration of NPs.
Photoalignment of azobenzenes is one of the most versatile tools for liquid crystal self-assembly for both the surface and bulk. An azobenzene component is homogenously mixed with a liquid crystal and irradiated with polarized visible light. By utilizing a simple two-step exposure method, we can induce photoalignment on the front and back of the liquid crystal sample independently. This back to front alignment, if varying in LC director orientation, translates into a twist through the bulk. In this talk, we discuss the two-step exposure method and how to control the twist as well as introduce characterization methods on twist quantification.
Blue phase liquid crystals (BPLCs) composed of double twisted cholesteric helixes are promising materials for use in next generation displays, optical components, and photonics applications. However, BPLCs are only observed in a narrow temperature range of 0.5-3 oC and must be stabilized with a polymer network. Here we report on controlling the phase behavior of BPLCs by varying the concentration of an amorphous crosslinker (PETA). LC mixtures without PETA display narrow temperature phase transitions from isotropic to BP-II, BP-I, and cholesteric phases, but the addition of PETA widens temperature window up to 10C. Above 3wt% PETA prevents the formation of BP-II and the mixture instead transitions from isotropic directly to BP-I phase. Blue phase I or II are stabilized using polymer networks via in-situ photopolymerization. Bandwidth broadening and red tuning responses are observed in polymer stabilized BPLCs with 10wt% and 15wt% polymer concentrations when DC voltage is applied.
Thin film holographic liquid crystal gratings are of great interest due to their controllable photonic properties. Holographic polymer dispersed liquid crystal (H-PDLC) with a polymer concentration of 40-90% is an example of a system where nonuniform irradiation is used to template alternating polymer-rich and polymer-poor regions with a periodicity related to the illumination wavelength. Here, we present a tunable holographic polymer stabilized liquid crystal (H-PSLC) reflection grating using a relatively small amount of polymer (6-20 wt%). Switching behavior from transparent to reflective state will be discussed by heating or applying an AC field. Heating above the isotropic temperature of the polymer-poor regions leads to the refractive index mismatch between the ordered LC polymer-rich regions and the disordered isotropic polymer-poor. Alternatively, the application of an AC field can be used as an Ohmic heat source to induce the thermal color change.
We aim to realize a novel nanotechnology-based biosensor specifically utilized to detect harmful bacteria in potable water. The nano-inspired device makes use of a chemically functionalized gold nanorods array (for the selective selection of specific pathogens) layered with a photo-responsive nematic liquid crystal (NLC) film for real-time and high sensitivity detection. The first experimental results are presented and discussed.
Cholesteric liquid crystal (CLC) is a one-dimensional photonic crystal and is promising for various applications, including smart windows, optical components, and displays. Previous work has reported that polymer stabilized cholesteric liquid crystals (PSCLCs) have shown dynamic photonic properties with the application of direct current (DC) field, including bandwidth broadening, switchable scattering, red tuning and blue tuning. Recently, PSCLCs have been prepared upon exposure of a 363.8 nm Argon laser, and higher order diffraction peaks, such as the second and/or third order diffraction peaks, are observed. The higher order reflection bands are caused by the deformed helical structure of the polymer stabilizing network formed during the exposure of a single laser beam or under reflection grating conditions. The spectral position of the second-order reflection band, which is half the spectral position of the main CLC reflection band, is simply adjusted by chiral dopant concentrations in the CLC mixture. The selective main and higher order reflection notches can be red-tuned and broadened by the application of DC fields. A potential mechanism for higher order diffraction peaks in the PSCLCs will be discussed.
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