We explore the structures and confinement-induced edge dislocations in Grandjean-Cano wedge cells filled with the recently discovered chiral ferroelectric nematic (NF ∗ ) and chiral antiferroelectric smectic-Z (SmZA ∗ ). The chiral mixture is formed by DIO mesogen doped with a chiral additive. Wedge cells with parallel and antiparallel rubbing at the opposite plates show quantitatively different structures which is attributed to the polar in-plane anchoring of the spontaneous polarization at the rubbed substrates. The helical pitch shows a non-monotonous temperature dependence upon cooling, increasing as the temperature is lowered to the N ∗ -SmZA ∗ phase transition. The SmZA ∗ formed from an untwisted N ∗ in the thin portion of the wedge shows a bookshelf (BK) geometry, whereas the twisted N ∗ transforms into a twisted planar (PA) SmZA ∗ structure. In the NF ∗ phase, the untwisted N ∗ becomes twisted in a wedge with antiparallel assembly of plates and monodomain in wedges with parallel assembly. The twisted regions of NF ∗ show only one type of Grandjean zones separated by thick edge dislocations with Burgers vector 𝑏 = 𝑃; the neighboring regions differ by 2π- twist.
We demonstrate experimentally that diffraction of light incident obliquely at the oblique helicoidal cholesteric (ChOH) structure results in scattering at both the half- (P/2) and the full pitch (P) periodicities. Transmission spectra at P and P/2 are observed in the Vis spectral range, tunable by the electric field and the angle of light incidence. Diffraction at P/2 shows two distinct spectral bands for p- and s-polarized incident light. Diffraction at P is a single wide band. Unique optical properties of ChOH are attractive for applications such as electrically tunable band-pass filters, optical rotators, and beam-steering devices.
An oblique helicoidal cholesteric ChOH represents a unique optical material with a pitch that can be tuned by an electric or magnetic field in a broad range from sub micrometers to micrometers. In this work, we demonstrate that the oblique helicoidal cholesteric doped with azoxybenzene molecules and stabilized by an electric field could also be tuned by light irradiation. At a fixed voltage, UV irradiation causes a redshift of the reflection peak by more than 200 nm. The demonstrated effect has the potential for applications such as smart windows, sensors, tunable lasers, and filters.
Liquid crystal materials with tendency to bend are capable of forming a unique type of a helicoidal structure in which the director is titled with respect to the helicoidal axis rather than orthogonal to it, as in regular cholecterics. The new state occurs in presence of an electric or magnetic field. It is called a cholesteric with oblique helicoidal structure or simply a twist-bend cholesteric, since both twist and bend deformations are characteristic of the ground state supported by the external field. Tilted configuration of the director and absence of density modulation makes the twist-bend cholesterics a unique material for various electro-optical applications. The presentation discusses electrically and magnetically tuned selective reflection of light as well as tunable lasing enabled by these materials. It is demonstrated that the electro-optical performance of the twist-bend cholesterics depends strongly on the type of anchoring conditions imposed by the boundaries of the cell. The work is supported by NSF grant DMR-1410378.
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