Tuning selective reflection of light by surface anchoring in cholesteric cells with oblique helicoidal structures

Iadlovska, Olena S.; Maxwell, Graham; Babakhanova, Greta; Mehl, Georg H.; Welch, Chris; Shiyanovskii, Sergij V.; Lavrentovich, Oleg D.

doi:10.1364/ol.43.001850

Cited by 22 publications

(9 citation statements)

References 16 publications

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“…Several recent papers [14][15][16][17] focused on the study of the N * phases in mixtures comprising bend-shaped molecules of CBnCB type and chiral dopants. Such systems under electric field undergo transition to a peculiar cholesteric state with oblique helicoidal structure denoted in many papers as ChOH.…”

Section: Introductionmentioning

confidence: 99%

Twist-bend nematics and heliconical cholesterics: a physico-chemical analysis of phase transitions and related specific properties

2021

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Twist-bend nematics and heliconical cholesterics: a physico-chemical analysis of phase transitions and related specific propertiesCertain nematic liquid crystals, e.g., those formed by banana-shaped molecules, can exhibit a low-temperature twist-bend nematic (Ntb) phase. Upon addition of chiral dopants, a chiral version of the twist-bend phase (N * tb) can be observed below the conventional chiral nematic (N * ) phase, while under electric field the N * phase is transformed into a specific state known as "heliconical cholesteric". In this work, we have studied the well-known Ntb-forming system CB7CB:CB6OCB with addition of 5CB and chiral dopants (ChD) using optical microscopy, differential scanning calorimetry (DSC), and measurements of temperature-dependent optical transmission using optical microscopy. Effects of 5CB and different chiral dopants (of steroid or non-steroid nature) upon thermal stability of Ntb phase were analyzed, and selective reflection in the visible range could be observed in the N * phase with a sufficiently high ChD content. As a peculiar feature, sharp unwinding of the cholesteric helix was observed at lower temperatures when approaching the N * tb phase, which appears to be similar to the helix unwinding close to the smectic-A transition.

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Section: Introductionmentioning

confidence: 99%

Twist-bend nematics and heliconical cholesterics: a physico-chemical analysis of phase transitions and related specific properties

2021

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“…The existence of the ChOH state in a Ch acted upon by an electric or magnetic field has been envisaged theoretically a long time ago [5,6] and confirmed experimentally very recently [7][8][9][10][11].…”

Section: Introductionmentioning

confidence: 88%

“…As demonstrated in Ref. [10], the subsurface inhomogeneous regions make the acting electric field bulk E different from the average applied electric field /…”

Section: Theoretical Modelmentioning

confidence: 96%

“…Such an ideal structure can exist in an infinitely thick sample, in which the surface anchoring of the director at the boundaries can be neglected. In cells of a finite thickness d , surface anchoring distorts the twist-bend director configuration and causes spatially-varying dielectric properties at the boundaries and redistribution of the electric field within the cell [10]. Selective reflection of light 5 in these cells is determined by the central bulk region, in which P ,  , and the acting electric field bulk E are coordinate-independent [10].…”

Section: Theoretical Modelmentioning

confidence: 99%

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Temperature dependence of bend elastic constant in oblique helicoidal cholesterics

Iadlovska

Babakhanova

Mehl

et al. 2020

Phys. Rev. Research

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Elastic moduli of liquid crystals, known as Frank constants, are of quintessential importance for understanding fundamental properties of these materials and for the design of their applications.Although there are many methods to measure the Frank constants in the nematic phase, little is known about the elastic constants of the chiral version of the nematic, the so-called cholesteric liquid crystal, since the helicoidal structure of the cholesteric renders these methods inadequate.Here we present a technique to measure the bend modulus 33 K of cholesterics that is based on the electrically tunable reflection of light at an oblique helicoidal ChOH cholesteric structure. 33 K is typically smaller than 0.6 pN, showing a non-monotonous temperature dependence with a slight increase near the transition to the twist-bend phase. 33 K depends strongly on the molecular composition. In particular, chiral mixtures that contain the flexible dimer 1′′,7′′-bis(4cyanobiphenyl-4′-yl) heptane (CB7CB) and rod-like molecules such as pentylcyanobiphenyl (5CB) show a 33 K value that is 5 times smaller than 33 K of pure CB7CB or of mixtures of CB7CB with chiral dopants. Furthermore, 33 K in CB11CB doped with a chiral agent is noticeably smaller than 33 K in a similarly doped CB7CB which is explained by the longer flexible link in CB11CB. 2The proposed technique allows a direct in-situ determination of how the molecular composition, molecular structure and molecular chirality affect the elastic properties of chiral liquid crystals.

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“…, however it appears that the topic of planar films with a spatially-variable orientation of the helical axis gained some revival in 2017-18 by inspiring different research. [20][21][22][23][24][25][26][27] In 2011, tuning the orientation of the twist and, in return, of the macroscopic color of a CLC slab with a polygonal texture was demonstrated. 28 Polygonal textures (Figure 2(a)), made of an array of micrometer-scale polygonal cells, can be observed in flat CLC films when the helical axis is strongly tilted with respect to the substrates.…”

Section: State-of-the-art and Motivation Of The Researchmentioning

confidence: 99%

Nanoscale hyperspectral imaging of tilted cholesteric liquid crystal structures

et al. 2019

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Tuning selective reflection of light by surface anchoring in cholesteric cells with oblique helicoidal structures

Cited by 22 publications

References 16 publications

Twist-bend nematics and heliconical cholesterics: a physico-chemical analysis of phase transitions and related specific properties

Twist-bend nematics and heliconical cholesterics: a physico-chemical analysis of phase transitions and related specific properties

Temperature dependence of bend elastic constant in oblique helicoidal cholesterics

Nanoscale hyperspectral imaging of tilted cholesteric liquid crystal structures

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