Electric-field-induced nematic-cholesteric transition and three-dimensional director structures in homeotropic cells

Abstract

We study the phase diagram of director structures in cholesteric liquid crystals of negative dielectric anisotropy in homeotropic cells of thickness d which is smaller than the cholesteric pitch p. The basic control parameters are the frustration ratio d/p and the applied voltage U. Upon increasing U, the direct transition from completely unwound homeotropic structure to the translationally invariant configuration (TIC) with uniform in-plane twist is observed at small d/p < or = 0.5. Cholesteric fingers that can be either isolated or arranged periodically occur at 0.5 < or = d/p<1 and at the intermediate U between the homeotropic unwound and TIC structures. The phase boundaries are also shifted by (1) rubbing of homeotropic substrates that produces small deviations from the vertical alignment; (2) particles that become nucleation centers for cholesteric fingers; (3) voltage driving schemes. A novel reentrant behavior of TIC is observed in the rubbed cells with frustration ratios 0.6 < or = d/p < or = 0.75, which disappears with adding nucleation sites or using modulated voltages. In addition, fluorescence confocal polarizing microscopy (FCPM) allows us to directly and unambiguously determine the three-dimensional director structures. For the cells with strictly vertical alignment, FCPM confirms the director models of the vertical cross sections of four types of fingers previously either obtained by computer simulations or proposed using symmetry considerations. For rubbed homeotropic substrates, only two types of fingers are observed, which tend to align along the rubbing direction. Finally, the new means of control are of importance for potential applications of the cholesteric structures, such as switchable gratings based on periodically arranged fingers and eyewear with tunable transparency based on TIC.

Capillary and winding transitions in a confined cholesteric liquid crystal

“…Previous and more recent works have analysed the problem using simple elastic models [17][18][19][20][21][22][23][24]. There are various experimental works that deal with these problems [25][26][27][28][29][30][31][32][33].…”

Theory of helicoids and skyrmions in confined cholesteric liquid crystals

“…Because of this frustration, the confined cholesteric phase forms topological defect structures. Depending on the geometry and anchoring strength, these defects may be elongated string-like objects called cholesteric fingers or helicoids [3][4][5][6][7][8], or they may be localized point-like objects called cholesteric bubbles [9][10][11][12].…”

Optically generated adaptive localized structures in confined chiral liquid crystals doped with fullerene

“…When this LC is introduced into a thin cell of thickness d Ͻ p with vertical ͑homeotropic͒ surface anchoring of n͑r͒, the incompatibility of these boundary conditions with the cholesteric helix results in structural frustration and tends to unwind the helicoidal structure. Although n͑r͒ is typically untwisted for d / p Ͻ 1, a number of locally twisted metastable and stable structures occur spontaneously 3,4 and can be generated by the use of external fields. [1][2][3][4] Various localized structures called "Torons" can be generated using laser beams with optical phase singularities 2 that reorient n͑r͒ to transform the initially unwound structure to a twisted configuration with point or line singularities.…”

“…Although n͑r͒ is typically untwisted for d / p Ͻ 1, a number of locally twisted metastable and stable structures occur spontaneously 3,4 and can be generated by the use of external fields. [1][2][3][4] Various localized structures called "Torons" can be generated using laser beams with optical phase singularities 2 that reorient n͑r͒ to transform the initially unwound structure to a twisted configuration with point or line singularities. On the other hand, LCs doped with dyes and nanoparticles, such as fullerene ͑C 60 ͒, exhibit enhanced optical realignment effects, which may have bulk or surface origin ͑due to dye or fullerene segregation to surfaces in the later case͒.…”

“…4 The LCs are doped with 0.01-0.1 wt % of Since C 60 -doped samples show no detectable absorption at 1064 nm but strongly absorb at 488 nm, we use both blue and infrared light to induce Torons. Illumination at 488 nm is performed using an Ar laser and scanned mirrors of a confocal microscope ͑FV-300, Olympus, Olympus, Japan͒, while the spatial patterning of the focused 1064 nm beam ͑ytterbium-doped fiber laser͒ is achieved using a holographic optical tweezer ͑HOT͒ setup.…”