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Heliconical cholesteric liquid crystals are expected to be more sensitive to torque induced by light field since their structure allows both bend and twist in molecular orientations, differently from the conventional cholesterics in which only twist deformation is involved requiring much higher fields. We report here a demonstration of tuning the helical pitch in heliconical cholesterics induced by an optical torque. Experimental observations are in agreement with expectations of the classical theory extended to include the effect of the optical field. A dual control of the helical pitch is achieved including both the low frequency electric field applied along the helix axis and the optical field orthogonal to it.

Selective reflection of light by oblique helicoidal cholesteric (Ch) can be tuned in a very broad spectral range by an applied electric field. In this Letter, we demonstrate that the peak wavelength of the selective reflection can be controlled by the surface alignment of the director in sandwich cells. The peak wavelength is blue-shifted when the surface alignment is perpendicular to the bounding plates and red-shifted when it is planar. The effect is explained by the electric field redistribution within the cell caused by a spatially varying heliconical Ch structure. The observed phenomenon can be used in sensing applications.

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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