Abstract: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 … Show more
“…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%
“…Recent research [6] demonstrated that the bend elastic constant K33 of Ch could be determined in situ if the material forms the so-called oblique helicoidal cholesteric (ChOH) structure when exposed to external electromagnetic fields [6][7][8][9][10][11][12]. In ChOH, the director twists around a single axis that is parallel to the applied field, but unlike in a conventional Ch, it is also tilted with respect to the helicoidal axis, Fig.…”
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.
“…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%
“…Recent research [6] demonstrated that the bend elastic constant K33 of Ch could be determined in situ if the material forms the so-called oblique helicoidal cholesteric (ChOH) structure when exposed to external electromagnetic fields [6][7][8][9][10][11][12]. In ChOH, the director twists around a single axis that is parallel to the applied field, but unlike in a conventional Ch, it is also tilted with respect to the helicoidal axis, Fig.…”
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.
“…Besides conventional cholesterics, CLCs with oblique helicoidal state were also disclosed 127 . The pitch of this state has been reported to be able to continuously tuned by external electric 38 , magnetic 128 and light fields 129 . For instance, Xiang et al 38 produced electrically tunable selective reflection of light ranging from UV to visible and infrared regions by using the oblique helicoid CLC structure with low driving electric fields, Fig.…”
“…10b , which opened a new avenue for anti-counterfeiting technologies 197 . In addition, unique optical materials of oblique helicoidal CLCs are also appealing candidates for smart displays 129 , 198 , which possess both twist and bend LC orientations, and are highly sensitive to the electric and light fields, resulting in a feasible tunability of the structural color consequently 129 . Fig.…”
Self-assembled architectures of soft matter have fascinated scientists for centuries due to their unique physical properties originated from controllable orientational and/or positional orders, and diverse optic and photonic applications. If one could know how to design, fabricate, and manipulate these optical microstructures in soft matter systems, such as liquid crystals (LCs), that would open new opportunities in both scientific research and practical applications, such as the interaction between light and soft matter, the intrinsic assembly of the topological patterns, and the multidimensional control of the light (polarization, phase, spatial distribution, propagation direction). Here, we summarize recent progresses in self-assembled optical architectures in typical thermotropic LCs and bio-based lyotropic LCs. After briefly introducing the basic definitions and properties of the materials, we present the manipulation schemes of various LC microstructures, especially the topological and topographic configurations. This work further illustrates external-stimuli-enabled dynamic controllability of self-assembled optical structures of these soft materials, and demonstrates several emerging applications. Lastly, we discuss the challenges and opportunities of these materials towards soft matter photonics, and envision future perspectives in this field.
“…Recently a more accurate calculation of the nonlinear propagation based on Ambartsumian's layer addition modified method 11 confirmed such behavior for moderate intensities while showing the onset of optical instabilities leading to self-oscillations for increasing value of the light intensity. These peculiar optical properties have been shown to be attractive for developing tuneable optical devices such as optical notch and bandpass filters 12 and envisage the possibility of being exploited in advanced sensing devices in optofluidic configurations 13 or coupled with piezoelectric photo-mobile supports 14 .…”
Section: Light-induced Reorientation and Nonlinear Optical Propertiesmentioning
In this presentation the novelty represented by the heliconical cholesteric liquid crystals (Ch-OH) for easy electric and optical control of optical properties are highlighted. After a quick summary of their electro-optical properties, an account of the recent experimental and theoretical achievements about the nonlinear optical response of Ch-OH will be given. The peculiar conical structure allows an easy control of the spectral location of the Bragg resonance making possible effects never observed previously in pure liquid crystals, making these materials attractive for development of several optical devices.
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