Circular chiral nematics

M Voigt, M Chambers, and M Grell, Circularly polarized emission from a narrow bandwidth dye doped into a chiral nematic liquid crystal, Liq. Cryst., 29 653-656, 2002. 

H Shi, BM Conger, D Katsis, and SH Chen, Circularly polarized fluorescence from chiral nematic liquid crystalline films theory and experiment, Liq. Cryst., 24 163-172, 1998. 

Note 3 Chiral nematic mesophases exhibit Bragg scattering of circularly polarised light at a wavelength proportional to the pitch P (Xr = P, where is the mean refractive index). 

Recently, true circular polarizers have been developed and are available from commercial sources. These are fashioned by sandwiching a chiral nematic liquid crystal between parallel glass windows. These devices are characterized by a high transmission and extinction ratios greater than 1000. They operate over a fairly narrow range of wavelengths about a central wavelength. 

In this liquid crystal phase, the molecules have non-symmetrical carbon atoms and thus lose mirror symmetry. Otherwise optically active molecules are doped into host nematogenic molecules to induce the chiral liquid crystals. The liquid crystals consisting of such molecules show a helical structure. The most important chiral liquid crystal is the cholesteric liquid crystals. As discussed in Section 1.2, the cholesteric liquid crystal was the first discovered liquid crystal and is an important member of the liquid crystal family. In some of the literature, it is denoted as the N phase, the chiral nematic liquid crystal. As a convention, the asterisk is used in the nomenclature of liquid crystals to mean the chiral phase. Cholesteric liquid crystals have beautiful and interesting optical properties, e.g., the selective reflection of circularly polarized light, significant optical rotation, circular dichroism, etc. 

D. Katsis, H.P. Chen, J.C. Mastrangelo, S.H. Chen, T.N. Blanton, Vitrified chiral-nematic liquid crystalline films for selective reflection and circular polarization. Chem. Mater. 11, 1590 (1999)... 

D. Katsis, A.W. Schmid, S.H. Chen, Mechanistic insight into circularly polarized photoluminescence from a chiral-nematic film. Liq. Cryst. 26, 181-185 (1999)... 

Lyotropic Di-substituted Polyacetylenes that Exhibit High Dissymmetry Factors in Circularly Polarized Luminescence Through the Chiral Nematic Liquid Crystal Phase... 

B.A. San Jose, S. Matsushita, K. Akagi, Lyotropic chiral nematic liquid crystalline aliphatic conjugated polymers based on di-substituted polyacetylene derivatives that exhibit high dissymmetry factors in circularly polarized luminescence. J. Am. Chem. Soc. 134, 19795-19807 (2012)... 

K. Watanabe, I. Osaka, S. Yorozuya, K. Akagi, Helically n-stacked thiophene-based copolymers with circularly polarized fluorescence high dissymmetry factors enhanced by self-ordering in chiral nematic liquid crystal phase. Chem. Mater. 24, 1011-1024 (2012)... 

In circular dichroism (CD) spectra of the polyacetylene thin films synthesized under the (fi-2)- and (S-2)-chiral nematic LCs, positive and negative Cotton effects are observed, respectively, in the region from 450 to 800 nm corresponding to tt — it transition of polyacetylene chain (Figure 3.8), despite the absence of chiroptical substituent in side chains. This indicates that the polyacetylene chain itself is helically screwed. It is evident that the above Cotton effect is not due to the chiral dopant [( )- or... 

FIGURE 3.8 Circular dichroism (CD) spectra of helical polyacetylene films. The polyacetylene films synthesized in the (R-2)- and (S-2)-chiral nematic LCs including (R)- and (S)-6,6 -PCH506-2,2 -Et-Binol are designated as PA by R-2 and PA by S-2, respectively. 

F re 3. Schematic diagram of selective reflection from a right-handed chiral nematic planar texture for A= inside the liquid ciystal. The linearly polarized input light may be considered as counter-propagating right-handed (RH) and left-handed (LH) circular components. The RH component in which the E field matches the sense of the helicoidal structure is back reflected due to director fluctuations, whilst the LH component is almost totally transmitted. By convention, the handedness is delined in terms of the progression of the E field vector in time relative to the observer. The RH rotating wave therefore has the same spatial structure as the chiral nematic at any time. 

The selectively polarized light (for Aj -p) is circularly polarized with the same handedness as the chiral nematic helix. The other circular polarization is almost totally transmitted. 

The theory of light propagation in a chiral nematic for directions other than that of the helix axis is complex. Whilst for normally incident light the transmitted and reflected waves are, to a good approximation, circularly polarized, for oblique incidence these waves become elliptically polarized... 

Figure 36. Schematic diagram of a chiral nematic polarized light source [187], where L=lamp, M=.spherical mirror, C=condenser lens, CNm l h I ded chiral nematic filter, QW=quarter wave plate (A/4), Io=unpolarized light, I o=right-handed circularly polarized light, and lp=polarized output light.

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