Jagdish Vij

Trinity College Dublin, Department of Electronic and Electrical Engineering, Trinity College Dublin, Faculty Member

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Dublin Institute of Technology

K.L. Sandhya

Tiezheng Qian

Rohit Verma

Amity University, Noida

Ahmad Labeeb

Paulo Simeão Carvalho

Universidade do Porto

Ken Ishikawa

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Optical rotatory power of different phases of an antiferroelectric liquid crystal and implications for models of structure

Physical review. E, Statistical, nonlinear, and soft matter physics, 2001

The antiferroelectric liquid crystal (AFLC) under investigation possesses different helical smect... more The antiferroelectric liquid crystal (AFLC) under investigation possesses different helical smectic phases. The various phases have been identified through a texture under cross-polarizers with a homeotropic alignment of the AFLC. Measurements of the optical rotatory power (ORP) of these phases have elucidated the ability of this method for finding phase transitions between several phases and for investigating the helical structure of the antiferroelectric phases. The optical rotatory power as a function of temperature at a fixed wavelength of light was measured for different phases of the investigated AFLC material. The values of the pitch for some of the phases have been calculated from the ORP data. The results of the ORP rule out the simple "clock" model or a clock model with a long pitch superimposed on to it. The results can be explained only in terms of biaxial models, either Ising-type models or a highly distorted "clock" model. It is also found that in t...

$The ORP in cholesteric LC is given as [13] where P is the helical pitch, \ is the wavelength in vacuo, and n, and n, are the refractive indices parallel and perpen- dicular to the director. This expression can be extended to the tilted smectic phases using the relations [14] each phase. The value of the ORP in the SmC* phase is very low. This confirms the predictions of the clock model [11], according to which the SmC* phase is characterized by a short-pitch helical structure. But there is a discrepancy be- tween values of this pitch measured by different methods. Resonant x-ray measurements [3,4] give a decrease in the pitch from eight- to five-layer periodicity with decreasing temperature. An ellipsometric study [12] showed that the he- lical pitch increases from 20- to 40-layer periodicity when the temperature decreases. It is possible that this difference in the temperature dependence and pitch magnitude is related to different phase sequences in these two compounds. The sign of the ORP in the SmC* and SmC;,, phases is the same for short wavelengths, but is the opposite for SmCZ,, and SmC; phases. This means that the handedness of the chiral structure is changed during a transition from the SmC;)y phase to the SmCZ,, phase. The sign of the ORP in the SmC* phase is the same for short wavelengths (494 and 523 nm) but is the opposite for long wavelengths (587 and 619 nm). According to Eq. (2) this means that the wave- length of the selective reflection is located between wave- lengths of 523 and 587 nm. This fact is confirmed by visual observation.$

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