K. A. Suresh

Liquid Crystals of Disc-like Molecules

Thermotropic mesomorphism has been observed in pure compounds consisting of simple disc-like molecules, viz., benzene-hexa-n-alkanoates. Thermodynamic, optical and x-ray studies indicate that the mesophase is a highly ordered lamellar type of liquid crystal. Based on the x-ray data, a structure is proposed in which the discs are stacked one on top of the other in columns that constitute a hexagonal arrangement, but the spacing between the discs in each column is irregular. Thus the structure has translational periodicity in two dimensions and liquid-like disorder in the third.

The Pancharatnam phase as a strictly geometric phase : A demonstration using pure projections

We describe an interferometer based on the Pancharatnam phase in which the extra phase introduced is of purely geometric origin, with no contributions from the dynamic phase. We use a cholesteric liquid crystal film as a circular analyser, and a sheet polarizer as a linear analyser, to project each polarization state on the next one. This experiment can also be viewed as a demonstration of Berrys phase arising from quantum measurements.

An Experimental Study of the Anomalous Transmission (Borrmann Effect) in Absorbing Cholesteric Liquid Crystals

Abstract It has been shown previously that in analogy with the Borrmann effect in X-ray diffraction from crystals there is an anomalous increase in the transmitted intensity near the Bragg reflection band in an absorbing cholesteric liquid crystal. In this communication detailed experimental studies of this effect carried out on thin films of mixtures of cholesteryl nonanoate and p-azoxyanisole of different concentrations are reported. Numerical calculations based on the dynamical theory of reflection are also presented. The theoretical curves of circular dichroism versus wavelength are in qualitative agreement with the experimentally observed features.

Anomalous transmission at oblique incidence in absorbing cholesteric liquid crystals

We consider theoretically light propagation at oblique incidence in absorbing cholesteric liquid crystals in the Bragg mode. Using 2 × 2 matrix procedure, we analyze the eigenmodes in terms of the forward- and backward-propagating eigenwaves inside the isotropic medium bounding the cholesteric liquid crystal. The nature of the eigenmodes in the medium changes continuously from the circular to the linear state with increase in the angle of incidence. Using the appropriate eigenmodes, we studied anomalous transmission in first and second Bragg orders in these systems and its relevance to the reported experimental results. We find some interesting polarization features that arise as a result of the absorption in the cholesteric medium.

Optical properties of mixtures of right- and left-handed cholesteric liquid crystals

A rigorous theory is developed of the optical properties of a mixture of right-handed and left-handed cholesteric liquid crystals, taking into account the effect of absorption. The dependence of the rotatory power and circular dichroism on pitch and on sample thickness is discussed.

Temperature variation of the layer spacing in the smectic A, reentrant nematic and reentrant smectic A phases of 9 OBCAB

Precise X-ray determinations have been made of the temperature variation of the layer spacing in the smectic A, reentrant nematic and reentrant smectic A phases of a pure reentrant mesogen, viz., 4...

Optical diffraction in twisted liquid-crystalline media-phase grating mode

We have calculated the diffraction of light perpendicular to the twist axis in a chiral smectic C liquid crystal. In contrast to a cholesteric liquid crystal, in a chiral smectic C liquid crystal we find extra orders which form the odd orders in the diffraction pattern. For an incident linearly polarized light, at a general azimuth, these odd orders are linearly polarized and the even orders are elliptically polarized. The intensities of the odd orders are always independent of the azimuth of the incident light, while this is possible for even orders only at a particular tilt angle of the chiral smectic C liquid crystal. Also, for the incident vibration parallel or perpendicular to the twist axis the odd orders are polarized in the orthogonal linear state with respect to incident vibration, while the even orders are in the same linear state.

Effect of Skew Cybotactic Structure on the optical Properties of a Nematogen with a Lateral Cyano Substituent

Abstract We report an unusual thermal variation of the ordinary refractive index ( n o) of the nematogenic compound 2-cyano-4-heptyl-phenyl-4′-pentyl-4-biphenyl carboxylate, which has a cyano group making a large angle (~60°) with the molecular axis. As the sample is cooled from the NI transition point, n o initially decreases as usual, but on further cooling it shows an increasing trend after reaching a broad minimum. On the other hand, the extraordinary index n e shows the normal temperature variation. X-ray investigations of a monodomain sample show that the nematic has a skewed cybotactic type of short-range order, the tilt angle increasing considerably as the temperature is lowered from the NI transition point. We have made a model calculation to explain the temperature variation of n o in terms of the variation of the tilt angle.

Discogen-DNA Complex Films at Air-Water and Air-Solid Interfaces

We have studied films of an ionic discogenic (discotic mesogenic) molecule (pyridinium salt tethered with hexaalkoxytriphenylene (PyTp)) and DNA complex at air-water (A-W) and air-solid interfaces. We have formed an PyTp monolayer on an aqueous subphase containing a small amount of DNA to obtain a PyTp-DNA complex at the A-W interface. Compared to the pure PyTp monolayer, the PyTp-DNA complex monolayer exhibits a higher collapse pressure and lower limiting area, indicating condensation and better stability. A Brewster angle microscope was used for in situ observation of the morphology of the film at the A-W interface. The PyTp-DNA complex films on silicon wafers were prepared using the Langmuir-Blodgett (LB) technique. We find that several tens of layers of the PyTp-DNA complex monolayer can be transferred with good efficiency. Fourier transform infrared spectroscopy studies confirm the presence of DNA in the LB films of the PyTp-DNA complex. Nanoindentation measurements using atomic force microscope reveal that the PyTp-DNA complex films are about two times harder as compared to the pure PyTp films.

Optical and X-ray studies on the twisted smectic C and twisted nematic phases: Evidence for a skew-cybotactic type of cholesteric structure

Abstract The optical rotation p and the pitch P of mixtures of p-n-heptyloxyazoxybenzenea nd cholesteryl benzoate have been measured as function of temperature in the twisted smectic C and the twisted nematic phases for two compositions (10% and 15% of cholesteryl benzoate by weight). Both p and P change discontinuously at the twisted smectic C-twisted nematic point, the values being higher in the lower-temperature phase. The birefringence δn of pure HOAB has also been determined in the nematic and smectic C phases. The observed p in the twisted nematic phase cannot be accounted for in terms of the measured values of δn and P. The calculated value is too high indicating that, in contrast to the classical cholesteric structure, the molecules in this phase are not normal but inclined with respect to the helical axis. This has been confirmed directly by X-ray studies; the diffraction pattern has an inner ring characteristic of a skew-cybotactic structure with a tilt angle of 39° with respect to the layer nor...