S. S. Bawa

Novel alignment technique for surface stabilized ferroelectric liquid crystal

An alignment method for obtaining excellent surface stabilized ferroelectric liquid‐crystal devices is described. Alignment is obtained by obliquely depositing the SiO in two steps. The first deposition involves vacuum coating at 60° angle and the second at 85° (orthogonally to the first) with the substrate normal. The method induces a high (∼20°–25°) pretilt surface which depends on the thickness of the second layer. The glass substrates are matched with antiparallel direction of evaporation of second coating. This ensures a selective pretilt of molecules and the uniformly tilted smectic layer structure. The advantages of this new method are the ease of large‐scale production, defect‐free large uniform monodomains, good contrast, bistability, and microsecond switching speed response.

Bistability in deformed helix ferroelectric liquid crystal

Bistability in deformed helix ferroelectric liquid crystal (DHFLC) has been investigated by electro-optical and dielectric methods and compared with the bistability phenomenon in surface stabilized ferroelectric liquid crystal (SSFLC). Bistability in DHFLC as well as SSFLC is based on different physical phenomena. In SSFLC, it occurs due to surface effect and in DHFLC, by the critical deformation of the helix, under the application of time delayed square voltage pulse of known magnitude and frequency. Memory behavior of DHFLC and SSFLC cells has been compared by optical, electrical, dielectric, and hysteresis methods and the difference has been analyzed. It has been observed that memory effect in DHFLC is independent of surface effect but critically depends on the applied voltage and frequency of time delayed square wave pulse.

Collective dielectric relaxation in smectic A and smectic C⋆ phases near transition temperature of electroclinic liquid crystals

The collective dielectric relaxation processes of electroclinic liquid-crystal materials have been investigated in a thin (3.5μm) planar cell in the frequency range of 1 Hz–10 MHz. Measurement of complex dielectric permittivity versus temperature has been taken on aligned samples filled with different electroclinic liquid-crystal materials. The dielectric data were collected by using the measuring electric field perpendicular to the smectic layer planes. The behavior of the collective dielectric modes has been studied in smectic C*(SmC*) and smectic A(SmA) phases. However, near the transition temperature of SmC*-SmA phase, the dielectric permittivity due to the Goldstone mode and soft mode could not be separated. It has also been observed that the dielectric permittivity increases with temperature before the transition temperature of SmC*-SmA phase under bias field. Temperature dependence of dielectric increments before the transition temperature under bias field and relaxation frequency suggests that the...

Is Curie–Weiss law valid in every ferro-to-para transition?

The Curie–Weiss law is obeyed where there is a change in the order parameter and the symmetry of molecules at the transition temperature (Tc) of ferro-to-para phase. The ferroelectric transition is not strictly governed by the Curie–Weiss law. In fact, a number of solid ferroelectrics show deviation from the Curie–Weiss law. It has been shown in the present letter that the de-Vries electroclinic liquid crystals, with a wide smectic A (Sm-A) phase and a high electroclinic effect, do not obey the Curie–Weiss law. The nonlayer shrinkage leads to symmetry conservation that makes the observance of soft mode impossible and hence the deviation.

Optical memory effect in a deformed helix ferroelectric liquid crystal.

Optical memory in a deformed-helix ferroelectric liquid crystal is proposed by deforming the helix under the application of a square-voltage pulse of known magnitude and frequency. This effect is based on the electromechanical effect of helix deformation due to the electric field. When the interaction between the electric field and the dipole is sufficiently strong, all of the dipoles align along the electric field. In such a situation the interlayer dipole-dipole interaction is strong enough to balance the elastic deformation energy. When the electric field is switched off, the molecules remain in a static, balanced state owing to the dipole-dipole interaction and hence the memory effect.

Molecular reorientation processes and dynamics of polarization reversal in thin surface stabilized ferroelectric liquid crystal dobambc

Molecular reorientation processes in thin (3 μm) surface stabilized ferroelectric liquid crystal (SSFLC) DOBAMBC have been studied by applying symmetric and asymmetric triangular voltage pulses. For symmetric (-5V-OV-+5 V) triangular pulse a non-linear behaviour and two peaks of polarization reversal current and for asymmetric [(+ or -) 10 V-0 V] pulse four peaks of polarization change near 0 V region have been observed. For symmetric pulse two peaks originate due to composite effect of molecular change initially in the bulk and then at the surface under the influence of depolarization field. The non-linear current response to a linearly varying triangular pulse have been qualitatively explained by considering an intrinsic depolarization field. Dynamics of polarization reversal amongst the two uniform states and also amongst the twisted intermediate state to either of the two uniform states have been studied by applying symmetric and asymmetric square wave pulses. Switching response time corresponding to ...

Dielectric investigations of pure and carbon nanotube-doped deformed helix ferroelectric liquid crystals

The doping of carbon nanotubes (CNTs) into liquid crystals enhances their parameters for use in various applications. Here, an attempt is made to study the effect of CNTs on the dielectric properties of the deformed helix ferroelectric liquid crystals mixture FLC-6304, in different liquid crystal phases. The inclusion of CNTs significantly affect the dielectric parameters in the chiral smectic C phase with an inversion of the effect near the transition to, and in, the chiral smectic A phase

Thickness-independent memory effect in ferroelectric liquid crystals

Memory in ferroelectric liquid crystals (FLCs) is well known in the literature where thickness of the cell is less than the pitch value of the material. Here, we report a thickness independent memory in a class of FLCs called the de Vries electroclinic liquid crystals. Thickness independency of memory effect is observed by dielectric spectroscopy and texture observation. The memory observed in SmC* phase of de Vries material is entirely different from conventional FLCs. In the former case, it is the inherent property of the material but in the latter it is dependent on the cell geometry. In de Vries material, it is probably the randomization that is playing a major role.

Direct pulse technique for spontaneous polarization dynamics and molecular reorientation processes in ferroelectric liquid crystals

A method has been devised for studying the spontaneous polarization, dynamics of polarization reversal, and the molecular reorientation processes that occur in the bulk and surfaces of ferroelectric liquid crystals. These studies have been carried out by analyzing the current response to triangular‐ and square‐wave forms. The current response to a triangular wave yields the spontaneous polarization by conveniently subtracting the background contribution due to the conductive and capacitive currents. Molecular reorientation processes which take place within the bulk and the boundary surface layer can be studied by the appearance of polarization current peaks on the triangular‐wave response. The dynamics of polarization reversal between bistable states and of the polarization change between the zero‐field relaxed state and either of the saturation remanent polarization states have been studied by applying ‘‘symmetric’’ and ‘‘asymmetric’’ square waves. The technique can also be successfully applied to solid ...

Total-Reflection Switch Using High-Tilt-Angle Cholesteric-Smectic C* Phase Transition Ferroelectric Liquid Crystal

A total internal reflection (TIR) microoptic switch using high-tilt-angle materials showing cholesteric-smectic C* phase transition for multimode fiber is described. The device shows excellent temperature-independent electrooptic characteristics and exploits the maximum contribution of ferroelectric liquid crystal (FLC) birefringence. The switch shows asymmetric response and unusual nonlinear behaviour, which is analysed by polarisation reversal current. The nonlinear response is due to the polar surface interaction which behaves as if it were positively charged and has been analysed by the transmission characteristics and by current analysis on applying square and triangular waves of various frequencies. The switch has a switching response of ~20 ms limited due to monostable surface conditions, and shows a beam attenuation of ~5×107 in the off state.