Sudarshan Kundu

Enhancement of Contrast Ratio by Using Ferroelectric Nanoparticles in the Alignment Layer of Liquid Crystal Display

We demonstrate the effect of the ferroelectric nanoparticles dispersed in the alignment layer of a twisted nematic (TN) liquid crystal (LC) panel. In comparison to a conventional TN LC panel with a conventional polymide layer, the TN LC panel with an alignment layer doped with ferroelectric nanoparticles shows a much higher contrast ratio. The effects of barium titanate nanoparticles at several different concentrations have been studied, among which 2.5 and 5 wt % barium titanate nanoparticles mixtures show better performances. The surface texture of the alignment layers has also been studied by atomic force microscopy (AFM).

Ferroelectric liquid crystal cell versus dye doped ferroelectric liquid crystal cells: A comparison of dielectric properties

We report the dielectric dispersion and ferroelectric response of a ferroelectric liquid crystal (FLC) mixture after doping with anthraquinone dye at two different concentrations (0.5% and 2.5% w/w). Dielectric and polarization switching properties of the FLC mixture and that of the dye-doped FLC mixtures are investigated as a function of temperature. It is observed that the dielectric data in the low concentration dye-doped mixture are comparable to that of the undoped FLC compound. On the other hand the real part of dielectric permittivity is significantly reduced when the concentration of the dye is increased to 2.5%. Temperature dependence of spontaneous polarization and response time is measured and that supports the dielectric results. We also evaluated the rotational viscosity and activation energy. The SmC*-SmA* transition temperature is decreased by the addition of dye and that shift with temperature is discussed with the help of the Landau theory.

Effect of UV Curable Polymer on The Dielectric & Electro-Optic Properties of Ferroelectric Liquid Crystal

Results reported on the dielectric properties, spontaneous polarization (Ps) and switching time of a FLC material and a mixture of the FLC material after having doped with a small quantity of optical adhesive polymer. The mixture has been prepared by dissolving NOA65 prepolymer and the FLC compound in chloroform. A homogenous mixture of FLC and polymer formed after chloroform is being evaporated. Dielectric and Ps measurements of FLC-Polymer (PSFLC) mixture have been performed in a 10 w m ITO coated cell after the cell being irradiated with UV radiation for Photo-polymerization in situ. It has been observed that the spontaneous polarization of PSFLC mixture is unexpectedly reduced in polymer network formed as a result of photo polymerization. Similarly the Goldstone mode dielectric strength is also significantly reduced and switching time has been increased. The Soft mode dielectric strength however remains unchanged. The reduction of spontaneous polarization and Goldstone mode dielectric strength of the PSFLC mixture is not the accordance with the mixing ratio. The results have been discussed.

Spontaneous polarization and response time of polymer dispersed ferroelectric liquid crystal (PDFLC)

Abstract Results reported on the spontaneous polarization, switching time and rotational viscosity measurement of a FLC compound in neat phase and that in polymer matrix in the present paper. Polymer-dispersed ferroelectric liquid crystal film was formed by photopolymerization due to polymerization induced phase separation. A detailed study has been made on the spontaneous polarization, switching time at different temperatures and measuring field. It has been observed that the magnitude of spontaneous polarization of the PDFLC mixture is not in accordance with their mixing ratio. The switching time in PDFLC mixture is less than that in neat FLC mixture.

Electro-optical and dielectric properties of a high tilt antiferroelectric liquid crystal mixture (W-193B)

The electro-optical properties and dielectric relaxation have been investigated for an antiferroelectric liquid crystal mixture W-193B. The material exhibits smectic A ∗ , smectic C ∗ and a wide range of anticlinic smectic C ∗ phases. The high tilt and broad room temperature smectic C ∗ phase make it a good candidate for antiferroelectric display materials. Dielectric studies have been made in a planarly aligned cell in the frequency range 10 Hz–13 MHz. Dielectric spectroscopy reveals the existence of soft mode in the smectic A ∗ phase and Goldstone mode in the smectic C ∗ phase. In the smectic C ∗ phase the dielectric spectrum of the material exhibits two absorption peaks related to the rotational fluctuation around the short axis of the molecules and antiphase azimuthal angle fluctuation, respectively, and are separated by about two orders of frequency. Electro-optical response using a low frequency triangular wave showed a very high quasi-static contrast ratio of 132 : 1, threshold voltage of around 7 V and saturation of 17 V. Surface-stabilized, low thickness cells of this mixture showed a perfect double hysteresis loop with a 1 Hz triangular signal, reaching different transmission levels for different voltage amplitudes. These levels can be stabilized with a single holding voltage, making it possible for the material to be passively multiplexed at video rate. (Some figures in this article are in colour only in the electronic version)

Electro-optical properties of an orthoconic liquid crystal mixture (W-182) and its molecular dynamics

We observed that the perfect dark state problem could be solved by using orthoconic antiferroelectric liquid crystal (OAFLC) instead of normal AFLC by comparing the properties of isocontrast and dispersion chromaticity of W-182 OAFLC and normal AFLC CS-4001. We electro-optically observed that several subphases such as SmCγ*, SmC*β, SmC*α and antiferroelectric SmI*A phases exist in W-182 OAFLC. We dielectrically observed in 4μm thin cell that during heating, several new phases appeared. In the high temperature antiferroelectric region, a higher order than SmC* phase could be detected dielectrically, in the temperature range of 91–98°C, behaving similar to SmCγ* and also, another phase below SmC* region could be dielectrically detected in the temperature range of 103–1100°C, behaving similar to SmCα*, and an antiferroelectric, similar to SmIA* phase, was observed in the lower temperature region of the antiferroelectric phase; those are definitely arising due to surface force and interfacial charges interact...

Improvement of Electro-Optical Characteristics of Liquid Crystal Display by Nanoparticle-Embedded Alignment Layers

The effect of various metal-oxide nanoparticles embedded in the alignment layer of a liquid crystal (LC) display is experimentally investigated. The study mainly focuses on the electro-optical properties of a twisted-nematic LC display. It is confirmed that a certain type of nanoparticles contribute to the enhancement of contrast ratio. In addition, it is observed that for almost all types of nanoparticles used in the present study, the statistical error of electro-optical properties of LC test cells reduces as compared to that of cells without nanoparticles, indicating the improvement of LC molecular alignment. For a further clarification, AFM study and other related measurements are performed.

Electro-optic and dielectric properties of an antiferroelectric liquid crystal material

Abstract Results reported in this paper on the electro-optic properties and dielectric relaxation in the SmCA* and SmC* phase of a liquid material as a function of sample thickness. The measurements were carried out in 3μm, 6μm, and 10μm thick cells. In the whole SmC* phase two modes were obtained in the low frequency region of the dielectric spectra in each of the three cells without bias electric field. The relaxation strength of both the modes sharply decreases with the applied bias dc electric field. One of the modes is assigned as Goldstone mode, arises due to phase fluctuation and the origin of the other mode may be related to the space charge accumulated at the interface between liquid crystal and the polymer coating. In antiferroelectric liquid crystal (SmCA*) phase two switching current peaks were observed only in a 3μm thick cell by applying triangular wave of frequency around 30 Hz-40 Hz. Two antiferroelectric modes were obtained in the dielectric spectra of SmCA* phase in the low frequency region and the results have been dicussed. Both the results of spontaneous polarization and dielectric studies reveal that the antiferroelectric behavior in the SmCA* phase is squeezed in each of the three cells.

Experimental characterization of hexatic smectic phases through electro-optic studies and dielectric relaxation spectroscopy

The electro-optic and complex dielectric behaviour of an antiferroelectric liquid crystal 4-(1-methylheptyloxycarbonyl)phenyl 4′-(n-butanoyloxyprop-1-oxy)biphenyl-4-carboxylate, having chiral SmCA* and hexatic smectic phases, have been investigated. Complex dielectric permittivities were measured as a function of frequency, d.c. bias field and temperature. Spontaneous polarization was measured by the current reversal technique; tilt angle was measured under a polarizing microscope using a low frequency electric field. The electro-optic properties and dielectric behaviour of the material are compared with results obtained by DSC and polarizing optical microscopy. Dielectric relaxation processes in SmCA* and hexatic smectic phases were determined. The dielectric strength at the SmCA* to hexatic smectic phase transition is discussed in terms of coupling between the long range bond orientational order and smectic C director. It seems from the results of spontaneous polarization and dielectric relaxation spectroscopy that the material might possess an additional phase between the SmCA* and hexatic smectic I* phases.

Dielectric Properties and Electro-optic Characteristics of TN-LCDs Doped with Metal Nanoparticles Exhibiting Frequency Modulation Response Accompanying Fast Response

ABSTRACT TN-LCDs doped with metal nanoparticles such as Pd, Ag, and Au are shown to exhibit a frequency modulation response together with the ordinary root-mean-square response to operating voltages. These devices are called FM-LCDs. The frequency range in this phenomenon spreads from several tens Hz to several thousands Hz depending on the materials of nanoparticles, their diameters, and their concentrations. The central frequency of each FM-LCD almost coincides with the dielectric relaxation frequency. FM-LCDs shows a sensitivity to the high frequency component of the driving voltage, this in turn gives rise to a fast electro-optic response in ms and sub-ms order. The EO effect of the FM-TN-LCD effect is explained based on the Maxwell and Wagner effect and it is shown that the metal nanopatricles have the effective electrical conductivity of σ2 = 6.0 × 105S/m that is about 100 times smaller that of metal Ag.