Suppression of relaxation modes in dye dispersed SmC* phase

We report the results of dielectric and electro-optical properties of ferroelectric liquid crystal (FLC), Felix 17/100, exhibiting chiral smectic C phase and dye dispersed FLCs. The polarization measurement on pristine and dye dispersed FLC mixture shows decrease in the value of polarization, indicating the distribution of dye dipole in a direction opposite to the orientation of FLC molecule. The rotational viscosity also decreases accordingly as shown by the measurement of response time. Dielectric measurement shows existence of two relaxation modes both in pure FLC and dye dispersed FLC. The relaxation strength of Goldstone mode decreases with the dispersion of dye and the relaxation frequency of this mode shifts towards the high-frequency side. The second relaxation mode arises due to the formation of domains at the surface interface. The dispersion of dye into FLC suppresses the domains.     

Changes in material parameters for dye-doped ferroelectric liquid crystal

Addition of the dichroic anthraquinone dye molecules in pure ferroelectric liquid crystal (FLC) matrix results in many improvement in the various vital parameters of the pure FLC. However, addition of anthraquinone dye molecules in pure FLC matrix is not advantageous every time. There are certain constraints which are crucial for the application of these systems into many devices. In this article, we have discussed the concentration and temperature dependence of vital properties of dye-doped FLC. In this study there is improvement in contrast ratio by dye doping due to enhancement in plane switching for dye-doped FLC.     

Surface anchoring effect on guest–host ferroelectric liquid crystal response time – an electro-optical investigation

The effect is reported of surface anchoring on various electro-optical parameters of a pure ferroelectric liquid crystal (FLC) and five mixtures with different concentrations of dye. The Anthraquinone D5 dye was used as guest entity, whereas Felix 17-000 was used as host in this investigation. The presence of dye molecules in the FLC dramatically affects the electro-optical properties compared with those of the pure system. Electro-optical parameters such as spontaneous polarization and rotational viscosity not only change with the addition of dye but they are also strongly dependent on the concentration of dye in pure FLC. The effect of dye molecules on the anchoring energy of the pure FLC system was also investigated. Due to strong anchoring energy on FLC substrate, its effect on response time was also studied. An improvement in the contrast ratio after dye doping was observed. The value of spontaneous polarization increases due to addition of dye, suggesting that molecular alignment improves, which is very useful from the application point of view.     

Effect of dichroic azo-dye doped on ferroelectric liquid crystals

In order to study the effect of mixing dye in ferroelectric liquid crystal （FLC） materials, the phase transition temperature and electro-optical properties of azo dye doped FLC samples have been investigated. All the properties have been found to be changed drastically. The results have revealed that not only the SmC^＊- SmA^＊ transition temperature decreased markedly by the addition of azo-dye, but also dye-doped FLC had lower threshold voltage and saturation voltage than the pure FLC.     

Effects of nanoparticle doping on the phase transitional behaviour of ferroelectric liquid crystal Langmuir–Blodgett composite films

Langmuir–Blodgett films of ferroelectric liquid crystals (FLCs) doped with a low concentration of functionalized Al: ZnO (AZO) nanoparticles were prepared and characterized. Pressure–area isotherms show that the nanoparticles as well as FLC composite systems have the capability to form stable monolayers at the air–water interface. The molecular interaction between nanoparticles and FLC molecules increased during barrier compression, which resulted in increased surface pressure. We observed various phases in isotherms with increasing concentration of nanoparticles in the FLC matrix. An X-ray diffraction profile at a low angle confirmed that FLCs retain their layer structure at a low concentration doping of AZO nanoparticles in the FLC matrix. Atomic force microscopy images indicate that low wt% composites are uniformly deposited without disturbing the translation behaviour of SmC* liquid crystals.     

Holographic diffraction gratings using polymer-dispersed ferroelectric liquid crystals

By controlling the morphology of holographic polymer-dispersed ferroelectric liquid crystals (FLCs), highly aligned FLC domains are obtained for diffractive optical applications. Rapid, thresholdless switching is observed for various grating pitch sizes between approximately 3 and approximately 12 microm. A simple phenomenological model is presented encompassing a distribution of domain sizes and an effective field that stabilizes the FLC domains to reflect the observed thresholdless switching and optical behavior.     

SWCNT doped ferroelectric liquid crystal: The electro-optical properties with enhanced dipolar contribution

Effect of single wall carbon nanotubes (SWCNTs) on electro-optical performance of a ferroelectric liquid crystal (FLC) has been studied. Voltage dependent spontaneous polarization and response time measurement has been made for the pure and SWCNT doped FLC system. Dielectric measurement has also been performed to understand the existing interaction between SWCNTs and FLC molecules. The results have shown increase in the value of spontaneous polarization and relative permittivity with slight slower response for the doped system. The observed properties of doped system revealed that the SWCNTs can perform well with FLC at low applied electric field to enhance the performance of LC devices.     

Phase separated composite films of liquid crystals

Phase separation of liquid crystals from a solution with polymers has long been studied and used to prepare polymer stabilized and polymer dispersed structures. They are formed by spatially isotropic phase separation. A new mode, in which the phase separation proceeds anisotropically, has recently been discovered. Known as phase separated composite films (PSCOF), the resultant structures are made of adjacent parallel layers of liquid crystal and solidified polymer. PSCOFs have been made with nematic, ferroelectric (FLC), and antiferroelectric (AFLC) liquid crystals. Liquid crystals in PSCOFs exhibit electro-optical properties not observed in devices prepared by conventional methods, polymer dispersion, or polymer stabilization methods. Devices incorporating FLCs possess grey scale and switch 100 times faster at low fields than conventional surface stabilized devices. This method makes it possible to prepare very flexible devices and devices with liquid crystal film thickness comparable to optical wavelengths with great ease.     

Polymer-doped ferroelectric liquid crystal: UV absorbance,fluorescence and electro-optical study

In this study, UV absorption, fluorescence and electro-optical study have been performed for pure ferroelectric liquid crystal (FLC) and its three different concentrations, i.e. 1%, 3% and 5% of fluorescent polymer in the pure FLC. We observed that there is higher value of UV absorption for fluorescent polymer-doped system in comparison to the pure system. We have found excitation wavelength, emission wavelength and quantum yield for all the mixtures. Quantum yield shows the probability of excited state being deactivated by fluorescence rather than non-radiative mechanism. The higher value of spontaneous polarization and faster response has been observed for fluorescent polymer-doped system compared to the pure FLC system. The electro-optical study also suggests that there exists an optimum concentration of fluorescent polymer for which we have found highest spontaneous polarization and least response time.     

Smectic-A*-smectic-C* transition in a ferroelectric liquid crystal without smectic layer shrinkage

The smectic layer spacing of a nonfluorinated ferroelectric liquid crystal (FLC) compound with almost no shrinkage and only minor tendency to form zigzag defects was characterized by small angle x-ray diffraction. The material lacks a nematic phase. The smectic-A*-smectic-C* phase transition was studied by measuring the thermal and electric field response of the optical tilt and the electric polarization. These properties are described very well by a Landau expansion even without introduction of a higher-order Theta(6) term. This result suggests a pure second-order phase transition far from tricriticality and differs considerably from the typical behavior of the A*-C* transition in most FLC materials.     

Evaluation of structural and adhesive properties of nylon 6 and PTFE alignment films by means of atomic force microscopy

Atomic force microscopy (AFM) has become a powerful technique for submicron investigation of surface properties. In this work we use the capability of this technique to investigate dielectric films used to align ferroelectric liquid crystals (FLC). In fact, the final performance of a surface stabilized FLC (SSFLC) flat panel display strongly depends on the alignment layer properties and quality. This work focuses on a comparison of two alignment films: the more conventional polyamide, nylon 6, and polytetrafluoroethilene (PTFE, commercially known as Teflon), only recently used as a new aligning material. A micromorphological characterization of the sample surfaces has been carried out in order to correlate structure with alignment properties of both polymer films. The results show varying roughness and periodicity wavelengths for the two alignment layers. These different properties can be related to different anchoring forces between aligning surfaces and FLC molecules and therefore to a different electrooptical response of SSFLC cells. In addition to the topographic characterization, AFM non-conventional measurements have been performed on alignment layers deposited on different transparent conductive oxides, such as indium tin oxide (ITO) and SnO₂, used to make electrodes in SSFLC displays. These measurements provide local information on the adhesive properties of the studied alignment materials as a function of substrate coating. These observations indicate less adhesion of PTFE with respect to nylon 6. Received: 16 April 2000 / Accepted: 16 April 2000 / Published online: 13 September 2000     

Switching behaviour of ferroelectric liquid crystals probed by optical second-harmonic generation

Ferroelectric properties of thin (1.5–4 μm planar cells of a ferroelectric liquid crystal (FLC) mixture are studied using electro-optic measurements, second-harmonic generation (SHG) and SHG interferometry. A switching behaviour of the FLC cells in external dc electric fields is observed. It is characterised by rotation of the polarisation plane of the transmitted light and by changes in the SHG intensity, phase and anisotropy dependences, which are attributed to a collective motion of the system as a ferroelectric uniform state with C₂ symmetry. Received: 16 October 2001 / Revised version: 18 March 2002 / Published online: 6 June 2002     

Linear and nonlinear optical properties of dye-doped KDP crystals: Effect of thermal treatment

Potassium dihydrogen phosphate crystals (KDP, KH₂PO₄) doped with the organic xylenol orange (XO) dye are grown, the XO concentration in the crystal matrix is about 10 ppm. The spectral and luminescent properties of nominally pure, dye-doped and dye-doped/annealed at 150 °C crystals (KDP, KDP:XO and KDP:XO_an) were measured. The annealing temperature effect on the degree of dye protonation in the crystal matrix is established. Analysis of the IR-absorption spectra reveals a strong interaction between the incorporated dye molecules and the hydrogen subsystem of the matrix. The nonlinear optical (NLO) properties of KDP, KDP:XO and KDP:XO_an crystals are studied within the self-action effect of picosecond laser pulses at 532 nm. The mechanism of photoinduced bleaching and the effects of laser beam self-focusing (in KDP) and self-defocusing (in KDP:XO and KDP:XO_an) are supposed to be due to resonance excitation of the subsystems of intrinsic defects and dye molecules, correspondingly. For KDP:XO_an it is shown that thermal annealing of intrinsic crystal defects leads to domination of more effective NLO response of the subsystem of dye molecules that is correlated with photoluminescence data.     

Frustration between syn- and anticlinicity in mixtures of chiral and non-chiral tilted smectic-C-type liquid crystals

We study the effects of mixing ferroelectric and antiferroelectric liquid-crystal compounds (FLCs and AFLCs) when the former are strictly synclinic and the latter strictly anticlinic, i.e. one mixture component exhibits only SmC* and the other only SmC a* as tilted phase. Three different paths between syn- and anticlinicity were detected: transition directly between SmC* and SmC a*, transition via the SmC_β* and SmC_γ* subphases, or by “escaping” the clinicity frustration by reducing the tilt to zero, i.e. the SmA* phase is extended downwards in temperature, separating SmC* from SmC a* in the phase diagram. The most common path is the one via the subphases, demonstrating that these phases appear as a result of frustration between syn- and anticlinic and, consequently, between syn- and antipolar order. For assessing the role of chirality, we also replaced the FLC with non-chiral synclinics. With one of the AFLCs, the route via supbhases was detected even in this case, suggesting that chirality --although necessary-- does not have quite the importance that has previously been attributed to the appearance of the subphases. The path chosen in the mixture study seemed to be determined mainly by the synclinic component, the subphase induction occurring only when the SmA*-SmC* transition was second order.     

Nonlinear optical properties of a channel waveguide produced with crosslinkable ferroelectric liquid crystals

A binary mixture of ferroelectric liquid crystals (FLCs) was used for the design of a channel waveguide. The FLCs possess two important functionalities: a chromophore with a high hyperpolarizability and photoreactive groups. The smectic liquid crystal is aligned in layers parallel to the glass plates in a sandwich geometry. This alignment offers several advantages, such as that moderate electric fields are sufficient to achieve a high degree of polar order. The arrangement was then permanently fixed by photopolymerization which yielded a polar network possessing a high thermal and mechanical stability which did not show any sign of degradation within the monitored period of several months. The linear and nonlinear optical properties have been measured and all four independent components of the nonlinear susceptibility tensor have been determined. The off-resonant d-coefficients are remarkably high and comparable to those of the best known inorganic materials. The alignment led to an inherent channel waveguide for p-polarized light without additional preparation steps. The photopolymerization did not induce scattering sites in the waveguide and the normalized losses were less than 2 dB/cm. The material offers a great potential for the design of nonlinear optical devices such as frequency doublers of low-power laser diodes. Received 7 March 2000     

Red colored transparent PMMA-SiO2 hybrid films

Red colored transparent organic-inorganic hybrids films of a cross-linked polymethyl-methacrylate (PMMA) and silica were prepared via the sol-gel route using tetraethoxy-silane (TEOS) as precursor and a commercial organic red dye. 3-(Trimethoxysilyl) propyl methacrylate (TMSPM) was used to make compatible the organic and inorganic components of the precursor solution mixture. Four type of colored hybrid films were deposited using precursor solutions with the reactants molar ratio 1:0.5:1 for TEOS: TMSPM:MMA, respectively, and four different weight contents of the organic red dye. The hybrid films were studied by Fourier transform infrared spectroscopy, atomic force microscopy and optical transmission and reflection spectroscopy. The hardness of the films was determined from a pencil hardness test referred to ASTM Standard D 3363-92. The results showed that the colored hybrid films have a thickness of about 2 μm and consist of a homogeneous cross-linked organic-inorganic matrix with embedded dye molecules very well dispersed. The hardness of the hybrid films was enhanced with respect to that of the pure organic component. The AFM measurements showed very flat and smooth film surfaces with rms average roughness about 0.3 nm. The optical properties of the hybrid films including their color properties were determined from optical transmission and reflection spectroscopy. We found that the intensity of the color in the hybrid films and the corresponding color coordinates depend on the amount of red dye in the hybrid films. Photodegradation studies were performed by monitoring the optical density of the films as a function of the exposure time to illumination for several values of illumination intensity.     

Dielectric and electro-optic behavior of pure ferroelectric liquid crystal material and the isomeric mixtures

We report here the dielectric relaxation spectroscopy and electro-optical properties of a ferroelectric liquid crystal (FLC) CB470 (R) and isomeric mixtures containing CB470 (R) and its optical isomer CB480 (S) in two different concentrations. We have studied with two isomeric mixtures: mixing CB470 (R) and CB480 (S) in the ratio 95:5 (w/w) and 90:10 (w/w). With increase of percentage of CB480 (S) in the mixture the spontaneous polarization (P_S) value has been decreased due to cancellation of a part of the dipolar contribution, since the direction of P_S of the two isomers are in opposite direction. The tilt angles of the pure FLC materials and the isomeric mixtures remain almost unchanged. The dielectric strength of Goldstone mode is almost proportional to the spontaneous polarization (P_S). An additional collective mode has been observed in the SmC¹ phase due to the formation of domain with large periodicity in pure FLC material and in one of the isomeric mixtures by unwinding the helical structures, which is known as domain mode. The existence of domain mode has been studied extensively with the variation of P_S in the isomeric mixtures. The result has been discussed.     

Electro-optic properties of nematic and ferroelectric liquid crystalline nanocolloids doped with partially reduced graphene oxide

Flakes of partially reduced graphene oxide (PRGO) were doped in nematic liquid crystals (NLCs) and ferroelectric liquid crystals (FLCs), respectively. The dielectric and electro-optical properties of NLCs doped with those flakes have been investigated. Threshold voltage and switching times are reduced by 30%–50%. This is primarily due to the decrease of the elastic properties of the nanocolloids compared to the non-doped nematics. The influence of the PRGO flakes on the spontaneous polarization, tilt angle and switching time of FLCs was investigated too. Such flakes reduce the response time by 40%–60%, increases spontaneous polarization by 20%–25% and increase the tilt angle by 15%–20%.     

Dopant-enhanced ablation of nitrocellulose by a nitrogen laser

The photoetching behavior of pure nitrocellulose and of nitrocellulose dyed with stilbene-420, coumarin-120 and rhodamine 6G by 337 nm nitrogen laser pulses has been studied. Ablation with a low power nitrogen laser is hereby reported for the first time. A two step photochemical mechanism is proposed to account for the ablation of the pure material. With the addition of dyes strongly absorbing at 337 nm the photoetching rate of nitrocellulose can be increased significantly. This increase is proportional to the molar extinction coefficient of the dye at 337 nm and its concentration in the polymer. The photoetching mechanism and the energy transfer processes from the dye to the polymer are discussed in detail.     

Effect of conducting polymer poly (3,4-ethylenedioxythiophene) (PEDOT) nanotubes on electro-optical and dielectric properties of a ferroelectric liquid crystal

A detailed comparative study of the dielectric and electro-optical properties of a ferroelectric liquid crystal (FLC) and FLC after having doped with conducting polymer Poly (3,4-ethylenedioxythiophene) (PEDOT) nanotubes is done. The electro-optic study reveals a lower electrical response time, rotational viscosity and spontaneous polarization in the FLC/PEDOT nanocomposite system. By fitting the capacitance with voltage in a Preisach model, four dipolar species in both FLC and composites system have been obtained. The orientation of the four dipolar species in the composites system is such that the effective dipole moment in the transverse direction of the FLC molecule is less than that in FLC compound.