Polymer-stabilized Liquid Crystal Research Articles

Optical and electrical control of circularly polarised fluorescence in CdSe quantum dots dispersed polymer stabilised cholesteric liquid crystal shutter

ABSTRACTIn the present communication, we report on the synthesis and electro-optic investigation of photosensitive CdSe quantum dots (QDs) dispersed polymer stabilised liquid crystal (PSCLC) luminescent gel. The assembly of the fluorescence properties of CdSe QDs facilitate the anisotropy of PSCLC gel and hence manipulates the optical and electro-optic switching properties, which was further investigated using polarised fluorescence spectrophotometer. The circularly polarised fluorescence intensity was tuned electrically so as to affect the orientations of liquid crystal in the helix. It was found that the electro-optic switching behaviour of QDs-doped sample predicts the improvement in threshold voltage and hence makes them applicable for the switchable liquid crystal contrivances with low power consumption.

Dielectric and electro-optical properties of polymer-stabilized liquid crystal system

In this work, we report the results of dielectric and electro-optical properties as a function of temperature for both pure liquid crystal matrix and polymer-stabilized liquid crystal (PSLC). The threshold and saturation voltages have been determined from transmission–voltage curves. We have studied the polymer domains formation in PSLC with variation of concentration of polymer in liquid crystal matrix. It is observed that the dielectric anisotropy of PSLC is significantly influenced by the polar order present in the polymer domains environment. A delicate interplay between the orientational order of liquid crystal and polymeric domains determines the molecular orientations of PSLC with respect to the director of the LC system.

High-efficiency and fast-switching field-induced tunable phase grating using polymer-stabilized in-plane switching liquid crystals with vertical alignment

We demonstrate a high-efficiency and fast-switching phase grating based upon a polymer-stabilized liquid crystal device driven by in-plane switching electric field. Periodical modulation in field-induced refractive index profile is realized in the in-plane switching liquid crystal (LC) cells with vertical alignment, and voltage-dependent diffraction patterns are obtained. The diffraction efficiency of the LC phase grating is varied by voltage applied to the interdigitated electrodes, and the first-order diffraction efficiency of the device achieves 28.5% at 6.25 V μm−1. The dynamic response is improved by surface polymer stabilization. This device has potential advantages of versatile design and ease of fabrication.

Deformation-free switching of polymer-stabilized cholesteric liquid crystals by low-temperature polymerization

We investigate the fabrication of 'deformation-free' polymer/cholesteric liquid crystal (PChLC) nanocomposites by controlling the degree of phase separation in a precursor mixture containing mesogenic monomer and low molecular-weight LC molecules. Scanning electron microscopy investigations reveal that even in mixtures containing only 6.6 wt% of monomer, nano-sized LC domains can be formed by low-temperature polymerization of −20 °C that lead to the 'deformation-free' response. PChLC nanocomposites with reduced monomer concentrations exhibit improved tuning range of the refractive index and a short decay time of about 20 μs, making them more practical for real-life applications.

Effect of anti-parallel and twisted alignment techniques on various propertiesof polymer stabilised liquid crystal (PSLC) films

ABSTRACTThin composite films consisting of liquid crystal (LC) domains surrounded by polymer networks, termed as polymer stabilised liquid crystals (PSLCs), were prepared by photo-polymerisation of a pre-polymer dissolved in LC. Four composite films were prepared with different rubbing directions and with and without electric field during photo-polymerisation. Morphological characterisation carried our using a polarising optical microscope (POM) and a scanning electron microscope (SEM) reveal significant changes in LC domain morphology and associated polymer networks with the application of electric field during the fabrication of the films. The electro-optic (EO) properties of PSLC films placed between two crossed polarisers were studied using a He–Ne laser under an action of externally applied electric field. It was found that the PSLC film with twisted alignment and polymerised in the presence of electric field showed better EO properties than other films. Transmittance obtained by EO studies was verified with absorbance studies using a Ultraviolet-Visible spectrophotometer. The dielectric behaviour of PSLC films in the frequency range 20–20 MHz was investigated using a precision impedance analyser. The obtained data were modelled using Debye and Cole–Cole methods to calculate relaxation time and distribution parameter. The relaxation time calculated through the Cole–Cole model is in agreement with response time.

Electric Field-Driven Shifting and Expansion of Photonic Band Gaps in 3D Liquid Photonic Crystals

Highly tunable 3D liquid photonic crystals are demonstrated using low-dc-field-driven polymer-stabilized blue-phase liquid crystals. The central wavelength of the photonic band gap can be reversibly shifted to more than 200 nm away from the original position. Besides, by controlling the polymerization-induced morphology variations, the band gap can also be expanded from a bandwidth of around 30 nm to at least 310 nm, the first time a “white” blue phase is observed. Both types of band-gap modulation, namely, shifting and expansion, can be independently manipulated in any crystal axis without affecting the lattice spacings in the other dimensions. We envision polymer-stabilized blue-phase liquid crystals as a fascinating platform for photonic applications, such as 3D lasers, nonlinear optics, and photonic integrated circuits.

Polymer-stabilized blue-phase liquid crystal grating cured with interfered visible light.

In this paper, we demonstrate a holographic polymer-stabilized blue-phase liquid crystal grating fabricated using a visible laser. As blue phase is stabilized by the interfered light, polymer-concentration gradient is achieved simultaneously. With the application of a uniform vertical electric field, periodic index distribution is obtained due to polymer-concentration gradient. The grating exhibits several attractive features such as polarization-independency, a broad temperature range, sub-millisecond response, simple fabrication, and low cost, thus holding great potential for photonics applications.

Photoinduced polymer-stabilised chiral nematic liquid crystal films reflecting both right- and left-circularly polarised light

A phototunable polymer-stabilised liquid crystal film reflecting both right- and left-circularly polarised light has been successfully fabricated by a washout/refill method. The film was obtained by prefabricating the polymer network with a left-handed helical structure and then refilling a light-driven chiral nematic liquid crystal with a right-handed helical structure into the network. Interestingly, the reflection wavelength and the reflectivity properties can be precisely tuned by UV-light irradiation. A hyper-reflective notch, the reflectivity of which approaches 100%, was achieved in a layer when the pitch lengths of the two opposite helical structures are almost the same. Moreover, their original state can be properly returned by visible-light irradiation.

Phase modulation using different orientations of a chiral nematic in liquid crystal over silicon devices

ABSTRACTModulation of the intensity of light by high quality reflective micro-displays is predominantly carried out by liquid crystal over silicon (LCoS) spatial light modulator (SLM) technology for applications such as pico-projectors. Wider use of these devices, in applications such as computer-generated holography and optical correlation, is limited by their phase modulation ability and illumination polarisation state dependence. These devices rely on planar or twisted nematic liquid crystals to modulate the light, but due to their viscoelastic properties they are inherently slow. Research into the use of the polymer stabilised blue phase has already shown that it can offer high speed phase modulation. However, other chiral nematic orientations are yet to be compared in LCoS devices. In this article, we demonstrate that polymer-stabilised chiral nematic liquid crystal electro-optical effects can offer phase modulation in silicon backplane devices. The uniform standing helix and focal conic textures are shown to be independent of the input light polarisation state and the uniform lying helix is shown to be polarisation dependent. These optical responses are then compared with that of the blue phase to identify a suitable orientation for further development in LCoS technology in order to find a high-speed, full phase modulating material.

Nematic Director Configuration, Local Order and Microviscosity in a PSLC Cell

The director configuration, the local order and the molecular-level reorientational dynamics (microviscosity) of the 5CB liquid crystal (LC) inside a polymer-stabilized LC (PSLC) cell prepared with the 4,4′-bis(6-(acryloyloxy)hexyloxy)biphenyl (BAB6) diacrylic monomer have been studied with the EPR spin probe technique across the nematic and the isotropic phase of the LC. The cells made from glass slides, either clean or coated with an aligning layer, were filled with pure 5CB or with 0.5, 1.0 or 2.0 wt% of BAB6, either in the monomeric state or polymerized. Local orientational order parameter (<P2>) in the non treated cells was always slightly lower than in the coated ones and both were lower than in the bulk LC. Order appeared to be independent of polymerization state or BAB6 concentration up to 1.0 wt%. Reorientational dynamics was essentially bulk-like in all cases, indicating that in our PSLC cells a full molecular mobility is retained. At the concentration of 2.0 wt%, in the coated cell with polymerized BAB6, the monodomain did not reform after a 90° rotation of the cell in the magnetic field, suggesting that the polymer network is, as expected, stabilizing the preexisting director configuration.

Vertically-Aligned Polymer Stabilized Liquid Crystals (VA-PSLC) with a Curing Voltage for Fast-Response Wavelength-Tuning Applications

We present experimental results obtained for a Vertically-Aligned Polymer Stabilized Liquid Crystal (VA-PSLC) with a curing voltage. The curing voltage was found to help reduce the otherwise strong scattering effect of the VA-PSLC. This liquid crystal mode was placed inside a Fabry-Perot cavity to achieve a wavelength tunable filter. Wavelength tuning range was found to decrease as curing voltage increased, which is consistent with what we expected since molecules had a larger pretilt angle when the curing voltage was higher. Shortening of response time (>10X) was found as polymer concentration increased since the polymer effect helped improve the response speed. These filters can have potential applications in wavelength tuning applications (e.g. WDM) in telecommunication systems where high speed is desirable. This new liquid crystal mode of VA-PSLC with a curing voltage can also find applications where both fast response time and pure phase modulation are desirable.

Low-voltage and fast-response polymer-stabilized hyper-twisted nematic liquid crystal

We report a low-voltage and submillisecond-response polymer-stabilized hyper-twisted-nematic (HTN) liquid crystal cell with a large dielectric anisotropy host mixture. To correct the measured voltage-dependent transmittance, we have to take the voltage shielding effect of the alignment layers into consideration. Both Kerr effect and flexoelectro-optic effect contribute to the observed induced birefringence. To evaluate the dynamic responses of these two effects, we fit the decay time data with a double relaxation model. A good agreement between the experiment and simulation is obtained. Such a HTN cell still exhibits fast response time (<2ms) even at low temperature (0°C). Potential applications for display and photonic devices are foreseeable.

Laser-pointer-induced self-focusing effect in hybrid-aligned dye-doped liquid crystals.

Nonlinear optics deals with phenomena where “light controls light”; e.g., there is mediation by an intensity-dependent medium through which light propagates. This field has attracted much attention for its immense potential in applications dependent on nonlinear processes, such as frequency conversion, multiple-photon absorption, self-phase modulation, and so on. However, such nonlinearities are typically only observed at very high light intensities and thus they require costly lasers. Here, we report on a self-focusing effect induced with a 1 mW handheld laser pointer. We prepared polymer-stabilized dye-doped liquid crystals, in which the molecular director orientation gradually changes from homeotropic at one surface to homogeneous at the other. This is referred to as hybrid alignment. In such films, the threshold intensity needed to form diffraction rings was reduced by a factor of 8.5 compared to that in conventional homeotropic cells, which enabled the induction of the self-focusing effect with a laser pointer.

PSV-FLC Fabricated Using Photo-Alignment Technique

Polymer-stabilized V-mode Ferroelectric Liquid Crystals (PSV-FLCs) are attractive for a next generation of LC display. However, in the rubbing technique which is conventionally used for LC molecular alignment, the rubbing scratches form on the alignment film surface, and then, the rubbing scratch pattern is clearly observed in the texture of FLC. Therefore, the high contrast ratio cannot be obtained due to the optical leakage. In this research, we introduce the photo-alignment technique to PSV-FLC. As a result, the optical leakage can be largely suppressed by the photo-alignment technique, and thus, the photo-alignment technique is effective to PSV-FLCs.

High transmittance blue-phase liquid crystal display with improved protrusion electrodes

A high transmittance polymer-stabilised blue-phase liquid crystal display with improved protrusion electrodes is proposed. We add a small in-plane switching electrode on the top of each protrusion electrode in order to create the fringe fields above the protrusion electrodes. These fields contribute to decrease dead zones above the traditional protrusion electrodes so that the transmittance increases. Compared with the traditional protrusion structure, the proposed structure improves the transmittance by ~23%. Moreover, this structure has a wide viewing angle and a great tolerance property of misalignment.

Effect of polymer concentration on self-focusing effect in oligothiophene-doped polymer-stabilized liquid crystals

Nonlinear optics (NLO) has attracted much attention for its immense potential in applications. However, it typically requires very high light intensities. Recently, we reported that polymer stabilization efficiently enhances the self-focusing effect for dye-doped liquid crystals (LCs). Here we studied the effect of polymer concentration on self-focusing in the dye-doped polymer-stabilized LC (PSLC). Oligothiophene (TR5)-doped PSLCs with different polymer concentrations were prepared, and self-diffraction ring measurement was performed. As polymer concentration increased, the threshold light intensity for self-focusing effect first decreased then slightly increased. The lowest threshold light intensity of 3.4 W/cm2 was obtained at 10 mol%. Polymer stabilization of the LCs decreased the threshold intensity by a factor of 7. We also discussed the possible mechanism for the polymer concentration dependence of NLO response in the TR5-doped PSLC.

Maximizing Orientational Order in Polymer-Stabilized Liquid Crystals Using High Magnetic Fields

Polymer-stabilized liquid crystals (PSLCs) are materials composed of a polymer mesh in a continuous phase of liquid crystal. The polymer mesh provides an anchor point for alignment of the liquid crystalline bulk. The macroscopic extent of order in such systems depends on the order parameter of the liquid crystal (given by the temperature) and the domain order parameter, induced by external stimuli, such as (rubbed) substrates or magnetic or electric fields. We studied thick PSLCs where substrate interactions cannot be employed and used magnetic fields instead. We show how the polymerization conditions, i.e., the temperature and the magnetic field, influence the overall order parameter in 4-octyl-4′-cyanobiphenyl (8CB)-based PSLCs. Optimal macroscopic alignment was obtained in samples polymerized at room temperature and at magnetic fields in excess of 5 T. The effect of mesh network can be quantified by introducing a phenomenological constant, which is correlated to the order parameter at the polymerizatio...

Low voltage blue-phase liquid crystal display with triple-penetrating fringe fields

A low voltage polymer-stabilised blue-phase liquid crystal display (PSBP-LCD) with triple-penetrating fringe fields (TPFF) is proposed. The PSBP-LCD employs in-plane switching (IPS) electrodes and two etched substrates to generate the TPFF. These fields including strong horizontal electric fields are symmetrically distributed across the entire LC layer, so large phase retardation along the beam path is accumulated. As a result, the proposed structure successfully reduces the operating voltage below 10 V. Moreover, the proposed structure can obtain a wider viewing angle and a higher transmittance than the traditional IPS structure because of the generated multi-domain structures in the etched areas.

Polarisation switching and molecular relaxation behaviour of anthraquinone dye-dispersed polymer-stabilised ferroelectric liquid crystal composites

Guest–host polymer-stabilised ferroelectric liquid crystal (GH-PSFLC) composite films have been prepared with dispersion of small concentration (0.1, 0.25 and 0.5 wt%) of anthraquinone blue dye in PSFLC host matrix via a polymerisation-induced phase separation (PIPS) process. The variation in alignment and size of twisted fibril has been observed in the optical textures of the guest–host composites with different wt/wt ratio of anthraquinone dye. The electrical and dielectric properties of PSFLC mixture and its guest–host derivatives are studied. Our results showed that an optimum amount of dye concentration (0.1 wt%) enhances the dielectric permittivity as well as the spontaneous polarisation of the GH-PSFLC material in the SmC* phase.

Electro-optical properties of photochemically stable polymer-stabilized blue-phase material

Polymer-stabilized blue-phase liquid crystal (BPLC) comprising fluorinated compounds with high resistivity and photochemical stability is demonstrated. The Kerr constant, driving voltage, and response time of this BPLC are measured using an in-plane switching liquid crystal cell. At 20 °C, the measured total response time is faster than 0.7 ms and Kerr constant is 2 nm/V2. This fluorinated BPLC material is a promising candidate for next-generation photonic and display devices, because it can be used in active matrix addressed devices.