LC Layer Research Articles

Low voltage liquid crystal microlens array based on polyvinyl alcohol convex induced vertical alignment

ABSTRACT In this study, low driving voltage liquid crystal microlens array (LCMLA) based on polyvinyl alcohol (PVA) convex array was fabricated by facile method. The PVA convex array served as a vertical alignment layer led to formation of hybrid-aligned LC layer with concentric nonuniform distribution refractive index in planar LC cell, conducive to lens usage. The virtual focal length of LCMLA was short during the initial state. At voltages from 0 to 0.6 V rms, the focal length was tuned from – 4.5 cm to ∞. The low surface tension of the surface outside the lens unit reduces the anchoring force of LC which led to very low driving voltage. In sum, the proposed LCMLA possessed low power consumption, compact structure and simple fabrication, uniform structure in large scale, conducive to future development and applications of high-quality diffusers, image processing and displays.

Polymer-sustained-alignment liquid crystal cell produced from the monomer containing benzophenone-moiety and its image-sticking property

ABSTRACT As a model of polymer-sustained-alignment liquid crystal display (PSA-LCD), the LC cells having the polymer layers produced from mixed monomers of 4,4ʹ-dimethacryloyl-oxy-biphenyl (4,4ʹ-DMABiph) and 4,4ʹ-dimethacryloyl-oxy-benzophenone (4,4ʹ-DMABenz) were fabricated and investigated image-sticking phenomenon with evaluations of residual direct current voltage (V rDC) and difference in pretilt angle before and after application of alternate current voltage (∆tilt). A rate constant of the polymerisation became large with an addition of a small amount of 4,4ʹ-DMABenz into 4,4ʹ-DMABiph. The V rDC was effectively improved with the addition of a small amount of 4,4ʹ-DMABenz because the concentrations of radical and ion in the LC layer were decreased due to enlargement for the rate constant of the polymerisation. The Δtilt was small enough only below 14% of 4,4ʹ-DMABenz in the mixed monomers of 4,4ʹ-DMABiph and 4,4ʹ-DMABenz. We confirmed that the PSA-LC cell produced from the mixed monomers of 4,4ʹ-DMABiph and 4,4ʹ-DMABenz with the weight ratio of 4,4ʹ-DMABenz between 9% and 14% was useful for obtaining the small level of image sticking.

Microwave Liquid Crystal Enabling Technology for Electronically Steerable Antennas in SATCOM and 5G Millimeter-Wave Systems

Future satellite platforms and 5G millimeter wave systems require Electronically Steerable Antennas (ESAs), which can be enabled by Microwave Liquid Crystal (MLC) technology. This paper reviews some fundamentals and the progress of microwave LCs concerning its performance metric, and it also reviews the MLC technology to deploy phase shifters in different topologies, starting from well-known toward innovative concepts with the newest results. Two of these phase shifter topologies are dedicated for implementation in array antennas: (1) wideband, high-performance metallic waveguide phase shifters to plug into a waveguide horn array for a relay satellite in geostationary orbit to track low Earth orbit satellites with maximum phase change rates of 5.1°/s to 45.4°/s, depending on the applied voltages, and (2) low-profile planar delay-line phase shifter stacks with very thin integrated MLC varactors for fast tuning, which are assembled into a multi-stack, flat-panel, beam-steering phased array, being able to scan the beam from −60° to +60° in about 10 ms. The loaded-line phase shifters have an insertion loss of about 3 dB at 30 GHz for a 400° differential phase shift and a figure-of-merit (FoM) > 120°/dB over a bandwidth of about 2.5 GHz. The critical switch-off response time to change the orientation of the microwave LCs from parallel to perpendicular with respect to the RF field (worst case), which corresponds to the time for 90 to 10% decay in the differential phase shift, is in the range of 30 ms for a LC layer height of about 4 µm. These MLC phase shifter stacks are fabricated in a standard Liquid Crystal Display (LCD) process for manufacturing low-cost large-scale ESAs, featuring single- and multiple-beam steering with very low power consumption, high linearity, and high power-handling capability. With a modular concept and hybrid analog/digital architecture, these smart antennas are flexible in size to meet the specific requirements for operating in satellite ground and user terminals, but also in 5G mm-wave systems.

Polymer-sustained-alignment liquid crystal cell produced from the monomer containing benzilketal-group and its image sticking property

ABSTRACT As a model of polymer-sustained-alignment liquid crystal display (PSA-LCD), the LC cells having the polymer layers produced from mixed monomers of 4,4ʹ-dimethacryloyl-oxy-biphenyl (4,4ʹ-DMABiph) and 4,4ʹ-dimethacryloyl-oxy-benzilketal (4,4ʹ-DMABzk) were fabricated and investigated image sticking phenomenon with evaluations of residual direct current voltage (V rDC) and difference in pretilt angle before and after application of alternate current voltage (∆tilt). A rate constant of the polymerisation became remarkably large with an addition of a small amount of 4,4ʹ-DMABzk into 4,4ʹ-DMABiph. The V rDC was effectively improved with the addition of a small amount of 4,4ʹ-DMABzk because the concentrations of radicals and/or ions in the LC layer were decreased due to enlargement for the rate constant of the polymerisation. The Δtilt was small enough only below 10% of 4,4ʹ-DMABzk in the mixed monomers of 4,4ʹ-DMABiph and 4,4ʹ-DMABzk. We confirmed that the PSA-LC cell produced from the mixed monomers of 4,4ʹ-DMABiph and 4,4ʹ-DMABzk with the weight ratio of 4,4ʹ-DMABzk between 6% and 10% was useful for obtaining the small level of image sticking.

Stabilisation of reverse twisted nematic structure by forming polymer matrix in the vicinity of the alignment layer

ABSTRACT Reverse Twisted Nematic (RTN) LCDs, which use the liquid crystalline materials twisting opposite from the one for usual TN-LCDs, have been reported to be driven by low driving voltage. RTN molecular arrangement can be formed by applying electric voltage to the splay twist state. The formed RTN state can be maintained tentatively, however, returns into splay twist state gradually without applied voltage. For stabilisation of RTN state, polymer matrix formation in the whole LC region was effective but caused driving voltage to increase. The usage of high pretilt angle alignment layers and the introduction of wall structures into LC layer were proposed. However, these methods had the defects like low transmittance and the complicated procedures, respectively. In this article, it is reported that RTN state can be stabilised by the polymer matrix formation just in the vicinity of alignment layer. This phenomenon is considered to be the first successful example in which the LC molecular arrangement twisting opposite from the intrinsic twist direction can be stabilised by fixing the structure near alignment layer surface. By this method, the usage of RTN-LCDs of low driving voltage can be realised.

Temperature-independent quarter wave plate compensated liquid crystal shutter with a self-diagnostic shutter monitor

A temperature-independent quarter wave plate compensated liquid crystal shutter (QWP-LCS) with a self-diagnostic shutter monitor is proposed. During the QWP-LCS operation, the QWP is mechanically switched to become a positive (P-) QWP. When the bottom LC layer is switched to become a negative (N-) [P-] QWP by applying a suitable electric field, the QWP-LCS becomes a null [λ/2] plate and is closed [opened]. When the QWP-LCS is in close state and the embedded optical sensor detects light leakage caused by the temperature variation, the LCs in bottom layer are automatic tuned electrically by the self-diagnostic shutter monitor to compensate for the change in phase retardation resulting from the temperature variation and keep the QWP-LCS closed. Moreover, when the QWP-LCS is in open state, the exposure of the QWP-LCS is corrected by the self-diagnostic monitor by using its built-in database. Hence, the exposure of the proposed QWP-LCS is temperature-independent. The complete operation flow of the self-diagnostic shutter monitor is discussed in detail. The monitor can be extended to all nematic LCSs with specific limitations. The temperature-independent shutter can be applied to various fields, such as laser and imaging systems.

Optical linear and nonlinear properties of hybrid liquid crystal cells containing gold island films

We present a new type of LC cell with a gold nano-island film deposited on one of electrodes. A nematic LC layer (a mixture E7) is 20.7 μm-thick. Nonlinear optical characteristics were measured in dependence on a low dc voltage (0.096-0.59 V/μm) and for two angle of the cell rotation: and Our cell exhibits excellent nonlinear properties, namely, high diffraction efficiency, strong energy transfer, low switching voltage, its nonlinear refractive index cm2/W and susceptibility esu. The first-order diffraction efficiency achieves the maximum (more 10%) at 0.24 V/μm. Exclusion is a long self-erasure time – the most fast time (a few second) is observed at 0.096 V/μm. We believe it should be decreased with the decrease of LC thickness.

Electronically Tunable Liquid-Crystal-Based F-Band Phase Shifter

We propose an F-band phase shifter based on the nematic liquid crystals (NLCs). The proposed phase shifter is formed by a voltage-controlled cavity through introducing an NLC layer between a dipole structure array and a metal floor. Under the action of electric field, the orientation of the NLC molecules will be deflected. We adjust the resonant frequency and phase of the reflected electromagnetic (EM) wave by tuning the permittivity. The transmission characteristics and the LC parameters are calculated and analyzed for EM waves within the frequency range from 85 to 115 GHz. The LC-based device with a $30\times 30$ array of two parallel unequal dipoles is printed on a quartz substrate, with 4 cm $\times {4}$ cm area and $490~\mu \text{m}$ thickness. The experimental results show that phase shift of zero to 350.7° is achieved at 104.2 GHz by changing the applied bias voltage on the LC layer from 0 to 20 V. Considering the anisotropy and inhomogeneity of the LC, an improved electrification model is established and compared with the test results. The proposed phase shifter is expected to find several applications in millimeter wave and terahertz reconfigurable antenna systems.

Deformed lying helix transition and lasing effect in cholesteric LC layers at spatially periodic boundary conditions

ABSTRACTThe field-induced orientational transition in layers of cholesteric liquid crystals with spatially periodic modulation of the surface anchoring is studied by numerical simulations and experimentally. The modulation of the surface anchoring is implemented using high-resolution focused ion-beam treatment of a polymer film providing planar alignment conditions. A specific feature of the orientational transition is that the thermodynamically stable initial planar structure with the helix axis along the normal to the layer transforms under an electric field into an equilibrium structure with a deformed lying helix (DLH) in the plane of the layer. For such a transition to occur the natural pitch of the cholesteric helix must be substantially less than the anchoring modulation period. The appearance of the DLH with a pitch corresponding to the anchoring modulation period results in strongly enhanced first-order diffraction efficiency. The orientational DLH transition is characterised by a rather narrow driving voltage range with a pronounced hysteresis. The waveguide lasing effect with characteristic of the deformed helix spectral modes is demonstrated in a range of the DLH transition.

Fast and low loss flexoelectro-optic liquid crystal phase modulator with a chiral nematic reflector

In this paper, we demonstrate a flexoelectro-optic liquid crystal phase-only device that uses a chiral nematic reflector to achieve full 2π phase modulation. This configuration is found to be very tolerant to imperfections in the chiral nematic reflector provided that the flexoelectro-optic LC layer fulfils the half-wave condition. Encouragingly, the modulation in the phase, which operates at kHz frame rates, is also accompanied by low amplitude modulation. The configuration demonstrated herein is particularly promising for the development of next-generation liquid crystal on silicon spatial light modulators.

Highly sensitive liquid crystal optically addressed spatial light modulator for infrared-to-visible image up-conversion.

A liquid crystal optically addressed spatial light modulator based on an InGaAs photodiode array operating at low light levels is investigated in the short wavelength infrared (SWIR) spectral band to serve as a SWIR-to-visible imaging upconversion device. It consists of InGaAs/InP heterojunction photodetectors array sandwiched with a nematic LC layer. The photodiode array is composed of a 640×512 InGaAs/Inp heterojunctions, grown on InP substrate with a 15μm pitch. Full up-converted visible images in stills and video modes were demonstrated with SWIR light intensities as low as 70 nW/cm2 or less than pW/pixel. The influence of operation frequency on the performance of the device was found theoretically and experimentally to be crucial for a proper operation of the device. The optimum sensitivity and contrast of the device was found at a frequency around 70Hz. To the best of our knowledge, this is the first time that such a high performance upconversion device is presented and that actual visible images are obtained.

Paper] Evaluation of Capability to Maintain Thickness of LC layer of Flexible LCDs with Bonding Polymer Spacers

We measured the thickness of the LC layer of flexible liquid crystal displays (LCDs) in non-curved and curved states and evaluated a capability to maintain the thickness of the LC layer of flexible LCDs with bonding polymer spacers. We found that both the rigid spacers suppressing a compressive force and the spacers bonding two substrates were required to maintain the thickness of the LC layer of curved LCDs. We developed the composite spacer structure using the columnar photo spacer and the bonding polymer spacer; these maintained a constant thickness of LC layer over the entire liquid crystal cell and will improve the contrast ratio of flexible LCDs even in a small radius of curvature.

Transparent conductors for Mid-infrared liquid crystal spatial light modulators

Transparent conductors (TCs) are required for liquid crystal spatial light modulators (LC-SLMs) in order to set up an electric field across the LC layer. In the middle infrared (Mid-IR) range (λ = 2 to 5 μm), LC-SLMs can offer a low-cost, non-mechanical, random-access and compact alternative to the gimbaled mirrors used currently for Mid-IR laser beam-steering. Indium tin oxide (ITO) is the industry standard for applications in the visible spectrum but it performs poorly in the IR, with a transmittance <20% for Mid-IR wavelengths. Little work has been done to develop a comparable material which fulfils the required properties in the Mid-IR: A sheet resistance allowing operation at typical frequencies (≈1 kHz) and, if patterned, with minimal voltage drop along the electrode, a transmittance >50% in the target range, chemical, thermal and mechanical robustness which can endure subsequent processing, and ability to be patterned at low-cost to a resolution comparable to the wavelengths investigated. Ni and Cu ultra thin metallic films (UTMFs), CuxO thin films (TFs), and chemical vapour deposition (CVD) grown mono-layer graphene were investigated. Ni UTMFs and graphene were found to have the best performance with sheet resistance values of 747±86Ω/□ and 360±34Ω/□ respectively for samples having a transmittance of 65% and 97% at λ = 2.3 μm. Both Ni UTMFs and CVD mono-layer graphene were found to be suitably stable with age. An increase of sheet resistance after baking was recorded due to oxidation and desorption of contaminating dopants respectively. Ni UTMFs were found to be patternable down to a 3 μm resolution, limited by the mask, using a standard photo-lithographic lift-off process. Transmissive LC cells with a maximum phase shift of 3π at λ = 2.3 μm were assembled with both Ni UTMFs and mono-layer graphene as TCs on sapphire, with the former having transmittance of 18.7% and contrast ratio of 25.0, and the latter having transmittance of 81.1% and contrast ratio of 50.7.

P‐153: High Contrast Ratio LCD with an In‐cell Polarizer

We develop a rigorous model to simulate an LCD's contrast ratio (CR) and viewing angle by considering the depolarization effect. To improve CR, an in‐cell polarizer is added between LC layer and color filter array. The obtained maximum CR of a multi‐domain vertical alignment LCD is boosted to &gt; 20,000:1.

Structure-property relationship and design of plasma-sprayed La2Ce2O7/8YSZ composite coatings for gas turbine blades

Novel lanthanum-cerium oxide/8 wt% yttria partially stabilized zirconia (LC/8YSZ) thermal barrier coatings (TBCs) were deposited by supersonic atmospheric plasma spraying. The thermal insulation temperature and thermal shock resistance of LC/8YSZ double-ceramic-layer TBCs (DCL-TBCs) were quantitatively evaluated by a burner rig test. The results showed that the thermal insulation temperature increased with the increase of LC layer thickness in DCL-TBCs. When the thickness ratio between LC layer and 8YSZ layer was close to 1:1, the DCL-TBCs had the highest thermal shock resistance. LC/8YSZ thickness ratio significantly affected the energy release rate and the stress induced by thermal gradient or sintering. The sintering stress was found to be the main reason that caused the delamination of LC layer, however, the stress induced by thermal gradient resulted in the spallation of YSZ layer.

Methods for polymer-stabilization of molecular orientation in LC-waveguiding structures

In this letter we present a new way for constructing and developing LC-based waveguiding structures. Specifically, liquid crystalline material enriched with monomer in the proper proportions is implemented in our approach and a waveguide geometry can be created in the LC cell thanks to the photo-polymerization process allowing for LC molecules to be suspended in desired position and orientation. The latter can be enforced by external fields and UV-irradiation can be performed trough the mask of desired geometry. In this way, regions with different molecular textures (e.g. planar and homeotropic) and thus waveguiding structures can be easily created within LC layer. The main idea of this letter is not only to show a prove-of-concept of the polymer-imposed LC stabilization for such planar optical devices but to discuss differences in methodology of their practical realization. Full Text: PDF ReferencesQ. Li, Liquid crystals beyond displays: chemistry, physics, and applications (John Wiley &amp; Sons, 2012). CrossRef J.P. Lagerwall, G. Scalia, Current App. Phys. 12, 1387 (2012). CrossRef A. Ciferri, Polymer liquid crystals (Elsevier, 2012). DirectLink D. Yang, Fundamentals of liquid crystal devices (John Wiley &amp; Sons, 2014). CrossRef L. Vicari, Optical applications of liquid crystals (CRC press, 2016). DirectLink I. Dierking, Adv Mater 12, 167 (2000). CrossRef K. Rutkowska, M. Chychłowski, M. Kwaśny, I. Ostromęcka, J. Piłka, U. Laudyn, "Light propagation in periodic photonic structures formed by photo-orientation and photo-polymerization of nematic liquid crystals", Opto-Electr. Rev. 25, 118 (2017). CrossRef K A. Rutkowska, M. Chychłowski, U.A. Laudyn, Proc. SPIE 10325, 1032506 (2017). CrossRef B. Turowski, K.A. Rutkowska, Phot. Lett. Poland 9, 82 (2017). CrossRef H. Ren, Y. Lin, S. Wu, Opt. Commun. 261, 296 (2006). CrossRef V. Mucci and C. Vallo, J Appl Polym Sci 123, 418 (2012). CrossRef U.A. Laudyn, M. Kwaśny, K. Jaworowicz, K.A. Rutkowska, M.A. Karpierz, G. Assanto, Phot. Lett. Poland 1, 7 (2009). CrossRef M.S. Chychłowski, S. Ertman, E. Nowinowski-Kruszelnicki, T.R. Woliński, Mol. Cryst. Liquid Cryst. 553, 127 (2012). CrossRef

Transmittance variable liquid crystal modes with a specific gray off-state for low power consumption smart windows

For low power consumption smart windows, we developed new ECB and VA mode LC devices with a specific off-state transmittance and an on-state transmittance variable from minimum to maximum values according to applied voltage. The off-state transmittance of the devices could be chosen to be a value suitable for most use time by selecting appropriate retardation values of LC layer and a retarder, which enables considerable reduction of power consumption when applied to devices, for instance, smart windows. The design rules of them were theoretically derived and the electro-optical properties stated above were exposed by corresponding theoretical simulations and experimental measurements.

A transflective polymer-stabilised blue-phase liquid display with partitioned wall-shaped electrodes

ABSTRACTA transflective polymer-stabilised blue-phase liquid crystal display (BP-LCD) with partitioned wall-shaped electrodes is proposed. The etched polymer layer contributes to balance the optical phase retardation between transmissive (T) and reflective (R) regions. The partitioned wall-shaped electrodes generate uniform and horizontal fields throughout the entire LC layer to induce isotropic-to-anisotropic transition in the blue-phase liquid crystal medium through Kerr effect. Consequently, the accumulated phase retardation along beam path is large, resulting in reasonable low operation voltage and high transmittance both in T and R regions. This approach enables the BP-LCD to be addressed by amorphous silicon thin-film transistors. Moreover, it exhibits wide viewing angle and a well-matched gamma curve.

Electrically tunable long-period fiber gratings with low-birefringence liquid crystal near the turn-around point

In this work, an electrically tunable long-period fiber grating (LPFG) coated with liquid crystal layer (LC) is presented. As a LC layer, a prototype low-birefringence 1550A LC mixture was chosen. As a LPFG host, two types of gratings were studied: the LPFGs based on a standard telecommunication fiber, produced by an electric arc technique with a period of 222 μm, and the LPFGs based on a boron co-doped fiber written by a UV technique with a period of 226.8 μm. The relatively short period of these gratings allowed exploiting unique sensing properties of the attenuation bands associated with modes close to the turn-around point. Experiments carried out showed that for the UV-induced LPFG with a LC layer, on the powered state the attenuation band could be offset from the attenuation band measured in the unpowered state by almost 130 nm. When the arc-induced LPFG was coated with the LC, the depth of the attenuation band could be efficiently controlled by applying an external E-field. Additionally, all experimental results obtained in this work were supported by the theoretical analysis based on a model developed with Optigrating v.4.2 software.

Light-induced electric field generated by photovoltaic substrates investigated through liquid crystal reorientation

Liquid crystal reorientation is exploited to analyze the electric field generated by light irradiation in iron-doped lithium niobate crystals. The evaluation of the strength of this light-induced field is based on the measurement of the phase shift induced in a probe light beam by a liquid crystal cell built with two z-cut iron-doped lithium niobate crystals as substrates. Then, the field profile has been determined starting from a Gaussian-like surface charge density following the model described in the text. The director profile corresponding to the modelled electric field is in good agreement with the director reorientation evaluated experimentally by means of the light-induced phase shift. This investigation gives a new approach to study the effects based on the photovoltaic response of lithium niobate crystals. Moreover, the characterization of the electric field optically generated inside the LC layer is highly desirable in view of the realization of new all-optical devices to be integrated in optofluidic platforms.