Blue organic seven segment display based on poly (9,9-dioctyfluorene)with β-phase emission

In this work, organic seven segment displays based on poly(9,9-dioctyfluorene), PFO, have been fabricated. PFO has consolidated as an attractive material for PLEDs due to its efficient blue emission [1] and high hole mobility. Additionally, PFO has a particular conformation, called β-phase associated to extended PFO chain conformation, which is of great interest for potential device applications because, among all others, it has the highest photoluminescence quantum efficiency [2] and the best colour stability [3]. The structure fabricated uses Indium Tin Oxide (ITO) as anode, Poly(3,4 -ethylenedioxythiophene) /poly(4- styrenesulfonate) (PEDOT:PSS) as hole transport layer and Ba:Al as cathode. After thoroughly cleaning the substrates (covered with ITO) a photolithography process is carried out in order to pattern the anode. Next, the organic layers (PEDOTT:PSS and PFO) are spin casted. Finally, metals (Ba~30 nm and Al~100 nm) are thermally evaporated in an atmosphere of 6x10 -6 Torr. PFO is dissolved in toluene at 1 % wt. A detailed description of the fabrication process can be found in [4]. Finally, the device is encapsulated (using an epoxy and a glass tap) and contacts are indium soldered on the pads. In figure 1, we can observe the shadow mask used for the anode photolitography process (left) and the final device lighting in a zero configuration (right)

Optically Transparent Beam-Steering Reflectarray Antennas Based on Liquid Crystal for Millimeter-Wave Applications

This study presents a method to realize an optically transparent beam-steering antenna. The RF and optical features of Liquid Crystal (LC) technology are used in combination with transparent metal mesh to realize the first optically transparent reconfigurable reflectarray (RA). Since the electric field of bias and Radio Frequency (RF) signals are highly non-uniform, the LC permittivity is both anisotropic and inhomogeneous thus the behavior of LC molecules needs to be obtained for accurate modeling prior to antenna design. A unit cell consisting of metallic mesh and LC is analyzed and LC director distribution is obtained. The director data are transformed into permittivity tensors in the entire LC volume and the LC is discretized in electromagnetic simulation software to perform full-wave periodic boundary simulation to model the anisotropy and inhomogeneity. The discretized model is approximated by a single dielectric block with a new permittivity range for GT7 LC material. A 10×10 RA is fabricated and measured in terms of optical and RF performance. The measured phase shift of the unit cell is 260˚ when the voltage is increased from 0 V to 40 V. The measured beam scans from -10˚ to 50˚ in the E-plane and from -50 to +50 in the H-plane with a 14.35 dBi maximum gain. The prototype optical performance is also measured. The benefits and drawbacks of current RF LC mixtures are discussed. It shows that with an appropriate LC mixture optimized for both RF and optical transmission, the LC-based optically transparent antennas are a viable solution for various new applications

An Optical Method for Pretilt and Profile Determination in LCOS VAN displays

Pretilt angle and cell thickness are two extremely important parameters in predicting the behavior of vertically aligned negative nematic liquid crystal (LC) displays. The accurate estimation of pretilt angle and cell thickness is not a trivial task when these devices work in reflective mode, as in liquid crystal on silicon (LCOS) vertically aligned nematic (VAN) displays. Usual experimental setups are based on the proportionality between the retardation of the polarization components of the incident light and the product effective birefringence times thickness. However, any attempt to separate the two product variables is cancelled out by symmetry from reflection. This work shows a relatively simple method capable of separating both variables, allowing accurate, independent measurements of pretilt and thickness, as well as other configurations details, such as residual twist. A simulation model based on the properties of actual reflective displays has been developed. An experimental setup specifically designed for measuring LCOS VAN cells has been prepared. Initial comparisons between experimental measurements of intensity and theoretical results showed some discrepancies that could be explained assuming that the LC profile contains a residual twist from the manufacturing process. Including that twist in the model, an excellent agreement between theory and experiment has been achieved. Matching simulations and experimental results yield separate determinations of pretilt angle and thickness, and give good estimates for the residual twist angle

Conductivity variations of multi-walled carbon nanotubes oriented in liquid crystal cells

The control of carbon nanotubes conductivity is generating interest in several fields since it may be relevant for a number of applications. The self-organizing properties of liquid crystals may be used to impose alignment on dispersed carbon nanotubes,thus control-ling their conductivity and its anisotropy. This leads to a number of possible applications in photonic and electronic devices such as electrically controlled carbon nanotube switch- es and crossboards. In this work, cells of liquid crystals doped with multi-walled nanotubes have been prepared in different configurations. Their conductivity variations upon switching have been investigated. It turns out that conductivity evolution depends on the initial configuration (either homogeneous, homeotropic or in-plane switching), the cell thickness and the switching record. The control of these manufacturing paramenters allows the modulation of the electrical behavior of carbon nanotubes

Asymmetric switching and charge transport in AFLC devices with dissimilar alignment layers

Surface stabilized antiferroelectric liquid crystal (AFLC) devices with dissimilar alignment layers on the two surfaces are characterized by a shift in the electro-optical response [1] so that it is symmetric with respect to a nonzero voltage (figure 1). Because of this shift, the switching state of a pixel which is initially set by applying a short, selection, voltage pulse, may be maintained without applying any holding, bias, voltage. The stability at zero volts, together with the inherent range of grey levels, typical for antiferroelectric liquid crystals, has some potentially very interesting applications. Generally, the magnitude of the voltage shift changes slowly over time, depending on the driving conditions and the materials used. This varying asymmetry is an important obstacle for the implementation of this technology in real devices. Fig. 1. Asymmetric transmission–voltage characteristic of an AFLC cell with dissimilar alignment layers. 2. The influence of charge in asymmetric AFLC devices Charges in AFLCs can considerably influence the switching behavior of the device [2][3]. Measurements of the electric current flowing towards the electrodes of the device when a voltage step is applied can elucidate the nature and the behavior of these charge

2D tunable graded index prism beam steering device based on nematic liquid crystals

Liquid crystal devices are being used in many non-display applications in order to construct small devices controlled by low voltage electronics without mechanical components. In this work, we present a novel liquid crystal device for laser beam steering. In this device the orientation of the liquid crystal molecules can be controlled. A change in the liquid crystal orientation results in a change of the refractive index. When a laser beam passes through the device, the beam will be deviated (Fig.1) and the device works a prism. The main difference between this device and a prism is that in the device the orientation profile of the liquid crystal molecules can be modified so that the laser beam can be deviated a required angle: the device is tuneable

Ellipsometric study of nematic alignment on silicon oxides for display

Vertical Alignment Nematics (VANs) displays are a form of LCDs in which the liquid crystals naturally align vertically to the glass substrates. In spite of their name, the liquid crystal (LC) director is never exactly vertical, rather it always show a small angle with the normal to the sample plane called tilt that may vary throughout the cell bulk. Its values are ultimately determined by the pretilt, defined as the tilt angle on the surfaces in the absence of voltage

Electrical response of liquid crystal cells doped with multi-walled carbon nanotubes

The inclusion of nanoparticles modifies a number of fundamental properties of many materials. Doping of nanoparticles in self-organized materials such as liquid crystals may be of interest for the reciprocal interaction between the matrix and the nanoparticles. Elongated nanoparticles and nanotubes can be aligned and reoriented by the liquid crystal, inducing noticeable changes in their optical and electrical properties. In this work, cells of liquid crystal doped with high aspect ratio multi-walled carbon nanotubes have been prepared, and their characteristic impedance has been studied at different frequencies and excitation voltages. The results demonstrate alterations in the anisotropic conductivity of the samples with the applied electric field, which can be followed by monitoring the impedance evolution with the excitation voltage. Results are consistent with a possible electric contact between the coated substrates of the LC cell caused by the reorientation of the nanotubes. The reversibility of the doped system upon removal of the electric field is quite lo

Tunable liquid crystal-photonic crystal fiber interferometer

In this work, we present a novel interferometer based on liquid crystal and photonic crystal fiber technology. The objective of this project is the development of a tunable (switchable) modal (Mach-Zehnder) interferometer for optical communications or sensing

Pretilt effect on a vertically aligned liquid crystal cell for optical displays

Vertical aligned (VA) mode liquid crystal (LC) cells are very popular in transmissive directview and reflective projection displays because of high contrast ratio and wide viewing angle. A nonzero pretilt angle is generally used to avoid domain formation during molecular reorientation by electric field, as well as to reduce both driving voltage and response time. On the other hand, increasing the pretilt the quality of the dark state will be reduced, and thus an optimal value of pretilt has to be sought. To this aim, even different pretilt values on the two surfaces, yielding an asymmetric tilt distribution, could be considered. The pretilt effect on electro-optical properties of a VA LC cell has been investigated. Pretilt was controlled changing the angle of SiOx deposition on the substrate surfaces [1]. We employed Variable Angle Spectroscopic Ellipsometry [2] to obtain the tilt angle distribution versus the applied voltage. Finally, we compared experimental findings with simulated data at different anchoring energies. The results will be discussed