F. Campoli

Design of an Ultrashort Directional Coupler With an SSFLC Coupling Layer

Abstract A design of a vertical directional coupler, operating at the wavelength λ= 632.8 nm, is presented. The coupler consists of a layer of the Hoechst ferroelectric liquid crystal mixture FELIX-M4851–025, homogeneously aligned and sandwiched between two single mode ion-exchanged BK7 glass waveguides. The design of the directional coupler was carried out by using the beam propagation method, taking into account also teflon alignment layer and ITO electrode contributions. Given the FLC mixture refractive indices, n o =1.611 and n o = 1.466 at λ= 632.8 nm and T= 25°C, calculations show that less than 60 μm coupling length, with an extinction ratio higher than 21 dB can be obtained by using single mode waveguides with a refractive index change at the surface Δn= 0.013, an FLC layer 2 μm thick, and cone axis tilted by about 19° from the propagation direction of the lightwave.

Use of Ptfe Alignment Layers in Passive Addressed Ssflc Displays

Abstract Polytetrafluoroethylene (PTFE or Teflon®) layers have been developed as orientation films in order to produce ferroelectric liquid crystals displays with improved addressing behaviour. PTFE layers have been friction deposited on glass substrate with ITO electrodes at various temperatures in the range 130 °C-200 °C. The PTFE layers have been used to assemble displays which were addressed and showed no hysteresis, reduced image sticking and reduced switching time with respect to standard test cells employing rubbed polymers as alignment layers. In order to assure the best electro-optical performance, PTFE films have been characterized from the point of view of uniformity and thickness as function of deposition parameters.

Influence of Charge Transfer Complex Doping of Polyamide Alignment Film on SSFLC Cell Performance

Abstract New alignment films for surface stabilized ferroelectric liquid crystals displays (SSFLCDs) have been produced by doping standard polyamide alignment films with an organic conducting charge transfer complex. The doping process exploits absorption equilibrium between a solution of the dopant and the polymeric solid phase. Dopant concentration in the solid state has been evaluated by spectrophotometric analysis in the UV-visible range. Test displays with ITO electrodes and doped alignment layers have been fabricated and their performance evaluated with new techniques. It has been found that optical response is a function of dopant concentration in the alignment layer and that display performances can improve at an optimum concentration. In fact addressing hysteresis is reduced at low dopant concentration, but increases at higher concentrations.

A construction for an analog grey scale SSFLC display

Abstract We introduce a promising low cost analog grey scale structure for SSFLC panels. The display under study consists of two glass substrates, one of which is covered as usual with ITO electrodes. The other substrate is covered with a high resistivity transparent film and with a pattern of thin, parallel metal conductors. Spacers can be formed by the photoresist stripes left on metal conductors with thickness controlled in spin coating process. The grey scale is produced due to variation of the FLC switching area beside the metal tracks according to the applied data voltage. The high resistivity coating leads to non-uniformity of the electric field which is responsible for domain creation and switching process growing gradually from the metal tracks. We also discuss addressing hysteresis and operation range problems and present our choice for FLC mixture, aligning layers and addressing waveforms. Finally we give addressing hysteresis measurements and preliminary results from our devices.

Passive matrix SSFLC display with analogue grey levels using PTFE alignment films

Fabrication and performance of a surface stabilised ferroelectric liquid crystal display are reported. Grey shades are obtained by modulating the width of switched FLC microdomains by means of an electric field gradient distributed over the pixel, between adjacent metal columns. Such a gradient, depending on the data voltages, is achieved through a high resistance film deposited on the column glass plate. The choice of PTFE (Poly-tetra-fluoro-ethylene) alignment layers, easily deposited by friction, provides very low hysteresis and sticking effect. Morphology of alignment layers, carried out by AFM, is briefly described and evaluated. Addressing mode voltage waveforms to obtain grey shades are also reported.

Anchoring conditions for different aligning materials from dielectric behavior of SSFLC cells

The dielectric response of thin surface stabilized ferroelectric liquid crystal cells with different aligning materials and filled with the same FLC mixture are studied and compared in the low frequency dielectric regime. For the measurements we adopted an improved cell structure with metallized pixel connections. In the FLC material in question, Merck SCE 13, only one absorption appears corresponding to a collective mode.It is ascribed to the Goldstone mode in the bulk FLC. From our measurements we calculate the surface anchoring energy density for the different aligning materials and discuss changes induced in dielectric spectra by a DC bias field.

Influence of alignment techniques on switching behavior of SSFLC cells

Dependence of SSFLC electrooptic switching characteristics and matrix addressing capabilities on cell materials was investigated. Four test cells were made with two FLC mixtures, ZLI 4851-000 and BDH SCE8, in combination with two alignment materials, rubbed polyimide and friction deposited teflon. The cells show a good bistability characterized by memory and symmetric switching. Delay and rise times have been measured and fitted using a uniform director model, allowing a determination of all parameters within this model. Based on this, computed regions of operation of normal and fast addressing modes are reported. For both mixtures, teflon aligned cells, compared to polyimide aligned cells, show higher elastic restoring torques in the model. For normal addressing modes, teflon cells feature a higher minimum and a lower maximum operating voltage. Furthermore, they allow higher addressing speeds for both normal and fast modes.

Video speed low total voltage matrix addressing technique for SSFLC displays

Novel computed and experimental results are achieved with addressing modes capable of video rate speed with equal selection and data voltages and a commercially available FLC mixtures. Total voltage requirements for integrated circuit drivers is strongly reduced and is now made compatible with existing STN driver technology. A new automatic fit procedure, necessary for fast evaluation of cell parameter and subsequent simulation of cell behavior under matrix addressing, is also described.

Properties and Stability of Bismuth Doped Tin Oxide Thin Films Deposited on Various Types of Glass Substrates

This work reports on deposition of transparent semi-insulating tin oxide thin films deposited by spray-pyrolysis over large area glass substrates. The precursors used are based on diluted chloride solutions. Bi-doped high resistivity tin oxide thin films are required in the fabrication process of an analog grey scale ferroelectric liquid crystal matrix display. Various parameters have been optimized in order to achieve good reproducibility and uniformity of the electrical and optical properties. Deposition temperature and the properties of the glass substrates have been found as the most critical variables which affect the deposition rate and the quality of the deposited films. The different substrates used, borosilicates and soda-lime glasses, lead to different electrical properties. A further investigation on the stability of the film properties versus different kind of aging procedures have been conducted. Preliminary results of the operation of the complete display making use of our films are shown.

SSFLC Polarization Switch for Zeeman Laser Stabilization Loop

Abstract Ferroeleciric liquid crystals (FLC) as light modulators in the stabilization system of a Zeeman laser increase the quality of stabilization due to the high switching speed and make the whole system very compact. We analyze the operation of the optical part of such a stabilization system and derive optimum values of the FLC cell parameters. Properties of available liquid crystals and their temperature dependence are considered as well as, in particular, the influence of a slight ellipticily of the laser radiation.