Towards a new horizon of optoelectronic devices with liquid crystals
J. HANNA
The liquid crystal having high viscosity is attracting increasing attention as a new type of quality organic semiconductor, i.e., self-organizing molecular semiconductor. In this article, its state-and-the-art in materials, properties related to charge carrier transport, and device applications, is reviewed briefly. In addition, the further researches needed towards its practical applications are discussed.
Keywords: liquid crystal, organic semiconductors, charge carrier transport, device applications.
Molecular engineering of triphenylene-based discotic liquid crystal conductors
R.J. BUSHBY, K.J. DONOVAN, T. KREOUZIS, and O.R. LOZMAN
2,3,6,7,10,11-hexakis(hexyloxy)triphenylene (HAT6) gives a columnar hexagonal mesophase between 70 and 100°C and a hole mobility of 7.1Y10n4 cm2Vn1sn1. Two methods are compared for extending the mesophase range to encompass the operationally-significant room temperature range and for enhancing the charge-carrier mobility: introduction of a lateral nuclear dipole and formation of a CPI (complementary polytopic interaction) ëcompoundí. Introduction of two fluorine substituents into the nucleus of HAT6 gives 1,4-difluoro-2,3,6,7,10,11-hexakis(hexyloxy)triphenylene (2F-HAT6). This has a lateral nuclear dipole and gives a columnar hexagonal mesophase from below room temperature to 121°C whereas the CPI mixture of 2F-HAT6 with 2,3,6,7,10,11-hexakis(4-nonylphenyl)triphenylene (PTP9) has a columnar mesophase from below room temperature to 129°C. At 100°C the time-of-flight hole mobilities of these two systems are increased to 1.6Y10n3 cm2Vn1sn1 for 2F-HAT6 and 1.5Y10n2 cm2Vn1sn1 for the CPI compound 2F-HAT6 + PTP9.
Keywords: liquid crystal, semi-conductor, time-of-flight, photoconduction, hole mobility, molecular engineering.
Numerical simulation of the transport of particles
in electrophoretic displays
T. BERT, H. De SMET, F. BEUNIS, and F. STRUBBE
Electrophoretic image displays (EPIDs) are a major contestant in the search for electronic paper. Working prototypes have been demonstrated, however, full understanding of the internal physical properties is not achieved yet. In this article, we will present, for the first time, a complete set of formulae that make it possible to simulate the electrical and optical properties of EPIDs. Starting from a theoretical model, that was presented in a previous article, this article gives numerical and graphical results. The set of formulae that was used and their output helps us understand how particles are transported inside a pixel and how this leads to the electrical and optical properties of the display.
Keywords: electrophoretic image display, simulation, electrical, optical.
Photorefractive cyclometalated complexes
R. TERMINE, M. TALARICO, I. AIELLO, D. DATTILO, D. PUCCI, M. GHEDINI,
and A. GOLEMME
Some of the most recent results on the photoconducting and photorefractive properties of cyclometalated complexes are reviewed. Pure compounds show good photoconducting properties, which can be increased by orders of magnitude by doping with fullerenes. In addition, photorefractivity has been observed both in pure materials in their glassy state and in mixtures with inert polymers. In such mixtures, at high concentrations of complexes, dispersions of small (less than 200 nm) crystals of the complex within a polymeric matrix can be obtained. This morphology has been clearly linked to an increase in photorefractive performance, measured through diffraction efficiency, by orders of magnitude. Moreover, complexes with proper ligands exhibit chiral smectic mesophases with ferroelectric properties which also show photorefractive properties.
Keywords: photoconductivity, photorefractivity, cyclometalated complexes.
Ambipolar charge carrier transport in liquid crystals
H. IINO and J. HANNA
We review the electron transport in both smectic and discotic liquid crystals with the updated experimental results, and discuss the ambipolar carrier transport in these materials. We indicate that the intrinsic nature of charge carrier transport in liquid crystals is probably ambipolar, in which both electronic and ionic charges are transported basically irrespective of their polarity, and that the ionic transport in these liquid crystals is extrinsic and induced by photo-ionization of chemical impurities. Furthermore, we discuss the importance of this unique features of charge carrier transport in the liquid crystals in the future development of new optoelectronic device applications that cannot be achieved with conventional organic semiconductors.
Keywords: organic semiconductors, liquid crystal, ambipolar charge carrier transport, mobility, electron, hole.
Fast charged carrier mobility of a triphenylene-based polymer film
possessing nematic order
M. INOUE, H. MONOBE, M. UKON, V.F. PETROV, T. WATANABE, A. KUMANO,
and Y. SHIMIZU
A polymer film was prepared by the photopolymerization in a discotic nematic (ND) mesophase of a mixture of the polymerizable triphenylene mesogen isomers. It was found that the film shows a relatively fast mobility of charged carrier (10n3 cm2Vn1sn1), which is probably faster than that of the monomeric ND mesophase. The microscopic domains in the film were found to keep the liquid crystalline order even after the polymerization evidenced by the IR spectra showing the small decrease in the local order parameter and by the X-ray diffraction (XRD) patterns comparable to those before the polymerization. It was indicated that in the photopolymerization, the long range order of molecules can be almost remained, the short range one is variant and, thus, such an in-situ liquid crystalline polymerization could lead to the enhancement of charge transport efficiency. Furthermore, the polymerized film was applied to a simple electroluminescent device to reveal that it certainly works as a hole transport layer because the luminescence is clearly induced under bias.
Keywords: liquid crystalline semiconductor, discotic nematic phase, photopolymerization, charged carrier mobility, triphenylene, electroluminescent device.
Fuzzy c-means clustering algorithm with a novel penalty term
for image segmentation
Y. YANG, Ch. ZHENG, and P. LIN
Fuzzy clustering techniques, especially fuzzy c-means (FCM) clustering algorithm, have been widely used in automated image segmentation. However, as the conventional FCM algorithm does not incorporate any information about spatial context, it is sensitive to noise. To overcome this drawback of FCM algorithm, a novel penalized fuzzy c-means (PFCM) algorithm for image segmentation is presented in this paper. The algorithm is formulated by incorporating the spatial neighbourhood information into the original FCM algorithm with a penalty term. The penalty term acts as a regularizer in this algorithm, which is inspired by the neighbourhood expectation maximization (NEM) algorithm and is modified in order to satisfy the criterion of the FCM algorithm. Experimental results on synthetic, simulated and real images indicate that the proposed algorithm is effective and more robust to noise and other artifacts than the standard FCM algorithm.
Keywords: image segmentation, fuzzy clustering, fuzzy c-means, expectation maximization.
Q-switched mode locking in diode pumped lasers
J.K. JABCZYNSKI, W. ZENDZIAN, and J. KWIATKOWSKI
The Q-switched mode locking (QML) regime provides the generation of relatively high peak power picosecond pulses train with energies of a few µJ each in a simple resonator. The critical review of QML methods and results including our investigations is given in the first part of presentation. The application of several types of saturable crystalline absorbers (Cr4+:YAG, V3+:YAG, LiF, GaAs) leads to chaotic, partial QML effect, with less than 100% modulation depth in principle. The fully modulated efficient QML laser was demonstrated in the next part. The acousto-optic cell playing a double role of Q-switch and mode locker was located near a flat output coupler. The two folding mirrors were mounted on the translation stages for matching the resonance frequency of the cavity to the radio frequency of acousto-optic modulator. The QML pulses with envelope durations of 100n150 ns and 100% modulation depth were observed for wide range of pump powers and repetition rates. In the preliminary experiments up to 3 W of output average power, 100 µJ of the envelope energy, having approximately 5n8 mode locked pulses were achieved.
Keywords: diode pumped lasers, mode locking, Q-switching, acousto-optic, saturable absorbers.
An efficient object tracking method based on adaptive
nonparametric approach
L. LI and Z. FENG
In this paper, an efficient method for object tracking based on nonparametric approach is presented. The density we estimated is based on an adaptive kernel model, which is driven by the intensity difference between the target and the background. The background-weighted histogram for statistics of feature takes into account the relevance between the target and background. What is more, this approach extends the range that is needed for searching object. The target model is updated according to the change of the object and environment. Experimental results on real image sequences demonstrate its robust performance in visual tracking and require less iteration computations when compared to other method.
Keywords: object tracking, adaptive, nonparametric, update, kernel.
Diffraction analysis by periodic structures using a method
of coupled waves
V.M. FITIO and Y.V. BOBITSKI
The coupled-wave method (CWM) has been used to analyse diffraction on some periodic structures, i.e., on relief gratings including metallized ones having sinusoidal relief as well as on one-dimensional and two-dimensional photonic crystals. Exact boundary conditions were taken into account in numerical calculations. Grating polarizers have been analysed, properties of dielectric gratings wherein coupled-mode resonance occurs have been studied, diffraction efficiency has been calculated for sinusoidal relief metallized gratings. Calculations were carried out for TE and TM polarizations. Band structure of a two-dimensional photonic crystal has been calculated using CWM.
Keywords: grating, diffraction, coupled-wave resonance, photonic crystal.
Numerical optimization procedure of TN LCD design process
M. OLIFIERCZUK
The main aim of this work is a presentation of the significance of the numerical optimization procedure for the TN liquid crystal display design process. The assumptions for calculations of such optical parameters as contrast ratio and luminance are presented. The way of measurements of optical parameters (refractive indices and absorption coefficients) for such the display elements as glass, conductive layer, polarizers, etc., which makes it possible to apply these results directly in the computer program worked out by the author, is described. The calculated results of the contrast ratio and luminance for TN transmissive and reflective display are presented. The optimization procedures for negative mode, and for transmissive and reflective type of the display were performed in an independent way.
Keywords: TN displays, numerical optimization procedure, contrast ratio.