Use In Electro-optic Devices Research Articles

Liquid-Crystalline Suspensions of Photosensitive Paramagnetic CeF3 Nanodiscs.

The design of high-performance energy-converting materials is an essential step for the development of sensors, but the production of the bulk materials currently used remains costly and difficult. Therefore, a different approach based on the self-assembly of nanoparticles has been explored. We report on the preparation by solvothermal synthesis of highly crystalline CeF3 nanodiscs. Their surface modification by bisphosphonate ligands led to stable, highly concentrated, colloidal suspensions in water. Despite the low aspect ratio of the nanodiscs (∼6), a liquid-crystalline nematic phase spontaneously appeared in these colloidal suspensions. Thanks to the paramagnetic character of the nanodiscs, the nematic phase was easily aligned by a weak (0.5 T) magnetic field, which provides a simple and convenient way of orienting all of the nanodiscs in suspension in the same direction. Moreover, the more dilute, isotropic, suspensions displayed strong (electric and magnetic) field-induced orientation of the nanodiscs (Kerr and Cotton-Mouton effects), with fast enough response times to make them suitable for use in electro-optic devices. Furthermore, an emission study showed a direct relation between the luminescence intensity and magnetic-field-induced orientation of the colloids. Finally, with their fast radiative recombination decay rates, the nanodiscs show luminescence properties that compare quite favorably with those of bulk CeF3. Therefore, these CeF3 nanodiscs are very promising building blocks for the development and processing of photosensitive materials for sensor applications.

Absorption properties of one- and two-dimensional semiconductor nanocrystals in the presence of an electric field

We have considered the broadening of optical absorption spectra of ensembles of randomly oriented nanorods and nanoplatelets that is caused by the action of a static electric field. It has been found that the linewidth in the spectra of the considered nanocrystals depends nonlinearly on the field strength and attains saturation in fields on the order of 100 kV/cm. We show that, due to a weak confinement, the electrooptical response of nanoplatelets is stronger than that of nanorods, which leads to a number of distinctive features in the field-induced broadening of the spectra of nanoplatelets and gives grounds to state that nanoplatelets are the most promising objects for use in electrooptical devices.

Photoluminescence, Enhanced Ferroelectric and Dielectric Properties of Pr3+ Doped SrBi4Ti4O15 Multifunctional Ceramics

Pr doped SrBi4Ti4O15 multifunctional ceramics were prepared by a conventional sintering technique and their luminescent, dielectric and ferroelectric properties were studied. The PL spectra at room temperature indicated that the red emission was excited by the ultraviolet and blue light, revealed their potential application in white-light LED. The investigation of dielectric and ferroelectric properties showed that the integration of luminescent, electrical properties by Pr ion doping would obviously enhance the ferroelectric properties. It is believed that the multifunctional ceramics have the great potential for application use in electro-optical devices, memory and display integrated chips.

An Electrochromic Tristable Molecular Switch.

A tristable [2]catenane, composed of a macrocyclic polyether incorporating 1,5-dioxynaphthalene (DNP) and tetrathiafulvalene (TTF) units along with a 4,4'-bipyridinium (BIPY(•+)) radical cation as three very different potential recognition sites, interlocked mechanically with the tetracationic cyclophane, cyclobis(paraquat-p-phenylene) (CBPQT(4+)), was synthesized by donor-acceptor templation, employing a "threading-followed-by-cyclization" approach. In this catenane, movement of the CBPQT(4+) ring in its different redox states among these three potential recognition sites, with corresponding color changes, is achieved by tuning external redox potentials. In the starting state, where no external potential is applied, the ring encircles the TTF unit and displays a green color. Upon oxidation of the TTF unit, the CBPQT(4+) ring moves to the DNP unit, producing a red color. Finally, if all the BIPY(2+) units are reduced to BIPY(•+) radical cations, the resulting CBPQT(2(•+)) diradical dication will migrate to the BIPY(•+) unit, resulting in a purple color. These readily switchable electrochromic properties render the [2]catenane attractive for use in electro-optical devices.

Time-Grating for the Generation of STUD Pulse Trains

Spike train of uneven duration or delay (STUD) pulses hold potential for laser-plasma interaction (LPI) control in laser fusion. The technique based on time grating is applied to generate an STUD pulse train. Time grating, a temporal analogy of the diffraction grating, can control the pulse width, shape, and repetition rate easily through the use of electro-optical devices. The pulse width and repetition rate are given by the modulation frequency and depth of the phase modulation function in theory and numerical calculation. The zero-chirped phase modulation is good for the compression effect of the time grating. A principle experiment of two pulses interfering is shown to verify the time grating function.

Increasing the flexoelastic ratio of liquid crystals using highly fluorinated ester-linked bimesogens

We present experimental results on the bulk flexoelectric coefficients e and effective elastic coefficients K of non-symmetric bimesogenic liquid crystals when the number of terminal and lateral fluoro substituents is increased. These coefficients are of importance because the flexoelastic ratio e/K governs the magnitude of flexoelectro-optic switching in chiral nematic liquid crystals. The study is carried out for two different types of linkage in the flexible spacer chain that connects the separate mesogenic units: these are either an ether or an ester unit. It is found that increasing the number of fluorine atoms on the mesogenic units typically leads to a small increase in e and a decrease in K, resulting in an enhancement of e/K. The most dramatic increase in e/K, however, is observed when the linking group is changed from ether to ester units, which can largely be attributed to an increase in e. Increasing the number of fluorine atoms does, however, increase the viscoelastic ratio and therefore leads to a concomitant increase in the response time. This is observed for both types of linkage, although the ester-linked compounds exhibit smaller viscoelastic ratios compared with their ether-linked counterparts. Highly fluorinated ester-linked compounds are also found to exhibit lower transition temperatures and dielectric anisotropies. As a result, these compounds are promising materials for use in electro-optic devices.

Syntheses of four new pyridinium phenolates with caged phenolate functionalities as chromophores for quadratic optics

Semi-empirical calculations as well as our preliminary studies indicate an increase of the nonlinear response of pyridinium phenolates with the raise of their interplanar angle. Unfortunately, the tendency of previously synthesized zwitterionic compounds to form aggregates prevents their use in electro-optical devices. In order to understand the process of aggregation and to circumvent it, the syntheses of new pyridinium phenolates bearing alkyl chains of various length, caging the phenolate functionality, have been achieved and are described herein.

Preparation of ferroelectric nanoparticles for their use in liquid crystalline colloids

In this paper we summarize our many years of experience in the preparation and optimization of stable colloids of ferroelectric nanoparticles dispersed in an isotropic carrier and in a liquid crystal host. The colloids are of interest for use in electro-optic devices, photorefractive hybrids and nonlinear optical elements. We also outline some of the most interesting features the nanoparticles bring to liquid crystals, along with the potential of these relatively new colloids.

Linear Electrooptic Coefficient of InP Nanowires

A novel fabrication procedure is developed that allows for the direct measurement of the linear electrooptic coefficient of semiconducting nanowires to determine their viability for use in electrooptic devices. Vertically aligned InP nanowires are transferred from their growth substrate to a glass substrate using a host polymer, while still retaining the alignment of the nanowires. The linear electrooptic coefficient of the InP nanowires exhibited a 1-2 orders of magnitude enhancement over bulk InP and ranged from 31 to 147 pm/V. The figure of merit, n3r, exhibited a factor of 20 enhancement over lithium niobate and ranged from 1010 to 4817 pm/V.

Bent-core liquid crystals in the route to functional materials

Several results on bent-core liquid crystals considered as new supramolecular functional materials with potential for applications are reviewed. Various topics are discussed where these materials present unique attributes with clear advantages over conventional mesogens. In particular the electrooptic responses of some mesophases are examined from the viewpoint of their possible use in electrooptic devices. Other issues are also treated such as nonlinear optical properties, flexoelectricity or the design of biaxial nematics from bent-core molecules. Future challenges and prospects for new possibilities with these materials are briefly outlined.

Order relaxation of a poled azo dye in a high Tg, fully aromatic polyimide

A guest-host polymer system with potential use in electro-optic devices is discussed. The polymer host is a high Tg, fully aromatic polyimide and the guest chromophore is disperse red 19. Relaxation mechanisms of polar order after electric field alignment procedure have been investigated by measuring the isothermal decay of the macroscopic nonlinear optical coefficient d333 at different temperatures below glass transition temperature (Tg), upon the removal of the poling electric field. All the decay curves can be fitted by a double exponential function. Below Tg, the slower relaxation time shows an Arrhenius temperature dependence. An extrapolation to room temperature allows to predict the time stability which results to be longer than 30years. In addition, absorption spectra measurements of the films were performed before and immediately after poling procedure to estimate independently the polar order of the dipoles through the decrease of the absorption coefficients.

Novel, highly polarizable thiophene derivatives for use in nonlinear optical applications

The synthesis of a variety of thiophene-containing materials for use in electro-optic devices requiring highly birefringent materials is reported. The refractive indices were measured with the use of an Abbé refractometer, and from these results the optical anisotropies, polarizabilities and order parameters were determined. The replacement of a phenyl ring by thiophene leads to large enhancements of polarizability. The most significant increases in polarizability anisotropy were observed when the rigid core was a collinear 5,5′-disubstituted-2,2′-dithienyl unit. The changes in the optical properties are discussed in terms of the structural units (thiophene, phenyl, isothiocyanate, nitrile, butylsulfanyl, alkoxy and alkyl moieties) used and their positions with respect to the molecular core.

Investigation of electrooptic modulation from organic-inorganic crystals

This paper presents the first investigation of the characteristics of organic-inorganic crystals for a possible use in electrooptic devices. Electrooptic and dielectric proper- ties in an electric field of frequency varying from 1 kHz to 1 MHz applied along the c-axis were determined in 2-amino- 5-nitropyridinum dihydrogenphosphate (2A5NPDP) and 2- amino-5-nitropyridinum dihydrogenarsenate (2A5NPDAs) around room temperature at a wavelength of 633 nm. The values of the thermooptic coefficients are also given. The whole set of properties are discussed in terms of figures of merit and compared to those of other crystals used for elec- trooptic modulation of a laser beam.

The Optics of Ferroelectric Liquid Crystals

Abstract Ferroelectric liquid crystals (FLCs) have been under investigation for use in electro-optic devices for over a decade. In this review the development of the studies of the optics of FLCs over this period is discussed. Starting from the first electro-optic switching studies undertaken in 1980 by Noel Clark and Sven Lagerwall, and the structure in a FLC cell proposed to explain the effects, the process of developing understanding is followed. The review concentrates on the structure revealed to be present in a FLC filled cell, as this is of central importance in the development of optical devices using this material. Problems encountered in device development, and the approaches taken to overcome them are discussed, including special surface alignments, AC stabilization, and treatment of cells with large electric fields. FLC continuum theory is also discussed as this is needed for calculation of the liquid crystal director profiles, and hence the determination of optical performance. More recent de...

Nonlinear Optical Properties of Chloronitroanilines

ABSTRACTA series of chloronitroaniline compounds were investigated as bulk crystals and in thin film form as potential nonlinear optical materials for use in electro-optic devices. Crystal structure determination and computational modelling were completed on 5- chloro-2-nitroaniline and electro-optic measurements were performed on crystals grown by physical vapor deposition. Second harmonic generation measurements were carried out on poled guest/host systems of 3-chloro-4-nitroaniline in poly(methylmethacrylate).