Self-organization drives the system to its internal attractors and so it is particularly effective in the soft matter. Self-organized systems are created in the presence of two at least simultaneous and competitive interactions of comparable strength. Such phenomena are the most exciting features of the liquid-crystal state and are frequently observed in multicomponent and multiphase systems as spontaneous periodic superstructures. If some of the periods of the superstructure are comparable to the wavelength of light and a sort of photonic band gap can be determined in it, then the superstructure behaves as a photonic crystal. In chiral structures, the band of a total reflection of circularly polarized light can play the role of an angle-dependent photonic band gap. The mechanisms of the formation of the superstructures as the result of minimization of the free energy, including cross coupling between gradients of the director field around the disclinations and gradients of the concentration of dopants (the thermodynamic force of a concentration gradient), are discussed here. The experimentally observed stable patterns, e.g., spontaneous diffraction gratings with uniformly oriented helical axis of N* at weak anchoring, two-dimensional hexagonal arrays of disclination lines perpendicular to nematic layers doped with small amount of chiral compound (bubble domains), and oriented arrays of TGBA in chiral smectics, are presented.

We have infiltrated photonic crystal fibers with liquid crystals, thus creating, to our knowledge for the first time, what we call Liquid Crystal Photonic BandGap (LCPBG) fibers. We have demonstrated thermal, electrical and all-optical tunability of the optical transmission properties of these fibers, thus providing broadband spectral switching and filtering functionalities which may be operated linearly as well as nonlinearly. For instance, in a continuous thermo-optic mode, the photonic bandgaps of a LCPBG fiber can be tuned with a sensitivity of a few nm/°C. In a thermo-optic threshold mode, a temperature change as small as 0.4°C can cause optical switching with an extinction ratio as high as 80 dB in the near-infrared. In another nonlinear mode, all-optical modulation up to 2 kHz frequency of an infrared probe signal has been achieved by using a pulsed, 532 nm pump source of a few milliwatts optical power. In addition, since the mode-field diameter of the LCPBG fiber is similar to that of standard, single-mode fibers, these devices can be inserted into existing telecom transmission links without significant additional losses. This paper gives a basic overview of the optics and the materials involved in LCPBG fibers, in particular "tuning in" on how the optical transmission properties of these fibers depend on various liquid crystal aspects, such as mesomorphism, optical and electrical parameters, and the alignment of liquid crystals in cylindrical geometry.

In these ten years after the discovery of electronic conduction in liquid crystals, a lot of effort has been made to characterize what the electronic conduction is in liquid crystals, understand the unique characteristics of charge carrier transport found, synthesize new materials having high mobility, and explore potential applications as a quality organic semiconductor. Judging from the accumulated data and understandings on properties in liquid crystals as a self-organizing molecular semiconductor, it is no doubt that the liquid crystal is very promising as a new type of quality organic semiconductor for opto-electronic devices, which enjoys both merits in molecularly disordered and ordered materials, i.e., large-area uniformity and high mobility that characterize each material, respectively. In this article, the new results found in this decade on charge carrier transport properties, new materials having a high mobility, theoretical modeling of charge carrier transport, which provides us with the physical basis to understand the unique features of charge carrier transport and with the scope and limitation of liquid crystals as an organic semiconductor, are reviewed briefly, and what the further study needs to be focused on for practical device applications are discussed.

As a new way to create promising LC network, the charge-transfer complexes (CTC) between an organic donor (monomer, polymer, nanoparticles) and fullerenes can be used. The new complex has a high dipole moment and hence it increases the polarizability of the compound. This effect improves the control of the NLC structure. The re-orientation of the NLC dipoles under light or field action can be observed with high speed. Thus, new principle of the speed enhancement of self-organized fullerene-doped nematic liquid crystal has been demonstrated. The switching time in the microsecond range has been obtained for all nematic LC devices.

The paper presents our latest experimental results on influence of temperature on the propagation properties of photonic liquid crystal fibers. We used solid-core photonic crystal fibers filled either with a low-birefringence prototype 1550 liquid crystal mixture or with PCB. It appeared that direction of temperature tuning in PLCFs is strongly dependent on thermal characteristics of both refractive indices.

High birefringence liquid crystals (LCs) play an important role for laser beam steering, tunable-focus lens, reflective display, cholesteric LC laser, infrared dynamic scene projector, and telecom variable optical attenuator applications. We have developed some high birefringence compounds and eutectic mixtures with birefringence in the 0.4-0.7 range. For some photonic devices where response time is critical, we have also developed high birefringence dual-frequency LC mixtures. The cross-over frequency is around 5-10 kHz. Using such a dual-frequency LC mixture, sub-millisecond response time is achieved.

We show how a tightly focused laser beam can serve as a tool to image complex patterns of the director using the technique of fluorescence confocal polarizing microscopy (FCPM). We expand the capabilities of FCPM into the domain of real-time scanning in order to study the dynamic processes at the time scale of about 1ms. In this approach which we call Fast FCPM, confocal imaging is performed using a modified Nipkow-disc scanning confocal microscope. In the Fast FCPM set up, we use a twisted nematic cell as a fast achromatic polarization rotator to change the polarization of probing light by 90°. The achromatic polarization rotator switches between two orthogonal polarization states when a sufficiently strong electric field is applied to reorient the director structure from the twisted to the homeotropic state. Both FCPM and Fast FCPM employ the property of anisotropic media to align fluorescent dye molecules. When observation is performed in polarized light, the measured fluorescence signal is determined by orientation of the dye molecules. As the dye molecules are aligned by the liquid crystal, the detected fluorescence signal visualizes the spatial patterns of the director rather than concentration gradients of dyes. Finally, we present 3D patterns of director associated with both static and dynamic processes in liquid crystals, anisotropic emulsions, and colloidal suspensions.

The liquid crystal (lc) filter, which produces distributed form of light transmission, has been described. Distribution of the transmitted intensity may be altered in filter window by means of an incident light polarization. To obtain such polarization sensitive transmission the special alignment of a liquid crystal layer is arranged in such a way, that on the one side, inside the filter, the lc layer has been aligned as planar or homeotropic, while on the opposite side of the filter it stay aligned in form of circles. Such liquid crystal cell may be called circular - homogeneous or circular homeotropic (CH LC). The filter transmission basis is adiabatic following in a twisted nematic lc layer (Mauguin regime). As Mauguin regime is fulfilled then CH alignment causes polarization sensitive transmission. Such filter provides new possibilities in a polarisation difference image analysis so may be convenient tool for a polarimetric image processing. Numerical simulation of the transmission has been presented as well as the experimental results. Even if Maugin regime is fulfil the examined filter transmission depends on the transmitted spectrum to some extent. It has been analyzed and potential application of this property of the filter has been discussed too.

We have developed a non-contact birefringence probing method for studying the dielectric heating-induced temperature rise of dual-frequency liquid crystals (DFLCs). The dielectric heating effects of three DFLC mixtures are investigated quantitatively. By properly choosing the molecular structures, the dielectric heating effect can be minimized while keeping other desirable physical properties uncompromised.

We have measured the twist viscoelastic coefficient of a lyotropic nematic liquid crystal composed of disc-like micelles of cesium perfluoro-octanoate (CSPFO) in water employing dynamic light scattering. We find that the twist viscoelastic coefficient increases with increase in temperature in the nematic phase. We have studied the viscoelastic response of the medium with the concentration of CSPFO in water. Twist viscoelastic coefficient rapidly increases with the concentration of CSPFO in water at a given reduced temperature. We have estimated the activation energy associated with the twist dynamics. We find that the activation energy is nearly independent of the concentration of CSPFO in water. We have also studied the effects of doping polyethylene glycol polymer on the viscoelasticity of CSPFO-water system.

The optic and electrooptic properties of recently prepared orthoconic antiferroelectrics have been revieved. Relation between their chemical structure and mesogenic properties, smectic layer structure and helical pitch is discussed.

Photoinduced bending and unbending behavior of cross-linked polymer films, prepared by in-situ polymerization of liquid-crystalline (LC) monomers with azobenzene moieties, is discussed. Various modes of bending have been achieved with various alignments of the photoactive mesogens in the elastomers. Exposure of a monodomain film to UV light to cause trans-cis isomerization of the azobenzene moieties leads to the bending of the film toward irradiation direction of the actinic light. Irradiation of a polydomain film with linearly polarized light results in the bending along the polarization direction of the light, which enables bending of the film along any direction. The LC alignment in the films significantly affected the photoinduced bending behavior.

Holographic gratings were recorded in hybrid cells of the nematic liquid crystal E7 doped with multi-walled carbon nanotubes in a degenerate two-wave-mixing experiment. The hybrid structure comprises doped nematic contained between photoconducting polymeric layers of poly(N-vinylcarbazole) spin-coated on two indium-tin-oxide-covered glass plates. Multiorder diffraction was observed and the first-order self-diffraction efficiency was obtained in order to investigate the properties of the gratings. The hybrid cells showed quite high diffraction efficiency up to 47%, which is far beyond the theoretical limit of 34% predicted by the Raman-Nath theory. In addition, such a structure was shown to exhibit the photorefractive effect due to the presence of two-beam-coupling gain. Under the condition for a fixed total intensity of the two interfering beams, the gain was found to increase with increasing pump-to-probe beam-intensity ratio.

Each hologram: optical, digital and computer generated represents the phase and amplitude of an object. Therefore the process of object reconstruction may rely on one of two alternative procedures: optoelectronic direct reconstruction of hologram, which is represented by diffractive structure intensity, and recombining of object phase and amplitude as calculated from wavefront propagation process (here the method is based on singular wave component that is derived from a rigorous Sommerfeld's solution) or numerical reconstruction of digital hologram. In the paper we present the comparison of these two ways of optoelectronic reconstruction. The pros and cons for application of these procedures from image quality point of view are given. The experimental systems based on a Twyman-Green interferometer with two phase and amplitude LCOS or on a single digital hologram or computer generated hologram reconstruction setup are presented. The conclusions of these experiments will be utilized for optimizing of the process of production of security holograms with use of LCOS modulators.

A Variable Optical Attenuator (VOA) based on a Polymer-Dispersed Liquid Crystal (PDLC) Cell is presented. The VOA's developed prototype has been successfully tested at 660nm, 850nm and 1300nm. This ability makes it strongly recommended for networks using Perfluorinated Gradual Index Polymer Optical Fiber (PF GI-POF) technology. The prototype has revealed a better than 12dBs dynamic range and losses of <1.2dB. In addition, the VOA presents a very low power consumption and non-dependendence with polarization. In order to avoid the dependence with temperature, an electrooptical feedback is incorporated to the device, by means of a microcontroller system. Electrically controllable intermediate transmission levels can be selected this way. The response time is in the milisecond range. The present feedback prototype, developed with POF technology, includes lenses, PDLC, 1x2 POF couplers and fotodiodes in the optical part, and a microcontroller system where the feedback processing is carried out. No polarizers are required so that optical losses are minimal. Polymer-Dispersed Liquid Crystals are formed by microdroplets of liquid crystal embedded in a flexible matrix, and sandwiched between transparent electrodes. This structure scatters strongly the light. When an AC electrical field is applied to the film the material becomes transparent. A largest dynamic range could be achieved designing conveniently the radius of the microdroplets. No-dependence with polarization, high transmittance when activated, and large dynamic range within a wide range of optical wavelengths make PDLC the most appropriate liquid crystal technology for VOAs fabrication.

Optical Phased Array technology will revolutionize optical systems. Within the last year or two we have had a demonstration of nonmechanical beam steering over a 45 degree x 45 degree field of regard. This demonstration had a response time of 6 - 40 millisecond, and a beam size of about a half of a centimeter. It had an efficiency of 15 -20 %. We are now working to increase beam size and efficiency, and to decrease speed. This demonstration was for narrow band radiation. We are also working on methods of decreasing dispersion so we can steer broadband radiation. We have approaches for all of the above areas. Another area of work is to move into the MWIR and LWIR regions. Currently we are working mostly in the visible and NIR. In the long run we would like to combine optical phased array technology with microwave phased arrays, so it would be possible to steer both 15 Ghz , or about 1 cm wavelength, radiation and 200 THz, or about 1.5 um, radiation.

Liquid Crystals (LC) are excellent low loss large area programmable materials for smart optical device design leading to agile photonics. This paper describes work by the Riza group to enable intelligent system design from the fiber-optic realm to the freespace optics domain of applications. Specifically, novels works will be described in fiber-optic attenuator design, freespace optical scanners, fiber-optic switches, fiber-optic delay lines, and laser beam profilers. Both theory and experimental demonstrations will be highlighted.

We report on the experimental demonstration of electrically tunable Bragg grating using liquid crystals. Such concept and device have been modelled in the past, but not realised experimentally. Using Direct UV writing technique, a waveguide and Bragg grating are simultaneously patterned in a planar silica on silicon substrate. Overcladding in the vicinity of the grating can be replaced by liquid crystals to exploit their electro-optic properties. The interactions of the evanescent field with the variable refractive index of the liquid crystal allow the tuning of the centre wavelength of the filter. In this way a 35 GHz tunable planar Bragg grating using liquid crystal and electric field was demonstrated.

A new phase contrast imaging system that permits a quantitative observation of phase distribution of a general object is proposed and experimentally demonstrated. This is realized by implementing the phase shifting technique in the phase contrast imaging system. A dye-doped nematic liquid-crystal (NLC) is newly employed for this purpose as a variable phase filter in contrast to the constant phase plate employed in the conventional phase contrast microscope (PCM). The dye-doped NLC provides a practical advantage to the system that realizes an alignment-free optical system for the phase plate. The self-alignment property of the phase filter is realized by utilizing the local phase transition from the liquid-crystal to liquid, which is induced by the relatively strong incident light of the specular component of the object. In the experiment, the fundamental phase modulation property and the response of phase modulation ability were measured as a function of incident power of light. Next, the phase measurements were performed with several phase gratings and composite objects having different phase amplitudes and absorption. As an experimental result, the usefulness of the proposed method was confirmed, and the accuracy of the phase measurement was estimated to be λ/40. In addition, it was also demonstrated that the phase information could be obtained independently from the absorption component of the object.

PDLC composites containing elongated ellipsoidal or flat liquid crystal droplets have been prepared by three different methods depending on properties of polymer matrix. The morphology of composites, i.e. concentration, size and shape of droplets have been changed during the preparation process and optimized after studies of optical properties. The light scattering of PDLCs, especially the optical contrast ratio in on- and off states, selective light reflection in case of chiral nematics dispersed in polymer as well the polarization properties of PDLC systems containing elongated liquid crystal droplets have been studied.

Mixing non-local process in a material with non-local response can yield to interesting phenomena. This is the case of spatial optical solitons excited in liquid crystals. It has been shown that spatial optical quasi-soliton can be generated either via a heat transfer process or a molecular reorientation. In both cases, light keeps propagating in the waveguide it created. Different parameters can be adjusted to optimize this waveguide and as a result the propagation of the quasi-soliton. Prior to control those parameters, it is interesting to measure the global non locality, i.e. the extension of the beam together with the induced index profile. Herein, it is reported first the main results on quasi soliton in liquid crystals, some technique to measure the induced index distribution. Then, a new method, based on Raman spectroscopy, is proposed to measure simultaneously the beam intensity distribution and the induced molecular reorientation in the case of a soliton generated via reorientation nonlinearity of a nematic liquid crystal. Finally, it is shown how the non-locality can be controlled and used. The case of a curved waveguide obtained this way is reminded.

Electro-optical Kerr effect data have been obtained for 6CB, 6CHBT, 6PCH and 6BOBT at temperatures above the nematic-isotropic transition temperature. All the compounds had a positive Kerr constant which increased with decreasing of temperature. The second order phase transition temperatures, T*, were determined for these compounds. The linear dependence of (T - T*)^-1 on the Kerr constant is found to be relatively in good agreement with the predications of the Landau-De Gennes model. The replacement of aromatic ring by a flexible cyclohexyl group and or by a bulkier bicyclooctane has a marked effect on the birefringence. Hexyl 4-(4-isothiocyanato-phenyl) bicycle [2,2,2] octane, 6BOBT, with the highest transition temperature had the lowest Kerr constant. In contrast, hexyl cyanobiphenyl,6CB, showed the largest Kerr constant (6BOBT → 6CHBT ≈ 6PCH → 6CB). The effect of the rigid core and polar head group of the liquid crystals was investigated.

The optical method to retrieve order parameter of liquid crystal droplets with rigidly fixed poles has been developed. The method is based on measurement of coherent transmittance of a monolayer of polymer-dispersed liquid crystal droplets and comparison with the results of coherent transmittance calculation. To calculate coherent transmittance of a polymer -dispersed liquid crystal monolayer the anomalous diffraction approximation and the approximation of effective refractive indexes for liquid crystal droplets are used.

A method for calculating transmission coefficient and characteristics of polarization state of light transmitted through polymer film with fine liquid crystal droplets is proposed. It is based on the Rayleigh-Gans approximation to describe scattering by an individual droplet, and the Foldy-Twersky equation to determine a coherent field transmitted through the polymer dispersed liquid crystal film. The results obtained relate the morphological characteristics of a film to its electro-optical response. The simple equations for scattering cross-sections are obtained. The accuracy of the method is estimated.

Effects of carbon-nanotubes on the transient current induced by a polarity-reversed electric field have been observed in lightly doped nematic liquid-crystal cells at the room temperature. Experimental results indicate that a minute addition of either single-walled or multi-walled carbon nanotubes in the nematic E7 alters the peak current and its time of occurrence for a given applied voltage. It is revealed that the lightly doped cells possess a higher charge mobility in comparison with their pristine counterpart. It is also found that multi-walled carbon nanotubes as a dopant modify the transient-current behavior more substantially than the allotrope single-walled carbon nanotubes do.

In our work the influence of the external light on the optical parameter of a twisted nematic (TN) display for transmissive and reflective modes are being analyzed. The numerical calculation of contrast ratio (CR) and luminance such the display are presented. Next, the computer optimization procedure for the reflective mode of a display is carried out. Thanks to these results the possibilities of the reflective mode of LCD are described. The influence of display elements on the display optical parameters are shown, and the way of the optimization are presented. The development of liquid crystal display (LCD) is very big in the last years. The quantity of produced displays is rising every year and their quality has improved. Because of the high resolution of displays and their electro-optical parameters they can be used in many applications. Unfortunately, despite these advantages these kind of a display has also some disadvantages. One of them is the fact that the LCDs are very sensitive on external light. This light can even, in some circumstance completely "blind" the display.

A nonlinear nonlocal model for beam propagation in a liquid crystal is analyzed. Using a perturbative approach interactions between in-phase and out-of-phase solitons is described analytically. Attraction of both in-phase and out-of-phase solitons is predicted. The perturbative approach also predicts the existence of a stable bound state of two out-of-phase solitons. The analytical results are verified by direct numerical simulations.

A simple model describing location of Raman-Nath index grating photoexcited in a planar cell with photoconductive orienting layers is proposed. We believe that index grating vector is parallel to cell boundaries. One of the most important features resulting from this model is that the index grating spacing equals to projection of the exciting optical grating spacing on cell boundaries. The experimental study was carried out with 9 micrometer thick planar cell filled with nematic liquid crystal 6CHBT, activated by anthraquinone dyes. DC or square-wave AC field has been applied to the cell. Holographic gratings were recorded by two (signal + pump) or three (signal + 2 pumps) coherent p-polarized beams from He-Ne laser operating at 632.8 nm. The crossing angle between signal and pump beams and the sample tilting angle were chosen to satisfy "angular resonance". The tilting angle varied from 20° to 45°, high diffraction orders were observed up to angle 77°. Calculations on the basis of our model and experimental results are in good agreement. We also discuss and present explanation of the observed effects in geometry with 2 pump beams.

The equivalence between a twisted-nematic liquid crystal cell and the combination of a retardation wave-plate and a polarization rotator can be used to calibrate a voltage-addressed liquid crystal display. We present a simple polarimetric procedure to determine the two parameters that define the optical properties of the equivalent retarder-rotator system for each value of the applied voltage. Once the calibration procedure is performed, the optical response of the liquid crystal cell can be predicted and optimized. In particular, we demonstrate the generation of a family of equi-azimuth polarization states with a liquid crystal display sandwiched by a polarizer and a quarter-wave plate, whose optimal orientations are evaluated by a numerical simulation. Laboratory results corresponding to a commercial liquid crystal display are pre-sented.

Dispersion of the refraction indices n (λ) as well as dispersion of the imaginary part of the refection coefficient r = r0*exp(-iφ_R(λ)) have to be determined to design band-pass Fabry-Perot filter. In the case when liquid crystal is placed in the filter gap then both ordinary and extraordinary indices have to be determined. Consequently one must find imaginary part of the coefficient r for ordinary and extraordinary waves transmitted by the filter. Imaginary part of the refraction coefficient is called also phase of reflection. Proposed method exploits a Fabry-Perot filter to obtain examined values. It has been proved that both values, phase of reflection, and refraction indices must be determined in the same procedure from data obtained in the same measurement. The procedure consists of: determination of the transmission peaks position in the Fabry-Perot filter spectrum; nonlinear fit procedure which optimize dispersions of the n(λ), and φ_R(λ) to minimize the differences between theoretical and experimental transmissions of the filter; simulation of the free spectral range (FSR), and finesse of the filter spectrum in dependence on dispersions of n(λ), and φ_R(λ). Results have been presented in the case of four different liquid crystal substances and two different mirrors applied in the filter. The way for determination of the φ_R (λ) seems to be interesting as this value is really hard to measure. Presented data for the examined liquid crystals are also new.

Remote controlled spatial light modulator (SLM) has been applied to design tuned "pin-hole" filter. The name "hole" means transparent area in the spatial light modulator (SLM) window. Transparent area in the SLM is remote controlled in the presented filter in accordance with the imposed "holes" distribution. As a result desired optical field diffraction has been obtained in the SLM window. Finally, one produces controlled movement of the focus in a resulting "pin-hole" filter. Proper optical field simulation has been described as well as presentation of the filter.

Low viscosity high birefringence nematic liquid crystalline compounds and mixtures are described. They belong to the families of 4'-alkyl-3-fluoro-4-isothiocyanatobiphenyls, 4"-alkyl-3,5-difluoro-4-isothiocyanatoterphenyls, 4'-alkyl-3-fluoro-4-isothiocyanatotolanes and 4'-(4-alkylphenyl)-3,5-difluoro-4-isothiocyanatotolanes.

Langmuir-Blodgett (LB) films formed of some discotic liquid crystals, namely 3,4,9,10-tetra-(n-alkoxy-carbonyl)- perylenes, have been studied. The absorption and fluorescence spectra were recorded. The results obtained have led to conclusions about formation of dimers both in ground and excited states at the air-solid substrate interface. Moreover, information about the molecular arrangement in LB films has been obtained.

In this work we design and build an optoelectronic demultiplexer to distribute audio and video signals transmitted through an optical carrier toward different users using a liquid crystal display (LCD) as a router. The output plane of our proposed device is located within the Fresnel Diffraction regime. With this feature we obtain a simultaneous distribution, high-speed of processing, easiness of path reconfigurations and a high density of interconnections. The optical carrier modulation includes a Frequency Shift Keying signal (FSK) to indicate the interconnection path. This last control signal is manipulated with a personal computer. We present some preliminary results of the feasibility of our proposal.

Our previous investigation regarding liquid crystal mixtures, used as a medium for dynamic holography, has proved the influence of addition of PVK (polivinylocarbazol) layer to the LC cell structure as a sensitivity amplifier [1]. This modification was very useful for bettering the dynamic properties of these modulators. In some cases (of LC Cells) we observe some kind of memory effect - holograms written before could be observed months after writing. We decided to examine different cells to determine CL Cell construction as rewritable holographic media.