R. MANIEWSKI, A. LIEBERT, M. KACPRZAK, and A. ZBIEC
Optical methods from the near infrared range, in particular the laser-Doppler flowmetry and the near infrared spectroscopy offer a new non-invasive, real-time technique for monitoring of the blood perfusion and oxygenation in a living tissue. In spite of some instrumental problems, e.g., relative calibration and unknown sampling measurement depth, these methods have been already used in clinical studies. In this paper, the principle of the methods and instrumentation have been described. The advantages and limitations of these techniques are also discussed and new trends in technical development of the laser-Doppler flowmetry and the near infrared spectroscopy, especially contactless perfusion scanning and photons time of flight measurement, have been shown. Finally, selected clinical applications of all presented methods have been described.
Keywords: laser-Doppler flowmetry, near infrared spectroscopy, blood perfusion, tissue oxygenation, clinical applications.
In the last few years, the unique features of antiferroelectric liquid crystals (AFLCs) have been explored to develop high-end displays. A number of passive- and active-matrix prototypes have been presented. However, although their use in a number of application areas has been suggested, no commercial products have been announced yet. This work reviews the state of the art of AFLC displays, the reasons for their present low incidence in display markets, and the latest developments aiming to overcome the main shortcomings that hinder their development. V-shape smectic displays are also included in this study. Although not considered strictly antiferroelectrics nowadays, V-shape materials behaviour and scope are similar to traditional AFLCs.
Keywords: antiferroelectric, display, V-shape, LCOS, active matrix, waveform.
A novel mechanism of photorefractivity in nematic liquid crystal (LC) cells is presented. Dynamic photorefractivity is based on temporal quasiperiodic modules excitation in the liquid crystal layer by applied external alternating electric field and their reorientation due to the charges induced by interference optical field on a surface of photoconducting orienting layer.
Keywords: liquid crystal, dynamic photorefractivity, novel mechanism.
We studied the electrooptical effects in ferroelectric liquid crystals at both the fundamental and the second harmonic frequency. The linear effects caused mainly by the changes in optic axis position give rise to the changes in the intensity of light passing the sample placed between crossed polarizers. The electric field causes also changes in the shape of the indicatrix. Both mentioned effects influence the second harmonic response. We observed that the defects present in ferroelectric liquid crystal sample affect the electrooptical response to a large extent. This influence is especially significant for the second order harmonics. The depth of light modulation depends on kind of alignment (bookshelf or chevron).
Keywords: liquid crystals, smectics, ferroelectricity, defects, alignment, electrooptical effects.
The sticking effect on photoaligning surfaces was investigated. We demonstrated that additional irradiation of photoaligning polymers with cinnamoyl side groups with not-polarized UV-light strongly decreases their sticking parameter. We associate this effect with cross-linking of the flexible side-groups by UV light and, as a consequence, with light-induced strengthening of the photoaligning surface. Restriction of mobility of the flexible groups on the photoaligning surface (surface strengthening) resulted in depressing the sticking effect. The method of the decrease of the sticking effect by the light-induced strengthening is rather general, and it can be applied for any photoaligning materials undergoing a light-induced cross-linking of polymer fragments. For poly(vinyl 4-fluorocinnamate) the light-induced strengthening allowed us to get the record value of the sticking parameter, S0 = 0.2%, which is better than traditional rubbed polyimide surfaces provide. Such a value of the sticking parameter along with other aligning characteristics allows considering poly(vinyl 4- fluorocinnamate) as very prospective material for modern liquid crystal display technologies.
Keywords: liquid crystal, photoalingnment, sticking effect.
Laser-induced change in refractive index and optical limiting effect have been studied in the liquid crystal systems based on fullerene-doped 2-cyclooctylamino-5-nitropyridine, polyimide, N-(4-nitrophenyl)-(L)-prolinol compounds. Experiments have been made under nano-, pico- and femtosecond pulsed laser irradiation at the wavelength of 532 nm and 805 nm. From the results obtained, both the optical limiting level and nonlinear refractive index n2 as well as the third-order susceptibility c(3) have been determined. Potentials of the systems studied to attenuate laser irradiation and to record thin amplitude-phase hologram over the visible and near-infrared spectral ranges have been evaluated to solve optoelectronic problems more efficiently.
Keywords: liquid crystal, fullerenes, optical limiting, laser-induced change in refractive index, optoelectronics.
Antiferroelectric liquid crystals have been applied to high-end multiplexed displays. The interest on these materials chiefly comes up from their capability of being multiplex addressed with no need of active matrices. Antiferroelectric grey levels arise from a double symmetric hysteresis loop that can be stabilized by a constant holding voltage. Driving schemes are compatible with passive multiplexing, but limitations appear when the multiplexing rate increases. To avoid these limitations, new driving schemes for high multiplexing level at video rate have been designed. The problem of accumulated voltage on bias level arising from data voltages is tackled as well.
Keywords: antiferroelectric, liquid crystal, display, multiplexing, waveform, video-rate.
A wedge cell of the wedge angle of the order of few milliradians was used to measure threshold magnetic fields for the magnetic Freedericksz transition [1-3]. A nematic liquid crystal filling the cell was of planar orientation enforced by the treatment of the flat boundary plates. A system of interference fringes appeared in the cell placed in normally incident light between analyser and polariser crossed. In the vicinity of each fringe, the cell could be considered as a flat-parallel one and hence it was equivalent to a system of flat cells of different precisely determined thickness, the same relates to any cell of slowly-varying thickness and flat cover plates. The threshold magnetic field magnitudes were interpreted as eigenvalues of the boundary eigenvalue problem for the operator of the second derivative; the interaction between the nematics and the substrate was described by the Rapini-Papoular formula [3] (i.e., weak coupling was considered). The resulting formulae were used to determine the polar anchoring energy coefficient and the anisotropy of diamagnetic susceptibility after the threshold fields measured. The method was applied to characterise the nematic liquid crystal 5CB and the coupling between it and the substrates made of poly(amic acid) MP2 [4, 5]. The estimates of material parameters agreed pretty well with those determined by the composite method [6].
Keywords: nematics elastic constants, magnetic susceptibility anisotropy, 4í-pentyl-4-cyanobipfenyl (5CB), polar anchoring energy coefficient, wedge cell method.
The effect of the preparation method and its parameters and system composition on the polymer-dispersed liquid crystal morphology are described including size, shape, and concentration of liquid crystal droplets. Three different methods of polymer- dispersed liquid crystal preparation – encapsulation, solvent-induced phase separation, and photopolymerization- induced phase separation have been studied. It has been confirmed that the mean size of liquid crystal droplets is proportional to the liquid crystal concentration and approximately inversely proportional to the solidification rate. Depending on the preparation parameters it is possible to obtain, from the same components, a composite in which liquid crystal droplets’ size differs by four orders of magnitude. The driving voltage decreases while switching time increases with a size of liquid crystal droplets. The composites containing elongated liquid crystal droplets exhibit different electrooptical properties than those containing spherical droplets. Generally, changing composite morphology one can adjust electrooptical performance of polymer-dispersed liquid crystal.
Keywords: polymer-dispersed liquid crystals, morphology, electrooptical properties.
Liquid crystal filled polymer structure (LiCFiPS) devices consist of a polymer structure, which performs the desired spatial phase modulation of the incident light, filled with liquid crystal to permit modulation of this optical function. Potentially fabrication of this type of device may be carried out at very low cost using established polymer hot rolling or embossing techniques. Devices incorporating complex polymer structures as the diffractive element will inevitably have liquid crystal aligned at varying orientations to the surface. Switchable gratings with the liquid crystal aligned in the plane of the grating but either parallel or perpendicular to the grating rulings have been investigated as the two extremes of alignment. Good quality optical switching has been achieved for parallel aligned devices, however devices with the liquid crystal aligned perpendicular to the grating lines tend to show defects and inhomogeneities. While such devices might not offer the ultimate versatility of matrix addressed SLM devices they do offer electronic control of diffractive optical devices at very low cost.
Keywords: liquid crystal device, switchable gratings, diffractive element, spatial light modulator.
Dynamic optical data storage and other applications in the field of optical data processing would be possible due to development of suitable nonlinear optical materials. In this paper, we present experimental investigations of an orientation photorefractive effect in nematic liquid crystal mixtures with different optical anisotropy and various layers of cell.
Keywords: dynamic holography, optical light modulator.
The liquid crystal (LC) filters with hybrid, circular-planar, and circular-homeotropic alignments of the LC layer have been analysed in detail. The exploited alignment of the LC layer assures the uncommon polarizing properties of the filter. We manage to observe that transmissions in such a filter create an orthogonal set. This set may be joined with a spectral component of light by the unique way. It is exhibited in the presented work. We proved here that such an orthogonal set is created by means of light polarization as well as by means of the light wavelength. It has been shown that a liquid crystal which has a high birefringence improves sensitivity of transmission on the transmitted wavelength and polarization.
Keywords: polarization, filter, liquid crystal.
The kinetics of the diffusion-controlled triplet-triplet annihilation of anthracene was investigated with the time-resolved laser spectroscopy. The analysis of the decays of delayed fluorescence with the standard Smoluchowski model leads to an artificial dependence of the annihilation radius from the delay (starting point of evaluation). The intuitive-empirical modification of the so-called short-time term in Smoluchowski equation (based on the treatment of the triplet-triplet annihilation as a non-Fickian diffusion) is proposed. A brief comparison is presented between standard Smoluchowski and modified models for anthracene. The limits of the approximation used are discussed and the way of further progress is suggested.
Keywords: laser spectroscopy, signal processing and mathematical modelling, non-Fickian diffusion, triplet-triplet annihilation (TTA).
In the paper, the novel idea of burst mode laser driver with high extinction ratio is presented. The concept is based on controlling the bias current very close to the laser threshold, what is obtained by stabilising the laser power between the data bursts in the range of a few µW. The at-threshold bias eliminates the laser turn on delay, making the driver suitable to transmit the bursts of data without any prebiasing procedure. The experimental driver design is described and measurement results are reported.
Keywords: laser driver, burst mode transmission, optical TDMA.