Volume 87, Issue 2, 15 January 2000
Index of content:
- LASERS, OPTICS, AND OPTOELECTRONICS (PACS 42)
Photorefractive holographic contrast enhancement via increased birefringence in polymer composites containing electro-optic chromophores with different alkyl substituents87(2000); http://dx.doi.org/10.1063/1.371917View Description Hide Description
The birefringence of a photorefractive polymer composite in response to an applied electric field is used to predict the holographic index contrast attainable. Predictions have been obtained for three different polymer composites based on the charge transfer complex matrix poly(N-vinyl carbazole)/2,4,7-trinitro-9-fluorenone, doped with electro-optic azo chromophores differing only in their alkyl substitutions. A comparison with experimental data indicates that unexpected large variations in the holographic contrast obtainable from different chromophores are, in most cases, accounted for by a corresponding variation in electric field-induced birefringence. This variation may be due to a combination of the chromophore number density and the composite viscosity. Where the birefringence does not correlate with holographic index contrast, a prediction based on a trap density limited space-charge field fits the data well.
87(2000); http://dx.doi.org/10.1063/1.371918View Description Hide Description
A new Mueller matrix approach is developed for the design of optical modes for all nematic liquid crystal displays (LCD). In particular, for linearly polarized light going into the LC cell, conditions for linear polarization and circular polarization outputs are obtained. By considering the switching between different polarization modes, new transmittive and reflective bistable twisted nematic (BTN) LCD operating conditions with optimized contrast and brightness are discovered. A passive matrix driven single polarizer reflective BTN display was fabricated with reasonably good measured optical performance and fast selection time.
87(2000); http://dx.doi.org/10.1063/1.371919View Description Hide Description
We demonstrate that a photochemical reaction can create various distributions of refractive index in polymer. When the polymer containing a photochemically active material is irradiated by UV light, the photochemical reaction which breaks the π-conjugated system in the material and decreases its linear polarizability can reduce refractive index of the polymer. We prepared a PMMA film added DMAPN ((4-N,N-dimethylaminophenyl)--phenylnitrone) with a rate of 23 wt % by use of spin coating. Electronic structural change of DMAPN and refractive indices of the film before and after UVirradiation were evaluated by UVabsorption spectra and m-line method, respectively. The UVirradiation decreased at 380 nm in the absorption spectra, which is attributed to nitrone, and the refractive indices exponentially with irradiation time. The change of refractive indices reached 0.028. The refractive index profile upon depth of the film was investigated by measuring refractive indices of stacked DMAPN/PMMA films. When UV with a power of irradiated upon three stacked DMAPN/PMMA films for 35 s, variation of the refractive index change showed a quadratic profile. The refractive index profile with various irradiation time can be accounted with the combination of the chemical kinetics with the steady state approximation and Lambert–Beer's law. Thus, the photochemical reaction can be used to control the refractive index distribution in polymer.
87(2000); http://dx.doi.org/10.1063/1.371920View Description Hide Description
The self-enhanced diffraction with oscillatory behavior for unfixed holograms is observed and investigated in new photorefractive crystals. The recorded diffraction efficiency can be increased 10 times during the readout process. The maximum peak diffraction efficiency obtained is close to unity. A closed solution of dynamic two-wave mixing equations and a coupling coefficient expression obtained from the standard band transport model are used to fit the observed results. The observed self-enhancement is caused by the interference between a new grating, formed by the reading beam and the diffracted signal, and the existing grating. The fitted results show that the internal space-charge field originating from the photovoltaiceffect and the pyroelectric effect can reduce the phase shift.
Approximate description of the three dimensional director and electric field in a liquid crystal display at a high voltage87(2000); http://dx.doi.org/10.1063/1.371921View Description Hide Description
An approximate analytical approach for describing properties of a liquid crystal display associated with a homeotropic to multidomainlike transition for a liquid crystal with a positive dielectricanisotropy was developed. The electrode and surface coating of the display are prepared in such a way that in the absence of an electric field (dark state) the liquid crystal alignment is homeotropic. The bright state corresponds to a situation when a high voltage is applied to the interdigitated electrodes located on both substrates, which causes a director deformation of a multidomain type. Approximate solutions for the three dimensional electric field and director configuration in the case of strong homeotropic anchoring were obtained to describe the bright state. It was found that two different defect structures are possible for the director configuration: one corresponds to a wall defect and the other to two disclination lines. A first order phase transition between the two structures is predicted at a particular value of the voltage. An estimate of this critical voltage is obtained.