We briefly review systematic and comprehensive studies on several chlorine-substituted bent-core liquid crystal materials
in their nematic phases. The results, in comparison to rod-shaped molecules, are both extraordinary and technologically
a) Electrohydrodynamic instabilities provide unique patterns including well defined, periodic stripes and optically isotropic
b) Rheological measurements using different probe techniques (dynamic light scattering, pulsed magnetic field, electrorotation)
reveal that the ratio of the flow and rotational viscosities are over two orders of magnitudes larger in bentcore
than in calamitic materials which proves that the molecule shape and not its size is responsible for this behaviour.
c) Giant flexoelectric response, as measured by dynamic light scattering and by directly probing the induced current
when the material is subject to oscillatory bend deformation, turns out to be more than three orders of magnitude larger
than in calamitics and 50 times larger than molecular shape considerations alone would predict. The magnitude of this
effect renders these materials as promising candidates for efficient conversion between mechanical and electrical energy.
d) The converse of this effect when the bent-core material sandwiched between plastic substrates 4 times thicker than the
liquid crystal material provided displacements in the range of 100nm that is sensitive to the polarity of the applied
field thus suggesting applications as beam steering and precision motion controls.
We report the results of our investigations on the nonlinear optical absorption and refraction of an organic chromophore, neutral red, both in solution and in solid film forms, using z-scan technique. Z scan experiments were performed with the samples in solution form, in methanol and in solid film form in polyvinyl alcohol (PVA) matrix, using 532nm, 20ns, Nd-YAG laser pulses. The pre-focal peak and post focal valley in the closed aperture z-scan for solution sample are suggestive of a negative optical nonlinearity; where as, the open scan for the same indicates that excited state absorption (ESA) takes over saturable absorption (SA) when the input intensity is increased. For the solid film sample, on the other hand, the closed scan exhibits a sharp pre-focal valley followed by a post focal suppressed peak showing the predominance of two photon absorption (TPA) process. The open scan of the solid film has a dip, which is characteristic of ESA. As both solution and solid samples of this chromophore exhibit nonlinear absorption as well as refraction, neutral red can be a promising material for optical limiting applications.
We report the optical limiting performances of a newly synthesized chromophore, 4 (2'-hydroxy- naphthyl azo ) antipyrine ( H-NAP ) and it's rare earth perchlorate complexes, La ( NAP )<sub>2</sub> ClO<sub>4</sub> and Dy ( NAP)<sub>2</sub> ClO<sub>4</sub>, in solution for 20ns pulses of wavelength 532nm. Comparing with the limiting response of C<sub>6</sub>0 in toluene (already reported optical limiter), the limiting threshold of H-NAP and it's complexes were found to be much lower. The origin of the optical limiting behavior observed for the samples are under investigation. To elucidate the various phenomena involved, z-scan experiments were performed in all the samples. From the closed aperture data, a negative refractive non-linearity (de-focusing effect) can be inferred in all the three cases. Larger magnitude of n<sub>2</sub> is expected to result in a more effective optical limiting, which implies a lower threshold for limiting and correspondingly lower output above threshold. The second hyperpolarizability of the compounds has a negative sign and is much larger than the reported values for fullerenes. The limiting in H-NAP can be attributed to the large nonlinear absorption of the material and in La (NAP)<sub>2</sub>ClO<sub>4</sub> and Dy (NAP)<sub>2</sub> ClO<sub>4</sub>, the main mechanism may be de-focusing caused by the nonlinear refractive index. H-NAP and its rare earth perchlorate complexes can hence be considered as very promising materials for optical power limiting.
Novel scattering-type displays using antiferroelectric smectic phases of liquid crystals of bent-shape molecules are reviewed and discussed. There can be two distinct states racemic and chiral that work in opposite ways. The racemic structure is scattering in the OFF state and is optically clear under sufficiently large (E~4-6V/m) electric fields. The chiral structure is transparent at zero fields and scattering in the field ON state. These two structures may be reversibly interchanged implying their use in devices that consume energy only during switching from one stable state to the other. After summarizing the previous results on the film thickness, driving voltage and temperature dependences of the light shutters, new results will be presented on a banana smectic material, which has an optically isotropic transparent antiferroelectric OFF state. We show that the optically isotropic and transparent OFF state can be reversibly switched to birefringent and scattering ferroelectric states in less than hundred microseconds.
Permanently microbent fibers are potential candidates for chemical sensing 1. They behave more or less like an unclad optical fiber in many respects and the basic mechanism involved in chemical sensing application is the evanescent wave absorption. In this manuscript we propose its usage for refractive index measurement of solutions. The sensing configuration employed here is the dark field detection configuration which essentially involves the detection of cladding modes generated in and near the microbent region.
We report the position dependent tuning of fluorescence emission from Rhodamine 6G doped plastic waveguide using side illumination technique . The transmitted fluorescence as a function of the distance from the point of illumination is measured by translating the waveguide horizontally across a monochromatic light source. This technique has proved to be a useful method for characterizing the light propagation properties of dye-doped waveguides. An important finding of the present studies is the nonlinear behavior of the loss coefficient as a function of propagation distance through the waveguide. It is also found that this type of nonlinear nature depends on the dye concentration and thickness of the waveguide.