L. Lucchetti

Dye-doped photorefractive liquid crystals for dynamic and storage holographic grating formation and spatial light modulation

The basic mechanisms of photo-induced space charge field formation, director axis re-orientation, and refractive index changes in fullerene C60- and dye-doped nematic liquid crystals films are presented. In particular, in aligned methyl-red-doped nematic liquid crystal film, we observe a nonlinear index change coefficient as high as 10 cm/sup 2//W, associated with purely optically induced liquid crystal director axis re-orientation. Experimental observations of dynamic and high-resolution storage holographic grating formation, two beam coupling with gain of nearly 3000 cm/sup -1/, optical limiting action at nanowatt cw laser power, and incoherent-coherent image conversion at /spl mu/W/cm/sup 2/ light intensity level are discussed.

All-optical steering of soliton waveguides in dye-doped liquid crystals

Soliton induced waveguides can be all-optically readdressed in planar cells by modifying the molecular anchoring with an external light source. Using an elliptically shaped control beam we demonstrate refraction and total internal reflection of spatial solitons in dye-doped nematic liquid crystals.

Colossal optical nonlinearity induced by a low frequency external electric field in dye-doped liquid crystals

We report on the effects of a low-frequency electric field on the optical nonlinear response of thin dye-doped liquid crystal cells. Experimental data show that the external field allows reaching extremely high values of the optical nonlinearity without any critical control of the cell interfaces. A qualitative interpretation of the collected data, based on the light-induced modulation of the bulk voltage through surface modifications, is proposed.

Pretransitional enhancement of the optical nonlinearity of thin dye-doped liquid crystals in the nematic phase

We report a critical enhancement of the nonlinear optical response of dye-doped liquid crystals obtained by approaching the clearing point in the nematic phase. The enhancement of the nonlinear diffraction efficiency has been measured in the range 0.2–0.3 deg below the nematic-isotropic transition temperature. It allows reaching a nonlinear refractive index higher than 40cm2∕W in all the tested cells.

Light-induced adsorption and desorption in dynamic and stable grating formation in methyl-red doped liquid crystals

A detailed study of the light-induced gratings in Methyl-Red-doped 4-n-pentyl-4′-cyanobiphenyl (5CB) homeotropically aligned is reported. In particular, the dependence of diffraction efficiency upon the pump and probe polarizations has been investigated both for transient and permanent gratings. Our results confirm the earlier observations on the role of light-induced adsorption of MR molecules on the cell boundaries and demonstrate the common origin of the supranonlinear response and the permanent recording effect.

Sine: Surface Induced Nonlinear Effects

Pump-probe and TIR experiments have been performed on the dye-doped nematic liquid crystal 5CB, in order to study the basic mechanisms of the extraordinarily large nonlinear response recently reported. Experimental data show that light-induced modifications of the anchoring conditions can be the origin of the observed effect. The bulk reorientation due to the collective elastic behaviour of the liquid crystal gives then rise to the nonlinearity, which occurs without a direct optical torque on the molecular director in the bulk. We have called this effect SINE (Surface Induced Nonlinear Effect).

Coarsening and phase separation in ultraviolet cured polymer dispersed liquid crystals

An extended investigation of the phase separation process in polymer dispersed liquid crystals is presented. A detailed analysis of the experimental results shows that the nucleation and growth of the liquid crystal droplets from the polymeric matrix can be described as the coarsening process occurring in binary alloys.

Fast optical recording in dye-doped liquid crystals

Light-induced director reorientation in dye-doped nematic liquid crystals was recently reported to be an efficient method of writing permanent holographic gratings with high sensitivity [Phys. Rev. Lett. 82, 1855 (1999)]. We report the achievement of stable director reorientation in the same materials by means of a single 4-ns pulse of the second harmonic of a Nd:YAG laser. Fast recording of high-resolution holographic gratings (more than 500 lines/mm) can be obtained with an energy density as low as 7x10(-3)J /cm(2) .

Effects leading to colossal optical nonlinearity in dye-doped liquid crystals

In this paper, we report our most recent results about the huge optical nonlinearity of methyl red (MR)-doped 5CB thin cells. The basic concepts of the surface-induced nonlinear effect interpretation are recalled and possible methods to obtain a better control of the samples response are discussed.

Surface reorientation induced by short light pulses in doped liquid crystals

Fast surface reorientation induced by a single 4-ns low-energy laser pulse in dye-doped liquid crystals is reported. The reorientation is due to light-induced modification of the surface anisotropy, which affects the liquid crystals director through the appearance of a preferred direction on the irradiated surface. The detected signals can be interpreted as being the result of light-induced desorption and adsorption of dye molecules.