S. Slussarenko

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.

Dye-doped liquid crystals as high-resolution recording media.

Light-induced anchoring of the molecular director is reported to be an efficient method for writing permanent holographic gratings in dye-doped liquid crystals. We have achieved higher sensitivity and spatial resolution in these materials with other methods. An energy density as low as 10(-1) J/cm(2) was sufficient to write gratings with a resolution higher than 100 lines/mm.

HIGH RESOLUTION POLARIZATION GRATINGS IN LIQUID CRYSTALS

Efficient recording of polarization gratings in dye-doped liquid crystals is reported. By exploiting the effect of light-induced anchoring of the molecular director, it has been possible to write stable holographic gratings with a diffraction efficiency of 8% at a resolution of 1000 lines/mm. The required surface energy density of 0.3 J/cm2 sets these materials among the most sensitive for optical storage.

Supra optical nonlinearities (SON) of methyl red- and azobenzene liquid crystal - doped nematic liquid crystals

Abstract In nematic liquid crystal films doped with methyl-red dye or azobenzene liquid crystal, we have observed extremely efficient optically induced director axis reorientation effects, and refractive index change coefficients >> 1 cm2/W. The basic mechanisms involved are determined to be optically induced space charge fields and flow reorientation, order parameter modification and molecular torques by the photo-excited dopant molecules. These supra-optical nonlinearities (SONs) enable nonlinear image conversion, optical limiting and holographic grating formation with μW power lasers.

Photo-induced space charge fields, photo-voltaic, photorefractivity, and optical wave mixing in nematic liquid crystals

Abstract We present a detailed discussion of the fundamental mechanisms for recently observed nonlinear electro-optical wave mixing effects in nematic liquid crystal films. Experimental results on photoconductivity, photorefractive-like two beam coupling, beam amplification, self-diffraction, and storage holographic grating formation in dye- and Fullerene C60 and C61 doped nematic films are presented along with a review of two- and four- wave mixing theories. Nematic liquid crystal films are shown to be potentially useful for adaptive and optical storage applications.

Lasing by Second-Order Bragg Diffraction in Dye-Doped POLIPHEM Gratings

A composite LC/polymer structure, patented as POLIPHEM, when properly doped with a dye is a promising material for compact tunable laser sources. We analyze the properties of distributed feedback system in a form of volume periodic grating in a medium with gain. The conditions required for lasing threshold achievement are considered for second-order Bragg diffraction regimes. First experimental results on the lasing in POLIPHEM DFB oscillator are reported.

Light-controlled anchoring energy in nematic liquid crystals

Abstract In this paper we show the possibility of regulating the dose of the anchoring energy under the control of the incident light. Using a dye-doped nematic sample where it was previously demonstrated the effect of light-induced anchoring, we measured the threshold for the Fred-ericksz transition for different irradiation times using a low power laser beam. Our data compare favourably to the existing theory and show that the amount of induced anchoring energy increases linearly with the irradiation time.

Director turning on isotropic surface due to light-induced torque in a bulk of nematic liquid crystal

The effect of the light-induced surface reorientation of the LC director in a cell filled with an LC-azo dye mixture is obtained. The surface reorientation is caused by LC director reorientation in the bulk, which is initiated by a consequent tans-cis-trans isomerization of azo dye molecules accompanied by a variation of their orientation with respect to the polarization of the exciting light. It is shown that the value and the kinetics of the effect depend on the type of the aligning surface.

Laser Beam Modulation Freezing on a Liquid Crystal Surface

Abstract We report the first observation of the formation of permanent light-induced annular patterns in a planar cell filled with dye-doped liquid crystal. The sample is placed between one rubbed surface, giving unidirectional planar alignment, and one isotropic surface, providing degenerated planar alignment. The patterns appear under the irradiation of the isotropic surface through the liquid crystal layer. The experimental results are explained in terms of spatial modulation of the light polarization over the isotropic surface, due to conformation nonlinearity of the mixture. The light-induced adsorption of azo-dye on the initially isotropic surface leads to “freezing” of this modulation and the appearance of a twisted director structure.

Photoalignment Effect Induced by Angular Momentum of Light in Dye-Doped Liquid Crystals

Abstract We studied the effective interaction of the angular momentum of light with a liquid crystal (LC) doped with azo-dye. The experiments were carried out in a combined cell with one substrate providing strong unidirectional planar alignment of LC and the other covered by iso-tropic polymer layer providing a degenerated planar alignment. The effect manifests itself as azimuth reorientation of the liquid crystal director under the action of circularly polarized light in the absorption band of azo-dye. The new orientation of LC was captured by the command surface because of a light-induced alignment memory effect. To describe the experimental dependencies we solved the self-consistent problem of the propagation of a circularly polarized light in the absorbing nonlinear media using the geometric optics approximation. Comparing the experimental data with numerical evaluation we obtain the basic parameters of the system: the orientation nonlinearly coefficient and the value of anchoring of the director wit...