Svetlana V. Serak

Laser-induced surface and bulk reorientation of the director in azo-dye-doped liquid crystal cells

The director reorientation in the liquid crystals (LCs) cells initiated by the trans‐cis isomerization of azo-dyes was studied with the aid of pump‐probe‐beam experiments. Two diAerent types of the samples were used, namely, a planar aligned LC layer, doped with an azo-dye, placed between substrates coated with rubbed poly(vinyl alcohol) (PVA) films, and an undoped LC layer placed between substrates, one of which (input) was coated with an untreated azopolymer film and other one was coated with rubbed PVA film. The cells were excited with nanosecond pulses of the second harmonic of an Nd:YAG laser. By comparing the reorientation dynamics with the help of dynamic holography method in both samples it was established that the values of development times of the orientational eAects were the same. The reorientation develops within 15‐17 ls and the eAciency of the orientational gratings is 13‐15%. In both kinds of samples the director orientation on the output surface with the rubbed PVA coating was not changed after irradiation. Contrary, the orientation of the director on the input substrate was changed in both cases. In case of a LC cell with azo-modified surface the reorientation is due to the photoisomerization of the azo-polymer film. In case of the cell with azo-dye-doped LC layer the photoisomerization of dye in the bulk of LC together with PVA film rearrangement on input substrate exert an influence on the director reorientation. ” 2000 Elsevier Science B.V. All rights reserved.

Space charge-induced reorientation in polymethyne dye-doped nematics under excitation with nanosecond laser pulses

Photoelectric phenomena and orientational nonlinearity have been studied which are induced in planar liquid crystal layers oriented by SiO and doped with polymethyne dyes (0.1 wt.%) under irradiation with nanosecond laser pulses. It was demonstrated that illumination of LC cells with Q-switched ruby laser pulses results in a drastic molecular reorientation which develops during 20 ms. This phenomenon is caused by photo-generation of surface and space charges in the LC cell and spatial distribution of space charges along the grating vector and beam propagation direction both with and without a DC field applied. A model is discussed which explains the orientational mechanism of photo-excitation by emergence of electrohydrodynamic instability at presence of both unipolar injection and anisotropy of conductivity and dielectric permittivity.

Four-wave phase conjugation in liquid crystals with photoisomerizable dopants

Abstract The stable and high quality wave front conjugation (WFC) of short laser pulses in azo-dye doped LC was obtained. The effect is based on the conformational nonlinearity and implies that the refractive index changes arise due to the dependence of the LC order parameter on the concentration of photoinduced dye-isomers. It was found that increasing of the pulse energy density in the range (7.5…12.5)×10 −2 J/cm 2 leads to the saturation of the conjugation coefficient. This effect was explained as a consequence of the light-induced changes of the absorption coefficient due to the saturation of the trans–cis transition. A phenomenological description of the nonlinear absorption was made on the base of the rate equations for the number of isomers. The nonlinear constant, which defines the WFC reflection coefficient, was estimated. It equals 0.25 cm 3 /J, that is ten times higher then the corresponding constant of the thermal nonlinearity.

Ordering of a.c. electric-field-induced domains in dye-doped nematics under photoexcitation

Electrohydrodynamic instabilities arising with a.c. electric field applied to planar-oriented cells filled with anthraquinone dye-doped multicomponent nematic mixture have been studied by diffraction methods. It has been found that c.w. He-Ne laser radiation of small power (~1 mW) exerts ordering effect on the LC layer domain structure. The ordered structure can persist in the presence of the same a.c. electric field for a few tens of minutes. The size of periodic optical irregularity has been calculated.

Thin waveplate lenses: new generation in optics

We present new lenses – waveplate lenses created in liquid crystal materials. Waveplate lenses allowed focusing and defocusing laser beam depending on the sign of the circularity of laser beam polarization. Using an electrically-switchable liquid-crystal half-wave retarder we realized switching between focused and defocused beams by the waveplate lens. A combination of two such lenses allowed the collimation of a laser beam as well as the change of focal length of optical system. Lenses of varied size and focal length are presented.

Liquid crystal near-IR laser beam shapers employing photoaddressable alignment layers for high-peak-power applications

Large-scale, high-energy Nd:glass laser systems require beam shapers to control the spatial distribution of the incident intensity. Commercially available liquid crystal (LC) electro-optical spatial light modulators (SLM’s) are frequently employed for this purpose, but their intrinsic requirement for conductive metal or metal-oxide coatings limits their 1054-nm laser-damage thresholds to 230 mJ/cm2 (2.4 ns, 5 Hz), relegating them for use only in low-fluence areas of the laser system. Previously, we demonstrated that passive near-IR LC beam shapers employing coumarin alignment layers patterned by contact photolithography are capable of high resolution and contrast and can withstand incident 1054-nm laser-fluence levels of <30 J/cm2 (1-ns pulse). An evolutionary step to expand the scope of this simple and robust device would be to identify and incorporate into the device structure photoalignment layers that trigger LC bulk reorientation by undergoing reversible optical switching between two predetermined alignment patterns using low-energy polarized UV/visible incident light and have a high near-IR laser-damage threshold. Such “optically driven” LC beam shapers offer the in-system write/erase flexibility of the electro-optical LC SLM’s while eliminating conductive coatings that compromise the laser-damage threshold and electrical interconnects that increase device fragility and complexity. To this end, we have recently identified and evaluated the 1054-nm laser-damage–resistance and coating properties of several commercial azobenzene-based photoswitchable alignment materials. In 1-on-1 and N-on-1 testing, these new materials displayed 1054-nm laser-damage thresholds that compare very favorably to those of previously tested coumarin photoalignment materials (30 to 60 J/cm2).

All-Optical Liquid Crystal Spatial Light Modulators

Nonlinear optical processes in liquid crystals (LC) can be used for construction of all-optical spatial light modulators (SLM) where the photosensitivity and phase modulating functions are integrated into a single layer of an LC-material. Such spatial light integrated modulators (SLIMs) cost only a fraction of the conventional LC-SLM and can be used with high power laser radiation due to high transparency of LC materials and absence of light absorbing electrodes on the substrates of the LC-cell constituting the SLIM. Recent development of LC materials the photosensitivity of which is comparable to that of semiconductors has led to using SLIM in schemes of optical anti-jamming, sensor protection, and image processing. All-optical processes add remarkable versatility to the operation of SLIM harnessing the wealth inherent to light-matter interaction phenomena.

Passive laser Q-switching using a dye-doped liquid-crystalline layer near total internal reflection

Abstract The process of optical switching of a modulator based on nonlinear variation of the reflection coefficient at the “glass–dye-doped liquid-crystalline layer” interface near the critical angle of total internal reflection that occurs as the layer is heated with a pulsed solid-state laser radiation has been studied. Various switching modes based on reflection of the s- and p-polarized waves from either thin ( d ∼ λ ) or thick ( d ≫ λ ) absorbing LC layer have been investigated. Q-switching of a ruby laser by means of the LC modulator has been realized, and giant pulses of 1 J energy and 60 ns duration have been obtained. Changing the LC layer orientation director by a DC field enables to control laser operation mode and giant pulse parameters.

Enhancement of photorefractive effect in nematic liquid crystals

We report observation of the dynamic enhancement of photorefractive effect in dye-doped nematic liquid crystal in the presence of an applied ac electric field. The multi- wave mixing efficiency in a planar cell with photoconductive orienting layers has been increased by two orders.

Bleachable dichroic dye-doped nematics as materials for intra- and extracavity laser elements

The paper presents the results of investigation of bleachable dichroic dye-doped liquid crystal (LC) cells operating as intra-cavity (passive shutter) and extra-cavity (wave-front conjugation mirror) laser elements. Regularities in the saturation of bleachable dichroic eye absorption in a LC have been established taking into account the order parameter. Control of monopulse parameters from free-running spiking mode to monopulse mode by changing the working temperature of the passive LC shutter or the layer director orientation has been achieved. It was shown that in case of wave-front conjugation of nanosecond pulses in LCs doped with bleachable dyes the thermal nonlinearity is considerably higher than the resonance nonlinearity in the range of temperatures close to the phase transition point ((delta) T degree(s)=3 degree(s)C), which results in higher total conjugation coefficient as compared with isotropic bleachable dyes solutions.