The optical power of diffractive waveplate structures is limited not as much by fabrication technology issues as by the fundamental features of light propagation in complex anisotropic structures. The infinitely thin two-dimensional film approximation does not apply, and the efficiency of 4G lenses, prisms, etc., is reduced for geometries corresponding to sharp focusing lenses and large diffraction angles. Due to thin-film nature, these films can be combined to reduce effective focal length, increase effective diffraction angle, topological charge, etc. Along with this, we will discuss the opportunity of increasing optical power of 4G lenses, prisms, etc. without compromising efficiency.
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.
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).
We study optical switching properties of novel azobenzene liquid crystal (azo LC) material systems based on mesogenic
azo dyes distinguished by enhanced absorption in the visible spectrum and a short lifetime of the photoexcited state. Due
to their mesogenic nature these azo dyes can be doped at high concentrations in room temperature LCs. This permits one
to obtain results using low energy density values required for observation of strong nonlinear optical processes and short
spontaneous restoration times of their original state. A photoinduced nematic-isotropic phase transition could be induced
with a single nanosecond pulse. Thin material layers of the order of radiation wavelength were used in the study.
Azobenzene liquid crystals (azo LCs) have been proven to possess the highest optical nonlinearity, bounding with photosensitivity, for cw laser beams. We show here that azo LCs are highly nonlinear for short laser pulses as well. Single as well as multicomponent room temperature nematic azo LCs were used in this study for single nanosecond pulses of the second harmonic of a Nd:YAG laser. These compositions demonstrate sensitivity starting from ~ 10 mJ/cm2 and exhibit response time at the nanosecond scale. The effect of material composition, layer thickness and pulse
energy on the nonlinear response of a system of crossed polarizers comprising planar oriented LCs are reported.
We report on the photoactuation of high modulus liquid crystalline polymers with side chain azobenzene mesogens (azo-LCP). Photomechanical deformation is actuated with the 442 nm line of a He-Cd laser. Early results show that this
system achieves displacement of up to 85° towards the actinic source when the He-Cd polarization is parallel to the
nematic director and approximately 30-40° when the polarization is orthogonal to the nematic director. A wide range of
materials have been synthesized by the photopolymerization of a liquid crystal diacrylate monomer (RM257) and an
azobenzene-containing monoacrylate liquid crystalline monomer. Interestingly, azo-LCP formed with holographic
photopolymerization have faster bending rates than polymers formed with flood-lit illumination of equivalent intensity.
We study the phenomena of laser beam propagation in azobenzene liquid crystals (azo LCs) in waveguiding configuration. We found that spatial solitons can be formed at microwatt power levels of a He-Ne laser beam. We have observed several well-known processes of nonlinear propagation such as undulation of solitons, their interaction and merging. In addition, we have shown that due to the memory of the nonlinear waveguide in azo NLC, the solitons are steered following mechanical displacement of the LC cell, a phenomenon that can be used for stabilized coupling of radiation into optical fibers. Cis-trans isomerization of azobenzene molecules and related change in the LC order parameter is the underlying mechanism of optical nonlinearity that makes possible formation of solitons. This mechanism is present in the mesophase as well as in the photoinduced isotropic phase of the material. We have shown that the photoinduced isotropic phase may be locally induced to form a waveguide that steers the solitons to large angles without noticeable attenuation of the beam at distances ~ 1 cm. We have presented a simple 3-level theory for describing the complexity of the effects of laser beams on azo LCs.
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.
The combination of liquid crystals (LCs) and reconfigurable nanoparticulate networks results in most versatile materials for controlling light beams. These material systems can be used for developing multi-functional reconfigurable photonics and opto-electronics components and spatial light modulators with ultimate light modulating capabilities. We review here our results in laser recording of one and two-dimensional diffraction gratings and gratings with variable pitch. Nonlinear optical properties of LC with nanoparticulate internal networks and diffraction gratings laser-recorded in these materials are discussed. Nanoparticulate networks are capable of stabilizing the thermodynamic relaxation of photoinduced cis-isomers of molecules in photosensitive azobenzene LCs used as host for the nanoparticulate network leading to bistability of the phase state (anisotropic and isotropic) of the material, and reversible all-optical switching between those states.
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.
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 LC's 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.
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.
Induced by nanosecond pulses of second harmonic generation from Nd:YAG laser the hydrodynamical instabilities in planar- and twist-oriented layers of the nematic 6CHBT doped with anthraquinone dyes under dc electric field in the region above the Freedericksz threshold were revealed. The dependence of diffraction efficiency on voltage is characterized by the two maxima which are corresponding to the two types of EHD-instabilities that have two characteristic decay times, one of them is of the order of 100 ms and second one is equal to several milliseconds.
This study examines the optical response and physical properties of the homologous series 4-n-butyl-4'-n-alkoxyazobenzene. The members of this series all exhibit liquid crystalline phase behavior, and have also been used as dopants in 4-cyano-4'- n'pentylbiphenyl (5CB), a room temperature nematic liquid crystal. The guest-host system and the azobenzene series have been characterized using optical microscopy and UV-vis spectrophotometry. Illumination of these systems with light of a suitable wavelength induces a trans-cis isomerization of the azo- dye molecules which results in a reorientation of the liquid crystal director, often to such an extent that the liquid crystal phase is disrupted, causing an in situ isothermal phase transition. The response of the liquid crystal system to a linearly polarized beam of exciting radiation is examined with the use of a probe He:Ne laser. Changes in light transmission are then detected with a photodiode. Responses are discussed in terms of homologue, cell thickness and temperature.
High power laser induced 'domain'-like reorientation in homogeneously oriented by silicon monoxide dye doped liquid crystals has been observed. Reorientation can be accompanied with writing of static gratings that are induced by radiation on monoxide layer surface.
Possibilities of creating a spatial-temporal light modulator (STLM) with matrix electrodes using energy reflection coefficient modulator at the glass-LC boundary have been studied. Ferroelectric smectic C* which as the optical structural angle between the director and the normal to the smectic layer near 45 degrees is proposed as an LC-medium. The results of investigation make it possible to construct an LC-valve characterized by high transmission (approximately equals 90%) of nonpolarized light in a wide spectrum range in the open state.
The method which allows us to increase the time interval during which an element is 'on,' using twist or S-effect in nematic-photosemiconductor structure is considered. The basis of this consideration is now that this structure exhibits the effect of memory in definite conditions. The memory organization is based on the hysteresis loop existence in an LC having a high Friedericksz threshold value.
Wave-front conjugation (WFC) in four-wave interaction (FWI) of monopulse radiation of ruby laser in dye-activated nematics has been studied. The effect of the absorption saturation of the dye and change in the LC order parameter on the WFC energy efficiency is discussed.
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