Photorefractive Liquid Crystal Research Articles

Impact of the Liquid Crystal Director Twisting on Two-Beam Energy Exchange in a Hybrid Photorefractive Inorganic-Liquid Crystal Cell

We studied the energy transfer between light beams on the director grating in a hybrid photorefractive liquid crystal (LC) cell assuming the propagation of light waves in the cell to be in the Mauguin regime. This approach makes it possible to trace the change of the gain coefficient dependence on the director grating spacing with the change of the LC director twist. Conditions for the LC flexoelectric parameters and the director helix pitch necessary for transformation the gain coefficient dependence from the nematic to cholesteric type are obtained. The influence of the director splay and bend deformations on the gain coefficient is also studied.

Squeezing of laser pulses in nonlinear-optical LC cell

Controllable compression of the temporal duration of light pulses takes place in a liquid crystal medium via self-action processes of beam mixing, namely, a dynamic Bragg grating formation by incoming light waves with their consequent self-diffraction on the same recorded grating. Wherein, the laser pulse duration should be comparable with the time relaxation constant of the medium while the extent of the temporal compression is controlled by the variation of the input pulse durations and the value of nonlocal response. Full Text: PDF ReferencesU. Bortolozzo, S. Residori and J. P. Huignard, "Beam coupling in photorefractive liquid crystal light valves", J. Phys. D: Appl. Phys. 41, 224007 (2008). CrossRef P. Guner and J.P. Huignard, Photorefractive Matrials and their Applications, 1, 2, and 3 (New York Springer) and references therein (2006). CrossRef S. Bugaychuk and E. Tobisch, "Single evolution equation in a light-matter pairing system", J. Phys. A: Math. Theor., 51 (12), 125201 (2018). CrossRef S. Bugaychuk and R. Conte, "Nonlinear amplification of coherent waves in media with soliton-type refractive index pattern", Phys. Rev. E 86, 026603 (2012). CrossRef A. Iljin, "Light-induced order modification – The way to speed up", Journal of Molecular Liquids 267, 38 (2018). CrossRef A. Iljin, "Transient Modulation of Order Parameter and Optical Non-Linearity in a Chiral Nematic Liquid Crystal", Mol. Cryst. Liq. Cryst. 543, 143 (2011). CrossRef D. Wei, A. Iljin, Z. Cai, S. Residori, and U. Bortolozzo, "Two-wave mixing in chiral dye-doped nematic liquid crystals", Opt. Lett. 37, 734 (2012). CrossRef S. Bugayhuk, A. Iljin, O. Lytvynenko, L. Tarakhan and L. Karachevtseva, Nanoscale Res. Lett., 12, 449 (2017). CrossRef

Enhanced photorefractive performance of the nematic liquid crystal cell containing photoconductive polymer film

Photorefractive (PR) nematic liquid crystals (NLCs) possess large optical non-linearity but coarse grating spacings (tens of microns) and slow response time (generally in the order of 1 s). Here we presented a great improvement in PR performance of NLC cells containing photoconductive films as aligning layers by varying the thickness and the sensitizer concentration of the photoconductive layer to enhance the surface electric field modulation. As a result of optimising, considerable diffraction efficiencies and the response time as fast as tens of milliseconds were obtained from steady-state PR gratings within a wide range of grating spacings from 30 to 0.7 μm. Besides, the grating response was enhanced significantly compared with the previous reports under similar experimental conditions.

Dynamic Amplification of Optical Signals in Photorefractive Ferroelectric Liquid Crystals

The photorefractive effect in ferroelectric liquid crystalline mixtures containing photoconductive chiral compounds was investigated. Ter-thiophene compounds with chiral structures were chosen as the photoconductive chiral compounds, and they were mixed with an achiral smectic C liquid crystal. The mixtures exhibit the ferroelectric chiral smectic C phase. The photorefractivity of the mixtures was investigated by two-beam coupling experiments. It was found that the ferroelectric liquid crystals containing the photoconductive chiral compound exhibit a large gain coefficient of over 1200 cm−1 and a fast response time of 1 ms. Real-time dynamic amplification of an optical image signal of over 30 fps using the photorefractive ferroelectric liquid crystal was demonstrated.

Optimization of plasmon–plasmon coupling in photorefractive layered media

In this paper we study grating-induced plasmon–plasmon coupling in photorefractive layered media using a weak-coupling approximation. The method used is applicable to general layered structures that support both plasmonic and optical modes, such as photorefractive liquid crystal cells. The approximate equations are accurate when compared to S matrix approaches and capture the plasmon propagation at the surface of the device along with the optical modes guided by the layered geometry underneath. Analysis of the resulting model provides insight into the effect of the control parameters in this device and the means to optimize the diffraction efficiency. For example, by considering the case in which the plasmon is spectrally separated from the guided modes it is possible to determine the optimum gold thickness and grating strength required to obtain the strongest possible diffraction.

Theory of hybrid photorefractive plasmonic liquid crystal cells

We use the theory of optical waveguides to study analytically the voltage-dependent response of a surface plasmon polariton (SPP) at the interface between a photorefractive liquid crystal cell and a semi-infinite gold layer. For sufficiently large electric fields the alignment of the liquid crystal can be calculated analytically. The resulting correction to the SPP dispersion relation is then determined in terms of the applied field and the liquid crystal surface alignment relative to the SPP propagation direction. The approximate analytic techniques developed here are shown to be accurate when compared to rigorous diffraction theory and experimental measurements. The approximate equations are a powerful tool of general application. They can be used to study SPP propagation at the interface between a metal and any nonhomogeneous or anisotropic dielectric and are also applicable to self-assembled monolayers and biosensing applications.

Nanoscale interlayer that raises response rate in photorefractive liquid crystal polymer composites

By depositing a nanoscale photoconductive layer on a stable photorefractive (PR) polymeric film, consisting of the polymer poly[N-vinylcarbazole] (PVK) doped with 4,4'-n-pentylcyanobiphenyl (5CB) and C60, both the response rate and beam coupling properties were improved greatly. Systematic measurements and observations unveiled the role played by the additive layer in preventing ion injection from the ITO layer into the PR film and hence in mitigating the charge compensation. A strong fanning effect and high diffraction orders at small angles have demonstrated the excellent PR property in the modified samples used. To demonstrate great potential of the PR composite in the updatable applications, real time double exposure interferometry was performed accordingly with good results.

Beam coupling in photorefractive liquid crystal light valves

Photorefractive liquid crystal light valves (LCLVs) are hybrid devices that combine a nematic liquid crystal layer with a thin monocrystalline Bi12SiO20 (BSO) photorefractive crystal in the form of a cell wall. The device behaves as an optically addressed spatial light modulator, where the photoconductive layer is made of the BSO crystal. Differently from conventional types of spatial light modulators, usually working in retroreflective configuration, the photorefractive light valves work in transmission, thus allowing new applications related to the coupling of the optical beams when they pass through the liquid crystal layer. Here, we review some recent experiments of beam coupling in photorefractive LCLVs. After a characterization of the device in terms of its spatial resolution, which is related to the features of pattern formation in an optical feedback configuration, we present two-beam coupling and optical amplification in single pass experiments. Then, we develop a theoretical model by taking into account the Raman–Nath diffraction of the incoming beams over the thin liquid crystal layer. By using two or more light valves in cascade, we show, both experimentally and theoretically, that the two-wave mixing gain can be enhanced by the Talbot effect related to the multi-passage of the beams through the successive layers of the nematic liquid crystal. Finally, we show that self-pumped phase conjugation can be realized by placing the light valve in a tilted feedback configuration. In this case, the four-wave mixing is spontaneously created through the scattering of the signal beam onto the feedback induced grating.

The coupling and superposition behavior of higher-order diffraction image based on photorefractive nematic liquid crystal materials

We report some photorefractive higher-order diffraction phenomena of two kinds of photorefractive nematic liquid crystal materials. In the experiment, we observed the superposition of higher-order diffraction images, the coupling of higher-order diffraction lights, and the coupling between higher-order diffraction lights and incident beams. The observed behavior suggests the complexity of the coupling process. The properties of higher-order diffraction images are discussed theoretically, which are in good agreement with experimental results.

Dynamic Formation of Diffraction Grating in a Photorefractive Liquid Crystal Cell With Mesoporous $\hbox{TiO}_{2}$ Layers

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We report the photorefractive effect of a layer-structured liquid crystal cell using N3-chemisorbed mesoporous <formula formulatype="inline"><tex>$\hbox{TiO}_{2}$</tex> </formula> layers as photoconductive layers. Well-organized mesoporous <formula formulatype="inline"><tex>$\hbox{TiO}_{2}$</tex></formula> thin films were prepared using surfactant templates. N3 was chemisorbed on the mesoporous <formula formulatype="inline"><tex>$\hbox{TiO}_{2}$</tex></formula> layer to effectively generate excited electrons, which could be injected in the <formula formulatype="inline"> <tex>$\hbox{TiO}_{2}$</tex></formula> layer and participate in the generation of a space-charge field in the <emphasis emphasistype="italic">LC</emphasis> cell. The prepared liqud crystal cell shows unique characteristics of an inorganic photoconductive layer, a temporal diffraction grating formation. </para>

NONLINEAR OPTICAL APPLICATIONS OF PHOTOREFRACTIVE LIQUID CRYSTAL LIGHT-VALVES

We show a novel nonlinear optical medium, which is formed by coupling a liquid crystal layer to a photoconductive crystal. We show that this medium allows the control of the liquid crystal birefringence in a local and dynamical way, so that large optical nonlinearities are obtained on a large transverse section. The properties of the photorefractive liquid crystal light-valves are used to implement transverse pattern formation, two-wave mixing amplification and phase conjugation.

Self-pumped phase conjugation in a liquid crystal light valve with a tilted feedback mirror

We report self-pumped phase conjugation in a photorefractive liquid crystal light valve with a tilted feedback mirror. This is a new scheme for the self-generation of a phase-conjugate replica and can in general be applied to thin nonlinear layers, opening the way to distortion-correction applications of thin media.

Higher-order diffraction images in photorefractive materials

We report some photorefractive higher-order diffraction phenomena of a liquid crystal polymer composite. The photorefractive composite consists of the polymer poly[ N-vinylcarbazole] (PVK) doped with the chromophore 4,4′- n-entylcyanobiphenyl (5CB) and the sensitizer C 60. In two-wave coupling experiments, the signal beam was interfered by a mutually coherent reference beam within composite materials at a small incident angle. Results are given when the composite was placed behind the focal plane, in front of the focal plane, and in the focal plane. Higher-order diffraction images were obtained by amplification and reduction. A theory of the properties of higher-order diffraction images in a photorefractive liquid crystal polymer composite is developed. The theory is in good agreement with the results of experimental work. The experiments demonstrate the feasibility of optical image processing.

Optical control of orientational bistability in photorefractive liquid crystals

Materials with light-controlled properties are extremely attractive for information technologies and, among the wide range of approaches, that of photorefractive media is particularly relevant1,2. Photorefractivity is observed in photoconductors, where charge carriers photogenerated by non-uniform illumination redistribute over macroscopic distances, and where the resulting electric field modulates the refractive index. This effect has been used in a variety of devices for applications ranging from image treatment to beam steering and medical imaging. Compared with the more commonly studied inorganic crystals, photorefractive non-crystalline organic systems3,4,5,6 offer a more technologically appealing alternative, as these materials are mechanically flexible, can be easily modified chemically and are inexpensive. Here we show how a photogenerated space–charge field can be used to control the switching between the two stable orientational states of ferroelectric liquid crystals. The resulting refractive-index patterns are stable, easily erasable and rewritable. In contrast to more traditional organic photorefractive materials, where the refractive index is continuously modulated by the applied field, the liquid-crystalline device described here is a ‘binary’ system, in which the refractive index n switches between two values (Δn∼3×10−2 in this case, but Δn of the order of 10−1 is, in principle, possible).

Pattern formation and direct measurement of the spatial resolution in a photorefractive liquid crystal light valve

In a photorefractive liquid crystal light valve, acting as a Kerr-like nonlinear optical medium, we show the appearance of optical patterns induced by a single mirror feedback. The spatial wavelength of the patterns scales with the distance between the mirror and the valve and the contrast of the patterns decreases for decreasing this distance. We use these properties to setup a new optical scheme for the measurement of the spatial resolution of the nonlinear device.

Characterization of an inorganic photorefractive liquid crystal hybrid cell

High birefringent liquid crystals are attractive for photorefractive applications but device operation, usually restricted to the Raman–Nath regime, can be degraded by small phase shifts between the optical and refractive index gratings and coarse grating spacings with narrow beam intersection angles. Applying external electric fields and having to tilt the cell at an angle to the grating k-vector cause further complications. In this paper, two-beam coupling in hybrid photorefractive cells comprising a nematic liquid crystal layer between inorganic photorefractive windows is described. Photorefractive properties are determined by the windows, while the liquid crystal amplifies the overall refractive index modulation. Bragg matched liquid crystal gain coefficients >1600 cm−1, grating periods <300 nm and wide beam intersection angles with perfect 90° phase shifts between the optical and refractive index gratings at normal incidence without the need for an external field are demonstrated.

High diffraction efficiency and a quasi-permanent grating in photorefractive nematic liquid crystal at low temperature

At a temperature lower than the melting point of pure pentyl-cyanobiphenyl liquid crystal (5CB) we studied the photorefractive effect of 5CB heavily doped with fullerene (C60) and obtained a diffraction efficiency of the photorefractive grating of more than 40%, which is more than twice as high as the best data reported to our knowledge. A quasi-permanent grating was observed with a diffraction efficiency of 11.2% after the grating was kept in the dark for a month.

Reconstruction of two-dimensional optical image from nonlocal gratings in a photorefractive mesogenic composite

Optical image reconstruction, a type of intensity amplification of a two-dimensional optical image, was demonstrated using photorefractive polymer-dissolved liquid crystals. Image amplification originated in nonlocal gratings generated in a photorefractive mesogenic composite consisting of a low-molar-mass nematic mixture, a functionalized copolymer, and sensitizing dye. This process was well-explained using Fourier optics and two-beam coupling theory for photorefractive effects.

Theory of beam coupling in a hybrid photorefractive-liquid crystal cell

A theory of beam coupling in a hybrid photorefractive liquid crystal cell is developed. The hybrid cell consists of layers of undoped nematic liquid crystal contained within wafers of photorefractive crystal or photoconductors. Reorientation of the liquid crystal is induced by the evanescent component of the space charge field propagating from the photorefractive layers, leading to a modulation of the liquid crystal refractive index. Using coupled wave theory, an analytic solution is obtained for the small signal gain in good agreement with previously published experimental work.

Beam coupling ratios and diffraction efficiencies of a layer-structured liquid crystal device; the effect of injection temperature and layer thickness

Beam coupling ratios and diffraction efficiencies of a photorefractive liquid crystal cell were measured while changing the injection temperature and the layer thickness of the liquid crystal in a layer-structured fabrication. A high beam coupling ratio and first-order Raman–Nath diffraction efficiency were observed at low injection temperature and small cell gap, which showed high order parameters. However, as the cell gap was reduced, the shear stress at the interfaces between the liquid crystals and the photoconducting layers increased, so that the reorientation of the liquid crystals by an internal field could be restricted by the geometry and interrupted by a highly-induced surface energy, reducing photorefractive effects. An optimum exists in the neighborhood of 7.5 μm when LCs are injected at 25 °C under our experimental conditions .