Orientational Photorefractive Effect Research Articles

Photorefractive surface waves in liquid crystal

We predict the formation of electrically and optically controlled photorefractive surfacewaves in liquid crystal, with orientational photorefractive effects. In contrast to the case forphotorefractive surface waves in inorganic crystals, the penetration depth of photorefractivesurface waves in liquid crystal can be controlled by means of the applied external electricfield, while the position of the photorefractive surface waves in the liquid crystal can becontrolled by means of the applied external electric field and the polarization of incidentlight beams.

DYNAMIC ORIENTATIONAL PHOTO-REFRACTIVE GRATINGS OBSERVED IN CdSe/CdS NANORODS DOPED NEMATIC LIQUID CRYSTAL CELLS

We observed an orientational photo-refractive effect in semiconductor CdSe/CdS nanorods-doped nematic liquid crystal cells homeotropically aligned. In order to study the photo-refractive and the orientational responses of this cell, we employed both asymmetric two-beam coupling (TBC) to measure two beam coupling gain under the influence of an applied electric field and forced light scattering technique. An optical index grating was written in the device in a holographic tilted configuration by overlapping two laser beams at 514 nm. Self diffraction of the writing beams and diffraction of a probe beam at 633 nm were measured. A high TBC gain coefficient of 270 cm-1 was achieved at a DC field applied of 3.4 V/μm. First-order diffraction efficiencies were evaluated with a maximum efficiency of 7% in the dynamic grating with a nonlinear refractive index, n2, 4 × 10-2 cm2/W for E0 = 3 V /μ m .

Anisotropic diffraction pattern formation from a nematic liquid crystals film induced by low-power linearly polarized beam

Anisotropic diffraction patterns were observed on far-field screen induced by a weak (0.16 W/cm2) linearly polarized collimated beam passing through a homeotropic C60-doped nematic liquid crystal (5CB) cell. Based on orientational photorefractive effect, two-wave coupling experiment was carried out to induce liquid crystal molecule re-orientation. The form of space-charge field with the Gaussian distribution induced by the Gaussian beam incident on the sample is investigated. The outline of far-field diffraction pattern is concentric ring, and shows symmetrical gaps in the direction perpendicular to the direction of polarization of light. In addition, diffraction pattern changes when the polarization of the incident light is changed, effective nonlinear index coefficient is n20.3 cm2/W. The results showed that our theoretical analyzsis was consistent with the observed experimental phenomena.

BEAM COUPLING CHARACTERISTICS IN NEMATIC LIQUID CRYSTAL DOPED WITH SILVER NANOPARTICLES

The orientational photorefractive effect was observed in the nematic liquid crystals E7 doped with silver nanoparticles. Both the self-diffraction and beam-coupling effects were studied in a two-wave-mixing experiment with an applied external DC field. The dependence of diffraction efficiency on the applied voltage, pump power, pump/signal ratio and grating constant were studied. Experimental results indicate that the optical non-linearity of NLCs can be enhanced significantly by doping silver nanoparticles. Further experiments demonstrate that the holographic grating formed in the sample can be switched on and off by the external DC field. PR two-beam coupling experiment was also performed in the Ag-doped NLCs cell. These results are of benefit to the applications of PR NLCs in holography and image processing.

AC electric field assisted orientational photorefractive effect in C60-doped nematic liquid crystal

Photorefractive gratings were produced in a C60-doped nematic liquid crystal cell under the application of two coherent beams and a nonbiased sinusoidal ac electric field. The beam coupling and diffraction of the ac electric field assisted gratings were studied systematically. A stable asymmetric energy transference was obtained. Diffraction was observed when the angle (between the normal of the cell and the bisector of the writing beams) was 0°, and the dependence of diffraction efficiency on the peak-to-peak value of the ac voltage was similar to that at an incidence angle of 45°, suggesting that the role of the ac field was to facilitate the charge separation, and the space-charge field (SCF) originated predominantly from the diffusion of the ac electric field assisted photo-induced carriers under the application of nonuniform illumination and an applied ac field. The grating was produced by director reorientation induced by the cooperation of the SCF and the applied ac electric field. A self-erasing phenomenon was observed in this cell. An explanation in terms of the movement of two kinds of carriers with opposite signs was proposed.

Orientational Photorefractive Effect in Undoped and CdSe Nanorods-Doped Nematic Liquid Crystal—Bulk and Interface Contributions

In the work, we revisited the orientational photorefractive effect generated in nematic liquid crystal (NLC) in a degenerate four-wave mixing interaction configuration. A detailed calculation of the bulk laser-induced space-charge fields and analytical expressions for the reorientation angles and the required dc threshold fields are obtained and compared with experimental measurements. While the bulk theory could describe the general features of the self-diffraction and threshold fields, there are significant deviations from experimental observation, particularly in an observed lowering of the threshold voltage by the optical intensity. This effect is due to an optically induced surface-charge modulation. A theoretical analysis including both the bulk and the surface-charge-modulation effects allows us to obtain better agreement with experimental observations of the threshold voltages on the optical intensity and sample thicknesses, and to estimate the relative contributions from the bulk and surface effects. Recently observed enhanced photorefractive effects of CdSe nanorods-doped NLCs are also reported to corroborate these analyses and to provide guidelines to optimize the photorefractive effect for dynamic holographic application.

Photorefractive effect in nematic-clay nanocomposites

The orientational photorefractive effect was observed in an organic-inorganic nanocomposite of nematic liquid crystal hybridized with montmorillonite clay. Both the self-diffraction and beam-coupling effects were evaluated in a two-wave-mixing experiment in conjunction with an externally applied dc field. The experimental results indicate that photoinduced generation was enhanced by the addition of smectite clay with adequate concentration. Physically, the drifting ion charges were trapped by clay layers and separated by interlayer cations, creating an internal, spatially modulated space-charge field, which led to nematic molecular orientation and, then, refractive-index modulation via the electro-optical response. The diffraction efficiency as well as the beam-coupling ratio of the phase gratings recorded in the cells of the nematic liquid crystal hybridized with montmorillonite clay was found to be two to three times higher than that in the pristine nematic cell.

Effects of applied electric field on orientational photorefraction in porphyrin:Zn-doped nematic liquid crystals

We observed an orientational photorefractive effect in porphyrin:Zn-doped nematic liquid crystals by measuring two beam coupling gain and diffraction efficiency under the influence of applied electric field. The gain and diffraction efficiency curves against applied electric field typically reveal resonant type curves, rapidly increasing to its maximum values and then gradually decreasing, but have clearly distinct peak positions. Based on the material equations and the torque balance equation of director axis reorientation of liquid crystals, we theoretically derived the expressions for the orientational photorefractive gain and diffraction efficiency, showing good agreement with the experimental results.

Optical amplification in nematics doped with carbon nanotubes

We present results on the orientational photorefractive effect in a mesogenic nematic doped with multiwalled carbon nanotubes. Efficient coherent-beam amplification of a signal beam by energy exchange with a pump beam was observed in homogeneous cells of the doped liquid crystal E7 at electric fields of ∼0.5 V/μm. Photorefractive gains as high as 103 cm−1 are achieved in the Raman–Nath regime. It is found that the gain coefficient depends strongly on the externally applied dc voltage, the pump/probe intensity ratio and the total input intensity.

Photorefractive Effect in Nematic Liquid Crystal Cell with Sandwiched Structure

ABSTRACTOptical responses and orientational photorefractivities of nematic liquid crystal cells doped with fullerene were investigated with a sandwiched structure. The photorefractive effect was attributed to the reorientation and switching of liquid crystals using a photoalignment layer for vertical alignment. When nematic liquid crystals were injected at a nematic phase, a better cell texture could be obtained due to the capillary flow-induced orientation effect. Samples with this method showed the higher photorefractive effects than those injected at an isotropic phase. Therefore, the state of molecular ordering is important for the orientational photorefractive effects.

Orientational photorefractive effect in nematic liquid crystal with externally applied fields

Light-induced space-charge fields and the related orientational photorefractive effects in nematic liquid crystal cells are studied both theoretically and experimentally. Analytical solutions for the refractive index change distribution are obtained under the single-elastic-constant approximation with various configurations of externally applied electric and magnetic fields and of the nematic liquid crystal director. The effects of the rigid boundary condition are investigated in detail. The grating spacing for which the refractive index change is maximized depends on the strength of external electric and magnetic fields. The responses of parallel cells differ from those of homeotropic cells, both upon the dependencies on the grating spacing and on the external fields. Under the same conditions, the orientational photorefractive effects are stronger in parallel cells than in homeotropic ones. It is also shown that the investigated effects will be greatly enhanced near the Freédericksz transition. Experimental verifications of the photorefractive response in parallel aligned cells confirm the necessity to consider the boundary conditions explicitly.

3D03 高分子液晶・低分子液晶混合系の分子再配列型フォトリフラクティブ効果

3D03 高分子液晶・低分子液晶混合系の分子再配列型フォトリフラクティブ効果

Orientational photorefractive effects observed in poly(vinyl alcohol)/liquid crystal composites

) as a photoconductive sensitizer under an applied dc field. Orientational photorefractivity was demonstrated by observing Raman–Nath diffraction beams with an external dc field. The photorefractive gratings were partially memorized even in the absence of the applied dc field.

Orientational photorefractive effects observed in polymer-dispersed liquid crystals

We present a novel type of photorefractive polymer-dispersed liquid crystal (PDLC) that contains a polymer, low-molecular liquid crystals (LC's), and a photoconductive sensitizer. Photorefractivity is demonstrated by two-beam-coupling experiments with an external dc field. This material shows highly sensitive photorefractivity, and a purely refractive-index grating (Dn >/= 3.0 x 10(-3)) is generated by irradiation of interference light (lambda=632.8 nm) . The grating in the photorefractive PDLC's shows strong anisotropy, and the dynamics depends strongly on the composition. In the case of a 40:60 weight ratio of polymer:LC's, the photorefractive PDLC's present a stable memory effect.

Orientational photorefractive effects in nematic liquid crystal films

We present a detailed theoretical discussion of the fundamental mechanisms for the recently observed nonlinear electro-optical effects in nematic liquid crystal films. Detailed expressions for the three principal mechanisms for space charge fields formation and various dielectric, conductivity and flow induced torques that are responsible for the liquid crystal director axis reorientation process are derived. We have observed strong optical wave mixing effects such as two-beam energy coupling and self-diffraction; the observed effects, their magnitude and dynamics, and dependencies on various physical parameters are found to be in good agreement with theories.

Observation of orientational photorefractive effects in nematic liquid crystals

We report the observation of nonlinear photorefraction in pure aligned nematic liquid crystals in the presence of an applied dc electric field. The effect is attributed to nematic axis reorientation created by the photoinduced space-charge field in combination with the applied dc field. Strong optical nonlinearity, self-diffraction, and beam-coupling effects are observed.