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.
©2005 Optical Society of America
Photorefractive (PR) materials have drawn great attention in past years due to the potential application of these materials in areas of high-density holographic data storage, optical image processing, optical phase conjugation, real-time holography, optical computing and pattern recognition . Early research on PR effects had been focused exclusively on inorganic materials, such as ferroelectric crystals and semiconductors. In 1990, Sutter et al.  first demonstrated the photorefractivity in an organic crystal. Since then, PR studies using organic base materials, such as polymeric composites, liquid crystal (LC) and organic or organically modified glass, were booming. LCs exhibit attractive qualities including a low electric field (~1V/µm) in the Raman-Nath regime for realization of wave mixing, a large refractive-index modulation arising from the large anisotropy, and are widely used in the display industry. These advantages have caused mesogenic materials to become the center of attention in the relevant studies [3–9]. Consequently, many investigations into the PR effect in dye-, fullerene- and nanotube-doped nematic liquid crystal (NLC) have been reported [6–9].
The principle mechanism of the PR effect in an electro-optical material is generally based on the photoinduced change in the refractive index of the material . First, two interfering laser beams create a spatial light-intensity modulation. Photogeneration and redistribution of charge carriers occur as a consequence of modulated light intensity and spatial modulation in conductivity and dielectric anisotropy, producing an internal space-charge electric field and resulting in a spatial modulation of the refractive index. Similarly, for a PR LC, the orientational PR effect is generally ascribed to a modulation of photogenerated charge density in the bulk, which reorients the molecular director through the space-charge field and, in turn, leads to a modulation in birefringence . A characteristic of the PR effect in LC is that the phase of the refractive-index grating is shifted from the interference pattern, yielding an energy exchange between the two propagating beams .
In this study, PR effects in cells of NLC hybridized with and without montmorillonite clay were investigated. When polarizable particles such as clay are suspended in a dielectric medium (e.g., in this study, NLC), either an applied alternating-current (ac) or direct-current (dc) electric field will induce a dipole on the particles in accordance with the Clausius-Mossotti relation [12–14]. The induced dipole gives rise to a rotational or translational force on the polarizable particles. Hence, for an NLC system impregnated with layer-structured nanoscale clay, it is expected that the electro-optical properties of the NLC device be modified by the hybridizing agent. To the best of our knowledge, degenerate two-wave-mixing effects, including two-beam coupling and Raman-Nath diffraction in an NLC film hybridized with inorganic clay, has never been reported on.
The clay used was PK-802, produced by PAI KONG Nanotechnology LTD, which was an octahedral substituted sodium-type montmorillonite with cation-exchange capacity of 116 meq per 100 g . The NLC used was the eutectic mixture E7 (from Merck) with dielectric anisotropy of 13.1 at 1 kHz and bulk resistivity of the order of 1011 Ω-cm. The organic-inorganic nanocomposites were prepared by adding PK-802, with a content ranging from 0.1 to 3.0% by weight, to E7 and by stirring the mixture for 48 hours at 90°C, followed by 2 hours of ultrasonication. Two indium-tin-oxide (ITO)-coated glass plates were spin-coated with polyimide which was rubbed in opposite directions to promote a planar alignment with a small pretilt angle (<2°). The ITO-glass cell of 30 µm gap was filled with the clay-hybridized NLC fluid by capillary action in the isotropic phase. In order to generate the holographic grating in the NLC cell, a conventional two-wave-mixing experiment was performed with two coherent p-polarized beams of equal intensity of I 1=I 2=45 mW/cm2, derived from an Ar+ laser operating at the 514.5 nm line. No self-diffraction was observed at the normal incidence of the writing beams. The sample cell, with the unperturbed nematic director lying in the plane of incidence, was tilted by 45° from the bisector of the writing beams to provide a projection of the grating wave vector in the direction of the applied dc electric field of 0.5 V/µm. An externally applied field must be present to induce refractive-index change. The schematic geometry of the two-wave-mixing experiment appears in a previous work .
3. Results and discussion
Figure 1(a) presents the representative scanning-electron microscopic (SEM) image of a montmorillonite-clay particle of 5–10 µm in size formed from stacked lamellae with a thickness of approximately 1 nm. Stirring the mixture of montmorillonite clay in the NLC delaminated the stacked lamellae of clay to a smaller size of ~100 nm and permitted the nanoscale particles to be well dispersed as seen in Fig. 1(b).
The first-order diffraction efficiency of the PR gratings, for characterizing the grating strength of a sample, is defined as
where I 1 is one of the incident intensities and I -1 is the intensity of the first-order diffracted beam originating from I 1. Figure 2 shows the dependence of the first-order self-diffraction efficiency on the two-wave-mixing angle. The diffraction efficiency varies with the wave-mixing angle and reaches a maximum at 1.4°, corresponding to a grating constant of 27 µm. The intermolecular elastic torques are associated with nematic director-axis reorientation. The torques produced by the space-charge field are dependent on the grating spacing . This dependence is a result of the balance between the space-charge field and the elastic restoring force. To achieve the considerable wave-mixing effect, the grating constant was fixed in the optimal condition of 27 µm in subsequent experiments.
For determining the type of grating, the dimensionless parameter (quality factor) Q was calculated as follows:
where L, λ and Λ are the grating thickness, wavelength and grating constant, respectively . Since Λ2 was much larger than dλ, the value of Q was smaller than one, indicating the diffraction to be of the Raman-Nath type as experimentally observed on the exit side of the sample cell. Figure 3 shows a typical multiorder self-diffraction pattern from a Raman-Nath grating established in a clay-hybridized NLC cell.
The grating recording dynamics of NLC, hybridized with or without clay, is shown in Fig. 4. The steady diffraction efficiency of the pristine E7 is close to 4% and increases to more than 10% as the content of clay increases to 1 wt% in an NLC cell. The index-grating amplitude for this hybridized NLC is calculated to be Δne=1.9×10-3 using η≈(πdΔne/λ)2 and the nonlinear-index coefficient n 2, defined by n2I=Δne, is thus 4.2×10-2 cm2/W. Note that, as shown in Fig. 4, the diffraction efficiency drops to ~6% as the clay concentration is increased to 3 wt%. Too much concentration (>1 wt%) of clay added to the NLC often causes sample flocculation, which hinders further promotion of the orientational photorefractivity.
Figure 5 briefly describes the formation mechanism of the PR grating recorded in an orientated NLC hybridized with nanoscale clay particles. At first, charge generation occurred in an inhomogeneously illuminated NLC cell in a dc electric field (Fig. 5(a)), then photo-generated charge carriers migrated in the LC bulk. Some were trapped by the clay layers (Fig. 5(b)). That is, photogeneration and, as a reasonable guess, redistribution of charges were enhanced as the NLC is hybridized with smectite clay, which provided extra trapping sites. The combination of light interference pattern and the applied dc electric field produced a space-charge field, which led to molecular reorientation and refractive-index grating in the device (Fig. 5(c)). There might have been other reasons for the increase of diffraction efficiency. For example, the decrease of the orientational order of the NLC, caused by the dispersion of nanoscale clay particles, could have resulted in the reduction of the elastic energy associated with the grating formation.
To demonstrate the PR nature of the gratings encoded in clay-hybridized NLC, an asymmetric two-beam-coupling experiment was conducted. Figure 6 illustrates the time-evolved beam-coupling ratios; i.e., the normalized beam intensities, in E7 and E7 hybridized with 1-wt% clay. When both coherent beams strike the cell at 90 s, the two-beam coupling occurs, and it disappears when one of the two is blocked at 210 s. An asymmetric energy transfer from beam 2 to beam 1 through the nonlocal dynamic grating is evident. In the NLC hybridized with 1-wt% clay, the asymmetric energy transfer is significantly higher than that in the neat counterpart indicating that the orientational PR effect is enhanced by the addition of clay in a mesogenic nematic. It is worth noticing that two-beam coupling within the nanocomposite occurs only when the external electric field is applied and p-polarized light is used, supporting the fact that the observed beam coupling is due to the formation of a PR grating and is not due to a thermal or absorption grating .
In a previous paper , we found that the transient dark current and the ion-charge density in the NLC cell, induced by the polarity reversal of an applied dc voltage, were dramatically reduced when hybridized with montmorillonite clay. Because of the enhancement of the resulting nonlinearity, it is expected that the adequate amount of clay as an additive is effective in promoting photoconductivity of the nematic system. To get a full understanding of the observed orientational PR effect in NLC hybridized with clay, theoretical results as well as extensive experimental data of material characteristics including steady-state photoconduction are certainly needed. Studies along these lines are currently being pursued in our laboratories.
In summary, the orientational PR effects in cells of homogeneously aligned NLC doped with various contents of clay have been observed by means of degenerate two-wave mixing. The Raman-Nath gratings were induced by the interference modulation of two coherent optical beams in conjunction with an applied dc electric field. The experimental results reveal that the Kerr-like optical nonlinearities were enhanced in NLCs impregnated with smectite clay. The clay acted as a dominant charge trap in the NLC matrix and the diffraction efficiency reached ~12% as the concentration of clay increased to 1 wt%. Further increase of the clay concentration resulted in a decrease of the diffraction efficiency of the NLC-clay system.
This research is supported by the National Science Council of the Republic of China under grants NSC-93-2113-M033-010, NSC-93-2745-M-033-004-URD, NSC-93-2112-M033-009 and NSC-93-2216-E-007-019.
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