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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 16, Iss. 6 — Mar. 17, 2008
  • pp: 4015–4022
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Nanoparticle Doped Organic-Inorganic Hybrid Photorefractives

G. Cook, A. V. Glushchenko, V. Reshetnyak, A. T. Griffith, M. A. Saleh, and D. R. Evans  »View Author Affiliations


Optics Express, Vol. 16, Issue 6, pp. 4015-4022 (2008)
http://dx.doi.org/10.1364/OE.16.004015


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Abstract

The gain characteristics of liquid crystal photorefractive cells doped with ferroelectric nanoparticles has been measured. The liquid crystal two beam coupling gain is found to reverse in sign and increase in magnitude through the addition of ferroelectric BaTiO3 nanoparticles, yielding gain coefficients up to 1100 cm−1 in the Bragg regime. We attribute the novel effects of gain reversal and magnitude increase to interactions between the ferroelectric particles’ spontaneous polarization and the local liquid crystal flexopolarization.

© 2008 Optical Society of America

1. Introduction

Photorefraction in liquid crystals has produced some spectacular advances in recent years. Of particular interest have been undoped nematic liquid crystal photorefractives in which the molecular alignment modulation is achieved through photo-generated space charges in adjacent photorefractive1

1. A. Brignon, I. Bongrand, B. Loiseaux, and J.P. Huignard, “Signal-beam amplification by two-wave mixing in a liquid-crystal light valve,” Opt. Lett. 22, 1855 (1997). [CrossRef]

or photoconducting layers2

2. F. Kajzar, S. Bartkiewicz, and A. Miniewicz, “Optical amplification with high gain in hybrid-polymer-liquid-crystal structures,” Appl. Phys. Lett. 74, 2924 (1999). [CrossRef]

,3

3. S. Bartkiewicz, K. Matczyszyn, A. Miniewicz, and F. Kajzar, “High gain of light in photoconducting polymer - nematic liquid crystal hybrid structures,” Opt. Commun. 187, 257 (2001). [CrossRef]

. Owing principally to effective trap density limitations, operation of these devices has been restricted to the Raman-Nath regime. However, recent work has demonstrated large liquid crystal gain coefficients can be obtained in the Bragg regime if the space-charge field originates from inorganic photorefractive crystals used as windows for undoped nematic liquid crystal cells4–6

4. G. Cook, C. A. Wyres, M. J. Deer, and D. C. Jones, “Hybrid organic-inorganic photorefractives,” SPIE 5213, 63 (2003). [CrossRef]

. Here we propose the use of ferroelectric nanoparticles as a general means of increasing the sensitivity of the liquid crystal matrix to external electric fields through coupling of the liquid crystal director with the nanoparticles. Using low concentrations of ferroelectric nanoparticles, we demonstrate a maximum gain coefficient of approximately 1100 cm−1, an extremely large value for Bragg matched two-beam coupling. We further show that the addition of the ferroelectric nanoparticles may result in a complete reversal of the sign of the beam coupling gain.

At first glance, hybrid photorefractive cells appear to be very simple devices. On exposure to optical intensity gradients (such as the interference between two mutually coherent laser beams) photogenerated charges diffuse to create a modulated space-charge field within the inorganic windows. Penetration of the surface space-charge field into the liquid crystal layer modulates the director alignment of the liquid crystal molecules, creating diffractive beam coupling within the liquid crystal layer4–6

4. G. Cook, C. A. Wyres, M. J. Deer, and D. C. Jones, “Hybrid organic-inorganic photorefractives,” SPIE 5213, 63 (2003). [CrossRef]

. In practice, this inherent simplicity belies a more complex nature. The beam coupling is uni-directional, leading to strong power coupling from one beam to another irrespective of the relative intensities of the beams. The origin of the large unidirectional gain in the liquid crystal layer arises from a combination of local surface induced pre-tilt of the liquid crystal molecules together with splay-induced flexopolarization of the nematic liquid crystal5–7

5. G. Cook, J. L. Carns, M. A. Saleh, and D. R. Evans, “Substrate induced pre-tilt in hybrid liquid crystal/inorganic photorefractives,” Mol. Cryst. & Liq. Cryst. , 453, 141 (2006). [CrossRef]

. A pre-tilt of the liquid crystal molecular surface alignment permits spatial frequency matching of the optical interference pattern and the resulting index grating, while splay induced flexopolarization enables the otherwise nematic material to become sensitive to the sign of the space-charge field4–6

4. G. Cook, C. A. Wyres, M. J. Deer, and D. C. Jones, “Hybrid organic-inorganic photorefractives,” SPIE 5213, 63 (2003). [CrossRef]

. Recent work has demonstrated that low concentrations of ferroelectric nanoparticles can have a pronounced effect on the gain characteristics of liquid crystal systems8–10

8. M. Kaczmarek, A. Dyadyusha, O. Buchnev, Yu. Reznikov, and V. Yu Reshetnyak, “Improved photorefractive response in liquid crystals with ferroelectric nanoparticles,” Nonlinear Opt., Quantum Opt. 35, 217 (2006).

. Our interest has been to investigate the effect such nanoparticles have on the beam coupling characteristics of hybrid photorefractives. In particular, the splayed nature of the liquid crystal director alignment in our cells creates a unique environment to study the underlying physics of nanoparticle interactions with their surroundings in response to an external electrical field.

2. Experiment

Two uncoated inorganic crystals of strontium barium niobate doped with 0.01 weight % cerium (Ce:SBN 60) were used as windows for the liquid crystal cell, each measuring 20 mm × 20 mm × 1.3 mm. The crystal c-axes were aligned parallel to one of the 20 mm long edges and the linear absorption coefficient for each window (light polarized parallel to the c-axis) was approximately 0.5 cm−1. Each crystal was electrically poled to create a single ferroelectric domain. The inside surfaces of the windows were spin coated at 4000 RPM for 30 seconds with a nylon multipolymer (Elvamide® 8023R, supplied by DuPont, prepared as a 0.125 weight % solution in anhydrous methanol), followed by uni-axially rubbing along the Ce:SBN negative c-axis direction with a low-speed nylon roller. Here we define the c-axis negative direction as being the direction opposite to that of beam amplification during two beam coupling experiments. The cell windows were aligned with parallel c-axes and held together using simple spring clips and the cell spacing was defined by 8 µm glass rods dispersed at low concentration in the liquid crystal medium.

The Ferroelectric nanoparticles were prepared by prolonged wet grinding of commercially obtained barium titanate (BaTiO3) powder in a zirconia ball mill. The grinding fluid was heptane, to which oleic acid was added as a surfactant. The addition of a surfactant was necessary to prevent agglomeration of the BaTiO3 nanoparticles and to enable small sizes to be achieved (~10 nm). Typically stock solutions were prepared using a 1:1:20 ratio of BaTiO3:oleic acid:heptane by weight respectively (equivalent to approximately 4.5 weight % nanoparticle stock solution). Immediately before use, the nanoparticle suspensions were placed in an ultrasound bath for a few seconds to ensure complete dispersion. A quantity of each solution was then added to TL205 liquid crystal (Merck) and the heptane solvent allowed to evaporate slowly while gently heating to just below the clearing temperature with occasional excitation in an ultrasound bath to ensure uniform dispersion of the BaTiO3 nanoparticles. Vacuum assisted evaporation of the heptane solvent was not used to ensure no loss of the eutectic mixture components of the TL205 liquid crystal. Periodic weighing of the preparation vessel allowed us to determine when the heptane had been completely evaporated. Each final solution of TL205 liquid crystal contained 0.5 weight % BaTiO3 nanoparticles. The liquid crystal TL205 was selected owing to its low ionic formulation, minimizing the possible occurrence of screening charge formation. The average particle sizes were measured using TEM imaging techniques.

The experimental arrangement is shown in Figure 1. A 532 nm continuous wave Coherent Verdi® laser together with a 50% reflective beam splitter provided the pump and signal beams. Two 100% reflective mirrors controlled the beam intersection angle, θ, creating an interference pattern in the hybrid cell with fringe spacing, Λ, given by Λ=λ/2 sin(θ/2), where λ is the laser wavelength in air. The pump beam power at the hybrid cell was 10 mW with a 4 mm 1/e2 diameter spot size, giving a local peak pump intensity of approximately 160 mW cm−2. Neutral density filters attenuated the signal power to 7 µW, giving a local peak signal intensity of approximately 56 µW cm−2 with a spot size identical to the pump beam. The low signal intensity ensured that the net gain for the hybrid cell remained in the small signal regime during the experiment.

Fig. 1. Experimental arrangement

The hybrid cell was placed at the overlap of the pump and signal beams and oriented approximately normal to the bisector of the two beams with the c-axis of each window in the plane of the incident polarization. The c-axes directions were such that the signal beam was amplified by the Ce:SBN. Prior to filling with liquid crystal, the gain of the cell filled with oil of a similar refractive index to the liquid crystal was measured at each grating spacing to provide reference gain characteristics of the Ce:SBN windows. After cleaning and re-coating the cell windows, the cell was filled with either pure or nanoparticle doped liquid crystal and the gain measurements repeated. For each grating spacing the gain of the liquid crystal was determined by dividing the liquid crystal filled cell gain by the oil filled cell gain. The gain coefficient, Γ, for the liquid crystal is then given by Γ=d −1 loge (G), where G is the small signal gain and d is thickness of the liquid crystal layer. Care was taken to ensure the assembled cells were free of any scatter or liquid crystal defects prior to making two-beam coupling measurements.

3. Results

The physical morphology of the prepared nanoparticle suspensions in heptane and oleic acid varied considerably with grinding time. For grinding periods varying from a few hours to 24 hours, the resulting suspensions progressively and gradually changed from rapid sedimentation to very slow sedimentation to no sedimentation with a gel-like consistency. TEM imaging showed the average nanoparticle size reduced asymptotically as the grinding time increased, reaching a minimum average diameter of approximately 9 nm. Figure 2 correlates the grinding times with the particles sizes and the corresponding morphology of the 4.5 weight % nanoparticle stock solutions.

Fig. 2. BaTiO3 mixture particle sizes and solution morphologies as a function of grinding time.

A comparative study was made to observe the photorefractive effects with different particle sizes used to make up the 0.5 weight % BaTiO3/TL205 liquid crystal solutions. Figure 3 compares the small signal gain measurements for six different source suspensions of BaTiO3/TL205 with the gain obtained by using pure TL205 (no nanoparticles).

Fig. 3. Small signal gain coefficient vs. grating spacing for 0.5 weight % TL205/BaTiO3 mixtures with different particle sizes.

It is clear from the data in figure 3 that the added nanoparticles dramatically influence the optical gain characteristics, increasing the magnitude of the gain and reversing the sign of the beam coupling. These effects become progressively more pronounced as the nanoparticle size is initially reduced, creating a maximum effect for particles of 11.6 nm in diameter. Further reductions in particle size then gradually reduce the gain.

The experiment was repeated using different concentrations of the optimum 11.6nm BaTiO3 nanoparticles. Figure 4 shows how the gain is influenced by nanoparticle concentrations varying between 0.05 weight % to 1 weight %. The trend clearly shows that the gain increases progressively with increased nanoparticle concentration. Concentrations beyond 1 weight % were not used owing to the onset of scatter at larger concentrations (≤1 weight % solutions remained scatter free). The maximum gain coefficient was approximately 1100 cm−1, an extremely large value for Bragg matched two-beam coupling. We did not observe any Raman-Nath diffraction during these measurements.

Fig. 4. Small signal gain coefficient vs. grating spacing for TL205/BaTiO3 mixtures with 11.6 nm diameter particles with different concentrations.
Fig. 5. Small signal gain coefficient vs. grating spacing for 0.5 weight % TL205/BaTiO3 mixtures for parallel and anti-parallel cell rubbing orientations. Filled points are anti-parallel rubbed data, open points are parallel rubbed data.

It is interesting to note that the points of zero gain coefficient in figures 3 and 4 correspond to grating spacings where the positive gain contributions from the TL205 liquid crystal exactly balance the negative gain contributions from the added nanoparticles. It is therefore tempting to consider the gain contributions from the nanoparticles and the liquid crystals as being independent quantities. To test this hypothesis, the gain for cells with and without nanoparticles was measured using parallel rubbed and anti-parallel rubbed Ce:SBN cells. Anti-parallel rubbing minimizes the liquid crystal alignment splay and corresponding gain. However, figure 5 shows that the difference between the nanoparticle doped and undoped maximum gain coefficients is not constant for the two rubbing conditions. The gain contribution from the nanoparticles is therefore directly linked to the gain contributions arising from the pure liquid crystal. The nanoparticle and liquid crystal gains therefore cannot be separated as independent variables.

4. Discussion

The dramatic effect of the nanoparticles is intriguing, especially the reversal of the sign of the beam coupling gain. We may speculate several reasons for this occurrence. The first possibility is that the process of adding nanoparticles increases the ionic contamination of the liquid crystal material. This could result in the accumulation of screening charges close to the surface of the Ce:SBN windows during illumination. To address this issue, we can estimate the penetration of a periodic space-charge field into the liquid crystal layer in the presence of charged impurities. For simplicity we have neglected any inhomogeneity of the LC dielectric function by assuming only small director modulations in a semi-infinite sample. Let us define the charged impurity distribution as ρ±(x, z). At the surface of the Ce:SBN window, z = 0, we assume the electric potential of the space-charge field has the form ϕ(z = 0)=ϕ 0cos qx, where ϕ 0 is determined by the photorefractive field in the SBN window and q=2πΛ . The electric field must obey Poisson’s equation so that Δϕ=ρε0εLC=(ρ+ρ)eε0εLC , where εLC is the relative permittivity of the liquid crystal and e is the electronic unit charge. If we assume the impurity charge distribution satisfies Boltzmann statistics ρ±=ρ0exp(eϕkBT) and the electric potential due to the photorefractive field is small so that eϕkBT<1 , we get ρ±(x,z)ρ0(1eϕkBT) and the solution to the Poisson-Boltzmann equation is then given by ϕ(x, z)=ϕ 0 exp(-q 1 z) cos qx, where q12=q2+ρ0e2ε0εLCkBT . The decay of the surface space charge field with distance into the liquid crystal layer increases with the concentration of charged impurities. At the optimum grating spacing of 1.7 µm, q≈4 × 106 m −1 and the additional term ρ0e2ε0εLCkBT1014m2 at room temperature with 1 weight % of 11.6 nm particles (ρ 0 ~2*1021 m −3). This means that q 1 ≈ 107 m −1 and the electric field decays faster (~exp (-q 1 z)) in ionically contaminated liquid crystal than for pure LC. Therefore the presence of any ionic contamination in the liquid crystal bulk can only diminish the photorefractive field induced torque onto the liquid crystal molecular director.

Although the space-charge field is spatially periodic, the local fields are D.C. in nature, so screening charge migration could account for reversal of the space-charge field within the liquid crystal layer. In steady state we may speculate that charged liquid crystal impurities may aggregate preferentially in the proximity of maximum charge densities close to the surface of the Ce:SBN, but with opposite sign. The possibility therefore exists for field reversal in the liquid crystal layer if the surface charge accumulation becomes sufficiently periodic above the background Boltzmann distribution to create a net surface field which is comparable to the Ce:SBN space charge field. This could be a mechanism to account for the gain reversal observed after adding BaTiO3 nanoparticles to the pure liquid crystal, but this should also result in a reduction of the net torque on the liquid crystal directors with distance into the liquid crystal layer. We therefore discount screening charge effects since this postulation would result in a reduced gain magnitude, conflicting with our observations.

A second possibility is that the addition of the nanoparticles somehow affects the sign and magnitude of the surface liquid crystal pre-tilt. However, measurements made in-situ with compensation for the Ce:SBN birefringence7

7. R. L. Sutherland, G. Cook, and D. R. Evans, “Determination of large nematic pre-tilt in liquid crystal cells with mechanically rubbed photorefractive Ce:SBN windows,” Opt. Express , 14, 5365 (2006). [CrossRef] [PubMed]

have shown that neither the magnitude nor the sign of the TL205 pre-tilt is affected by the presence of the nanoparticles.

We therefore propose that the most likely mechanism for the gain reversal and magnitude increase is to do with the ferroelectric domain structure of the BaTiO3 nanoparticles. If some (or all) of the nanoparticles are single domain, each will have a net spontaneous polarization. Since the splay-induced flexopolarization polarizes the liquid crystal molecules, we can speculate the nanoparticles may locally align approximately anti-parallel to the TL205 flexopolarization field, with their positive poles in contact with the cell windows. Assuming the external space-charge field is not sufficient to re-pole the ferroelectric nanoparticles, the external space-charge field will try to rotate the liquid crystal molecules and the nanoparticles in opposite directions. If we assume strong anchoring between the liquid crystal molecules and the dispersed BaTiO3 nanoparticles, it is plausible that the nanoparticles may drag the liquid crystal directors in the direction opposite to that which would apply for pure liquid crystal with the same flexoelectric polarization and alignment conditions, resulting in a reversal of the cell gain coefficient. A full theoretical treatment of the interaction between the liquid crystal flexopolarization and the nanoparticle spontaneous polarizations will be the subject of a later paper, but we can illustrate the interactions here by estimating and comparing the liquid crystal dielectric polarization, the liquid crystal flexopolarization and the effective polarization due to the presence of the ferroelectric nanoparticles.

The liquid crystal dielectric polarization is given by Pdiel = ε 0 (εLC-1) Esc, where Esc is the photorefractive space charge field. For typical values of the liquid crystal dielectric tensor (ε || = 9.1, εa = 5) and space charge field (~105 V m−1), Pdiel ≈ 10−5 Cm −2.

The liquid crystal flexopolarization is given by11

11. P. G. De Gennes and J. Prost, “The Physics of Liquid Crystals,” Second edition, Clarendon Press, page 136 (1993).

P flex = e 11 n∇·n+e 33 (∇×n×n), where e 1, e 3 ~10−11 Cm −1 are typical liquid crystal flexoelectric coefficients12

12. S. P. Palto, N. J. Mottram, and M. A. Osipov, “Flexoelectric instability and a spontaneous chiral-symmetry breaking in a nematic liquid crystal cell with asymmetric boundary conditions,” Phys. Rev. E 75, 061707 (2007). [CrossRef]

. The spatial derivatives introduce a multiplication factor of 2π/Λ, such that for our optimum grating spacing, Λ = 1.7µm, Pflexeii·2πΛ1011×2π1.7×1064×105Cm2 .

Accurately determining the effective polarization of a liquid crystal colloid containing ferroelectric nanoparticles is not a simple matter. However, we may roughly estimate the effective polarization using the relation Peffαfdv, where d ~0.26 Cm −2 is the spontaneous polarization of BaTiO3 nanoparticles8

8. M. Kaczmarek, A. Dyadyusha, O. Buchnev, Yu. Reznikov, and V. Yu Reshetnyak, “Improved photorefractive response in liquid crystals with ferroelectric nanoparticles,” Nonlinear Opt., Quantum Opt. 35, 217 (2006).

, v ≈ 8*10−25 m 3 is the volume of each nanoparticle (11.6 nm diameter), f ~2*1021 m −3 is the effective volume concentration of the particles (at 1 weight % of 11.6 nm diameter particles) and parameter α accounts for the particles orientational distribution function. The simplest model for the particle orientations distribution is a two level system where the possible particle spontaneous polarization orientations are either along electric field or in the opposite direction. With this simplification, αdvEkBT0.5 and Peff ≈ 2 × 10−4 Cm −2.

Comparing the estimates for the respective polarizations, we see that the dominant contributions come from the flexopolarization and the effective liquid crystal polarization due to the ferroelectric nanoparticles. Depending on the relative sign of these polarizations the net polarization can have a different sign and we may expect to observe a change of the LC director reorientation direction and therefore a reversal of the optical coupling direction. For our system, since PeffPflex, adding the ferroelectric nanoparticles also increases the optical gain.

The balance of torques onto LC director due to nanoparticles and due to flexopolarization depends on the grating spacing and on the particle concentration. This is because the flexopolarization interaction with the space charge field depends on the derivative of the space charge field (which is inversely proportional to the grating spacing), but the net spontaneous polarization is proportional to particles concentration. The torque arising from the nanoparticle spontaneous polarization is therefore proportional to the space charge field, while the competing torque arising from the splay-induced flexopolarization is proportional to the local rate of change of the space-charge field.

The existence of an optimum particle diameter in figure 3 is probably associated with the ferroelectric domain structure of the nanoparticles. If the particles are too large, there exists the possibility of multiple ferroelectric domains being present within the majority of particles, reducing the net spontaneous polarization field. If the nanoparticles are too small the ferroelectric properties vanish (e.g. the Curie temperature falls below room temperature13

13. T. Ohno, D. Suzuki, H. Suzuki, and T. Ida, J. Soc. Powder Technology, 41, 2, 86–91 (2004, in Japanese) and KONA, no. 22, 195 (2004, English translation). [CrossRef]

). Somewhere between these two extremes the particle size becomes energetically favorable to support a single ferroelectric domain, maximizing the particle sensitivity to the local space-charge and flexopolarization fields.

We should also note that other effects may prove to be important in this system. The permanent spontaneous polarization (and any additional space-charge field induced polarization) of the nanoparticles effectively creates tiny dipoles which may lead to particle migration in the presence of the fringing field gradient from the surface space-charge field. This could lead to a periodic concentration gradient of nanoparticles across the illuminated area.

Acknowledgements

We gratefully acknowledge useful discussions with Nelson Tabiryan, Malgosia Kaczmarek, Serguey Odoulov and Igor Pinkevich. This work has been partially supported by EOARD grant no 078001.

References and links

1.

A. Brignon, I. Bongrand, B. Loiseaux, and J.P. Huignard, “Signal-beam amplification by two-wave mixing in a liquid-crystal light valve,” Opt. Lett. 22, 1855 (1997). [CrossRef]

2.

F. Kajzar, S. Bartkiewicz, and A. Miniewicz, “Optical amplification with high gain in hybrid-polymer-liquid-crystal structures,” Appl. Phys. Lett. 74, 2924 (1999). [CrossRef]

3.

S. Bartkiewicz, K. Matczyszyn, A. Miniewicz, and F. Kajzar, “High gain of light in photoconducting polymer - nematic liquid crystal hybrid structures,” Opt. Commun. 187, 257 (2001). [CrossRef]

4.

G. Cook, C. A. Wyres, M. J. Deer, and D. C. Jones, “Hybrid organic-inorganic photorefractives,” SPIE 5213, 63 (2003). [CrossRef]

5.

G. Cook, J. L. Carns, M. A. Saleh, and D. R. Evans, “Substrate induced pre-tilt in hybrid liquid crystal/inorganic photorefractives,” Mol. Cryst. & Liq. Cryst. , 453, 141 (2006). [CrossRef]

6.

D. R. Evans and G. Cook, “Bragg-matched photorefractive two-beam coupling in organic-inorganic hybrids,” J. Nonlinear Opt. Phys. Mat. , 16, 271 (2007). [CrossRef]

7.

R. L. Sutherland, G. Cook, and D. R. Evans, “Determination of large nematic pre-tilt in liquid crystal cells with mechanically rubbed photorefractive Ce:SBN windows,” Opt. Express , 14, 5365 (2006). [CrossRef] [PubMed]

8.

M. Kaczmarek, A. Dyadyusha, O. Buchnev, Yu. Reznikov, and V. Yu Reshetnyak, “Improved photorefractive response in liquid crystals with ferroelectric nanoparticles,” Nonlinear Opt., Quantum Opt. 35, 217 (2006).

9.

O. Buchnev, A. Dyadyusha, M. Kaczmarek, V. Yu Reshetnyak, and Y. Reznikov, “Enhanced two-beam coupling in colloids of ferroelectric nanoparticles in liquid crystals,” JOSA B ,24, N7, 1512, (2007). [CrossRef]

10.

F. Li, O. Buchnev, C. I. Cheon, A. Glushchenko, V. Yu Reshetnyak, Y. Reznikov, T. J. Sluckin, and J. L. West, “Orientational coupling amplification in ferroelectric nematic colloids,” Phys. Rev. Lett. 97, 147801 (2006). [CrossRef] [PubMed]

11.

P. G. De Gennes and J. Prost, “The Physics of Liquid Crystals,” Second edition, Clarendon Press, page 136 (1993).

12.

S. P. Palto, N. J. Mottram, and M. A. Osipov, “Flexoelectric instability and a spontaneous chiral-symmetry breaking in a nematic liquid crystal cell with asymmetric boundary conditions,” Phys. Rev. E 75, 061707 (2007). [CrossRef]

13.

T. Ohno, D. Suzuki, H. Suzuki, and T. Ida, J. Soc. Powder Technology, 41, 2, 86–91 (2004, in Japanese) and KONA, no. 22, 195 (2004, English translation). [CrossRef]

OCIS Codes
(160.1190) Materials : Anisotropic optical materials
(160.3710) Materials : Liquid crystals
(160.5320) Materials : Photorefractive materials

ToC Category:
Materials

History
Original Manuscript: January 3, 2008
Revised Manuscript: February 26, 2008
Manuscript Accepted: March 3, 2008
Published: March 11, 2008

Citation
G. Cook, A. V. Glushchenko, V. Reshetnyak, A. T. Griffith, M. A. Saleh, and D. R. Evans, "Nanoparticle doped organic-inorganic hybrid photorefractives," Opt. Express 16, 4015-4022 (2008)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-6-4015


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References

  1. A. Brignon, I. Bongrand, B. Loiseaux and J. P. Huignard, "Signal-beam amplification by two-wave mixing in a liquid-crystal light valve," Opt. Lett. 22, 1855 (1997). [CrossRef]
  2. F. Kajzar, S. Bartkiewicz, and A. Miniewicz, "Optical amplification with high gain in hybrid-polymer-liquid-crystal structures," Appl. Phys. Lett. 74, 2924 (1999). [CrossRef]
  3. S. Bartkiewicz, K. Matczyszyn, A. Miniewicz, and F. Kajzar, "High gain of light in photoconducting polymer - nematic liquid crystal hybrid structures," Opt. Commun. 187, 257 (2001). [CrossRef]
  4. G. Cook, C. A. Wyres, M. J. Deer, and D. C. Jones, "Hybrid organic-inorganic photorefractives," SPIE 5213, 63 (2003). [CrossRef]
  5. G. Cook, J. L. Carns, M. A. Saleh, and D. R. Evans, "Substrate induced pre-tilt in hybrid liquid crystal/inorganic photorefractives," Mol. Cryst. Liq. Cryst.  453, 141 (2006). [CrossRef]
  6. D. R. Evans and G. Cook, "Bragg-matched photorefractive two-beam coupling in organic-inorganic hybrids," J. Nonlinear Opt. Phys. Mat.  16, 271 (2007). [CrossRef]
  7. R. L. Sutherland, G. Cook, D. R. Evans, "Determination of large nematic pre-tilt in liquid crystal cells with mechanically rubbed photorefractive Ce:SBN windows," Opt. Express 14, 5365 (2006). [CrossRef] [PubMed]
  8. M. Kaczmarek, A. Dyadyusha, O. Buchnev, Yu. Reznikov, and V. Yu Reshetnyak, "Improved photorefractive response in liquid crystals with ferroelectric nanoparticles," Nonlinear Opt., Quantum Opt. 35, 217 (2006).
  9. O. Buchnev, A. Dyadyusha, M. Kaczmarek, V. Yu Reshetnyak, and Y. Reznikov, "Enhanced two-beam coupling in colloids of ferroelectric nanoparticles in liquid crystals," J. Opt. Soc Am. B 24, 1512 (2007). [CrossRef]
  10. F. Li, O. Buchnev, C. I. Cheon, A. Glushchenko, V. Yu Reshetnyak, Y. Reznikov, T. J. Sluckin, and J. L. West, "Orientational coupling amplification in ferroelectric nematic colloids," Phys. Rev. Lett. 97, 147801 (2006). [CrossRef] [PubMed]
  11. P. G. De Gennes and J. Prost, The Physics of Liquid Crystals, Second ed., (Clarendon Press, 1993) pp. 136.
  12. S. P. Palto, N. J. Mottram, and M. A. Osipov, "Flexoelectric instability and a spontaneous chiral-symmetry breaking in a nematic liquid crystal cell with asymmetric boundary conditions," Phys. Rev. E 75, 061707 (2007). [CrossRef]
  13. T. Ohno, D. Suzuki, H. Suzuki, T. Ida, J. Soc. Powder Technology, 41, 2, 86-91 (2004, in Japanese) and KONA, no. 22, 195 (2004, English translation). [CrossRef]

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