Showing papers on "Organic photorefractive materials published in 2007"
TL;DR: In this paper, the authors present the concept of photorefractive beam coupling in hybrid photore-fractive cells comprising a liquid crystal layer adjacent to inorganic photore fractive windows, as well as the overall advantages of this architecture.
Abstract: This review article presents the concept of photorefractive beam coupling in hybrid photorefractive cells comprising a liquid crystal layer adjacent to inorganic photorefractive windows. The roles of the liquid crystal layer and the photorefractive windows, as well as the overall advantages of this architecture, are explained. The mechanism involved and the required window parameters to break the local alignment symmetry in order to achieve Bragg-matched two-beam coupling in a liquid crystal layer are discussed.
29 citations
TL;DR: In this paper, the photorefractive properties of a poly(Nvinylcarbazole) (PVK) matrix with different concentrations of 9-(2-ethylhexyl) carbazole (EHCz), and doped with the photosensitizer C60 and the electro-optic push-pull chromophore 4-piperidinobenzylidene malonitrile were analyzed by two-beam coupling experiments.
Abstract: We study by two-beam coupling experiments the photorefractive properties of a poly(Nvinylcarbazole) (PVK) matrix plasticized with different concentrations of 9-(2-ethylhexyl)carbazole (EHCz), so-called liquid carbazole, and doped with the photosensitizer C60 and the electro-optic push-pull chromophore 4-piperidinobenzylidene malonitrile. The steady state and dynamic photorefractive performances of these materials are optimized by tuning the concentration ratio between PVK and EHCz. In parallel, the trap density values are determined by a spectroscopic method based on the strong absorption of the C60− anion in the near infrared region and are compared with those obtained from the photorefractive measurements. The results obtained from both techniques are in good agreement and show the important role played by the liquid carbazole in the charge trapping processes and the photorefractive properties of C60 sensitized photorefractive polymers.
27 citations
TL;DR: It is revealed that a TE-like full band-gap exists for materials of refractive index as low as 1.6 and the Q-factor of L3 photonic crystal nano-cavities showed a super-linear increase withRefractive index.
Abstract: We model the optical properties of L3 photonic crystal nano-cavities as a function of the photonic crystal membrane refractive index n using a guided mode expansion method. Band structure calculations revealed that a TE-like full band-gap exists for materials of refractive index as low as 1.6. The Q-factor of such cavities showed a super-linear increase with refractive index. By adjusting the relative position of the cavity side holes, the Q-factor was optimised as a function of the photonic crystal membrane refractive index n over the range 1.6 to 3.4. Q-factors in the range 3000-8000 were predicted from absorption free materials in the visible range with refractive index between 2.45 and 2.8.
22 citations
TL;DR: In this article, a photorefractive reflection gratings with grating periods of about 0.2μm using a standard low-glass-temperature polyvinylcarbazole based composite was used to obtain the highest external diffraction efficiency and two-beam coupling gain at moderate applied fields.
Abstract: To date highest external diffraction efficiency and two-beam coupling gain at moderate applied fields (about 30% and 260cm−1, respectively, at 60V∕μm) are obtained in photorefractive reflection gratings with grating periods of about 0.2μm using a standard low-glass-temperature polyvinylcarbazole based composite. Reflection gratings exhibit five times faster growth/erasure rates than conventional transmission gratings. Further, their performance is very sensitive to changes in the saturation field. This allows a reliable calculation of the photorefractive trap density, which shows a significant enhancement with the field. Finally, the theoretical analysis reveals the geometry-dependent competition between the birefringence and electro-optic contributions.
16 citations
TL;DR: The investigation of the light-induced changes of refractive index in a LiNbO(3): Fe sample in case of strip-like illumination shows that character of the dependences of the refractiveIndex on the coordinate perpendicular to the illuminated strip is different for different strip orientation.
Abstract: The investigation of the light-induced changes of refractive index in a LiNbO3: Fe sample in case of strip-like illumination is presented in the contribution. The changes of the refractive index are visualized by interferograms of the sample obtained for various orientation of the illuminated strip and various polarization of the light used during interferogram creation. The investigation shows that character of the dependences of the refractive index on the coordinate perpendicular to the illuminated strip is different for different strip orientation. It indicates the possibility that for different orientation of gradient of the illumination the different mechanisms are responsible for changes of the refractive index.
13 citations
01 Jan 2007
13 citations
TL;DR: In this article, a theory of the properties of higher-order diffraction images in a photorefractive liquid crystal polymer composite is developed, which is in good agreement with the results of experimental work.
11 citations
TL;DR: The nonlinearities in the periodically poled LiNbO(3) induced by 532 nm continuous waves are believed to be mainly due to the photorefractive effect.
Abstract: Optical nonlinearities of periodically poled LiNbO3 crystals were investigated by the single beam Z-scan technique with a continuous wave (cw) laser beam at 532 nm. The nonlinear optical absorption coefficient and refractive index change are determined to be 8.1×10−6 cm/W and 2.6×10−4 at 0.5 MW/cm2 light intensity, respectively. Both sign and magnitude of the measured refractive nonlinearity are considerably different from the Z-scan results in congruent LiNbO3. The nonlinearities in the periodically poled LiNbO3 induced by 532 nm continuous waves are believed to be mainly due to the photorefractive effect.
11 citations
TL;DR: In this article, a polyvinyl carbazole based photorefractive composite was used to tune the near-infrared holographic recording speed by illuminating it at a wavelength of very strong absorption.
Abstract: The authors report on tuning the near-infrared holographic recording speed in a poly(N-vinylcarbazole) based photorefractive composite by illuminating it at a wavelength of very strong absorption. Due to the small penetration depth of the light under these conditions this approach allows to flood the material with charge carriers from the side of the sample. Even at light levels much stronger than the write beams, this additional illumination does allow for grating recording. However, under these conditions competition between positive and negative charges leads to sign inversion of the two-beam coupling gain coefficient during recording. An improvement of the recording speed is demonstrated.
10 citations
TL;DR: In this article, a novel optical medium formed by a liquid crystal layer coupled to a thin photorefractive crystal, allowing a dynamical control of the liquid crystal birefringence was presented.
Abstract: We present a novel optical medium formed by a liquid crystal layer coupled to a thin photorefractive crystal, allowing a dynamical control of the liquid crystal birefringence. We show two-wave mixing amplification, optical pattern formation and self-pumped phase conjugation.
10 citations
TL;DR: In this article, a two-beam coupling was performed in fullerene C60-doped nematic liquid crystal cells with an input pump power as weak as 16mW.
Abstract: Photorefractive two-beam coupling was performed in fullerene C60-doped nematic liquid crystal cell. Photorefractive gain coefficient as high as 1386cm−1 was obtained at an input pump power as weak as 16mW. However, the thin film nature led to a small gain of 16. By the use of several such cells stacked together to increase the overall interaction length, the optical gain was improved greatly, and the pump power to obtain the highest optical gain was reduced. At 1.2V, the highest gain of 141 was obtained for the 40μW signal at an input pump power of 12mW.
TL;DR: In this paper, the authors consider the modes of surface waves with the diffusion mechanism by the band transport model of the photorefractive effect and the nonlinear equation for the first time.
Abstract: We consider the modes of surface waves with the diffusion mechanism by the band transport model of the photorefractive effect and the nonlinear equation for the first time. We demonstrate the relationship between the modes of surface waves and the angles of incidence. The conditions for the realization of surface modes at the interface between photorefractive crystal and a metal, a linear dielectric with a lower refractive index, or another photorefractive crystal are discussed.
TL;DR: In this paper, the photorefractive effect of C70-doped bistable surface stabilized ferroelectric liquid crystals is investigated and an innovative approach toward the optical control of polarization switching in C70doped Bistable devices by means of a photogenerated space charge field is described.
Abstract: In this paper, results of studies regarding the photorefractive effect in bistable surface stabilized ferroelectric liquid crystals are reported. In particular, an innovative approach toward the optical control of polarization switching in C70-doped bistable devices by means of a photogenerated space-charge field is described. The photorefractive nature of the resulting refractive index gratings is confirmed by two-beam coupling and direct phase shift measurements. The influence of an applied field and of light intensity on grating formation is also studied. The photorefractive gratings obtained by using this approach are stable, easily erasable, and rewritable. The described devices are “binary” systems, in which the refractive index switches only between two possible values. Bistable systems constitute a completely new class of photorefractive materials, with peculiar properties which distinguish them from previously developed media.
TL;DR: In this paper, the photorefractive performance of bistable surface stabilized chiral smectic C cells doped with C70 was studied, and it was shown that the illumination efficiency depends on the combination of the application time of the electric field used during the writing process and the light intensity.
Abstract: The authors studied the photorefractive performance of bistable surface stabilized chiral smectic C cells doped with C70. Diffraction efficiency strongly depends on the combination of (a) the application time of the electric field used during the writing process and (b) light intensity. By choosing the conditions of the writing process, gratings with high efficiency can form only for certain levels of light intensity.
TL;DR: In this paper, the formation time of the photorefractive surface wave is related to the intensity of the incident beam and the background illumination, and the experimental results showed that an appropriate background illumination will shorten the photOREfractive response time.
Abstract: Photorefractive surface waves on the surface of Sr0.6Ba0.4NbO3 with a diffusion mechanism of nonlinearity have been observed. The formation time of the photorefractive surface waves is related to the intensity of the incident beam and the background illumination. The formation time of the photorefractive surface waves is about 40 min with a beam intensity of 3 mW, 25 min with 10 mW, and 30 s with 10 mW when an additional background illumination is applied. The experimental results showed that an appropriate background illumination will shorten the photorefractive response time.
TL;DR: In this paper, a nonlinear optical active tricyanopyrroline-based plasticizer was also synthesized and introduced into the prepared molecular glass so as to control the glass transi-tion temperature of the photorefractive matrix.
Abstract: The authors report the near-IR sensitive star-shaped molecule [D-(CBz-TCP)3] containing bifunctional chromophores. A nonlinear optical active tricyanopyrroline-based plasticizer was also synthesized and introduced into the prepared molecular glass so as to control the glass transi-tion temperature of the photorefractive matrix. In a doped glass, they observed significantly enhanced photorefractive performances with regard to the asymmetric energy transfer and the diffraction efficiency at 830nm. In the presence of a small amount of (2,4,7-trinitro-9- fluorenylidene)malononitrile, they could demonstrate a very large net two beam coupling gain higher than 300cm−1 and a diffraction efficiency of 58%.
10 Apr 2007
TL;DR: In this article, the photorefractive effect in LiNbO 3 crystals was investigated from light diffraction observed on photore-fractive records of periodic optical field and obtained from interference images of refractive index modulation in the records.
Abstract: The contribution presents results of the experimental investigation of the photorefractive effect in LiNbO 3 crystals obtained
from light diffraction observed on photorefractive records of periodic optical field and obtained from interference images of
refractive index modulation in the records. These results are compared with results following from equations describing the
process of the photorefractive record formation in cases when various mechanisms of generation of the internal electric
current responsible for photorefractive effect are taken into account. The comparison between the calculated and measured
dependences shows that the modulation of the refractive index is not caused by
the electro-optical effect (as the standard model assumes) but by the redistribution of carriers trapped at the donor centres and
by change of their polarisability.
TL;DR: In this paper, the authors investigated the dependency of photorefractive speed against ionization potentials of the photoconductive polymers and found that the composite with the lowest ionisation potential gives the highest photoconductivity.
Abstract: Photorefractive response time has been an important issue to be improved. We investigate the dependency of photorefractive speed against ionization potentials of the photoconductive polymers. The composite with the lowest ionization potential gives the highest photoconductivity. Fast component of the response time is found to be strongly dependent of photoconductivity of the composite.
TL;DR: In this article, the photorefractive properties of near IR-sensitive amorphous molecular glasses containing the tricyanopyrroline (TCP) chromophore were reported.
Abstract: We report the photorefractive properties of near IR-sensitive amorphous molecular glasses containing the tricyanopyrroline (TCP) chromophore Second-order nonlinear optical chromophores bearing the TCP acceptor were synthesized to improve the optical nonlinearity and the infrared sensitivity via an intramolecular charge transfer complex 2-(1-Allyl-4-{4-[bis-(2-ethyl-hexyl)-amino]phenyl}-3-cyano-5-oxo-1,5-dihydro-pyrrol-2-ylidene)-malononitrile (DEHA-TCP) and 2-[3-cyano-4(4-dibutylamino-phenyl)-5-oxo-1-propyl-1,5-dihydro-pyrrol-2-ylidene]-malononitrile (DBAP-TCP) have relatively high figures of merit compared to other conventional chromophores The photoionizable TCP chromophore was employed to fabricate thick film samples in the presence of (2,4,7-trinitro-fluorenylidene)malononitrile (TNFM) A two-beam coupling experiment at 830 nm provided clear evidence for asymmetric energy transfer phenomena and a reasonably high gain coefficient
TL;DR: In this paper, the two-wave mixing gain coefficient and time constant were measured at a wavelength of 457.8 nm, and the origin of photochromism was assigned to Fe-ion-related transition.
TL;DR: In this paper, hybrid materials, fabricated by combining liquid crystals with ferroelectric, inorganic nanoparticles, showed improved photorefractive response, including three-fold increase in dielectric anisotropy and optical anisotropic response.
TL;DR: In this paper, the photorefractive polymer with mixing two kinds of nonlinear chromophores was improved with 16% diffraction efficiency with 0.1 mm thick sample, and shelf lifetime of more than 2 months.
TL;DR: In this article, the relation between the mesophase structure and the photorefractive performance in transmission and reflection grating geometries of photore-fractive composites based on highly-ordered layered and less-ordered nematic-like photoconductive polymers was investigated.
06 May 2007
TL;DR: In this article, the Talbot effect can be used to enhance the two-wave mixing gain of two photorefractive liquid crystal light-valves in cascade and self-pumped phase conjugation can be obtained in an optical feedback scheme.
Abstract: We show that Talbot effect can be used to enhance the two-wave mixing gain of two photorefractive liquid crystal light-valves in cascade and that self-pumped phase conjugation can be obtained in an optical feedback scheme
TL;DR: In this paper, the authors show that by doping Sn2P2S6 crystals, the near-infrared photorefractive sensitivity can be enhanced or suppressed, and they also show that fast self-pumped optical phase conjugation with a rise time below 100 ms at 1.06 µm has been demonstrated.
TL;DR: In this paper, the photorefractive nonlinearity possesses a big potential for manipulating light pulses, including their slowing down, long term storage and release on demand, and it is shown theoretically and experimentally that it possesses the potential to manipulate light pulses.
TL;DR: The dependence of self-pumped photorefractive two-beam coupling in Fe:KNbO3 with crystal angle has been calculated and measured at 532 nm for the a-c and b-c crystal planes using a combination of oil-immersion and relay imaging as mentioned in this paper.