Organic Photorefractive Materials Research Articles

Review of Organic Photorefractive Materials and Their Use for Updateable 3D Display.

Photorefractive materials are capable of reversibly changing their index of refraction upon illumination. That property allows them to dynamically record holograms, which is a key function for developing an updateable holographic 3D display. The transition from inorganic photorefractive crystals to organic polymers meant that large display screens could be made. However, one essential figure of merit that needed to be worked out first was the sensitivity of the material that enables to record bright images in a short amount of time. In this review article, we describe how polymer engineering was able to overcome the problem of the material sensitivity. We highlight the importance of understanding the energy levels of the different species in order to optimize the efficiency and recording speed. We then discuss different photorefractive compounds and the reason for their particular figures of merit. Finally, we consider the technical choices taken to obtain an updateable 3D display using photorefractive polymer. By leveraging the unique properties of this holographic recording material, full color holograms were demonstrated, as well as refreshing rate of 100 hogels/second.

A hyperbranched polymer with enhanced photorefractive effect at low and zero applied electric field

Organic photorefractive (PR) materials are widely applied in electro-optical technologies. However, most PR materials require a high external electric field to show PR effect, which would cause materials dielectric fatigue and electrical breakdown. It is significant to explore materials that demonstrate high PR effects under low and even zero applied electric field. In this work, through Suzuki polymerization, a hyperbranched conjugated polymeric PR materials is prepared to show high PR effects with coupling gain coefficients (Г values) of 86.3 cm−1 and 62.5 cm−1 at low applied electric field of 2.5 V μm−1 and even 0 V μm−1, respectively, which are superior to the performance of its linear analogue. These results verify the advantages of branched structure in producing high PR effect, providing a new approach for PR material design.

High-performance organic photorefractive materials containing 2-ethylhexyl plasticized poly(triarylamine)

We report high-performance photorefractive materials containing poly(triarylamine), chromophore, and photosensitizer, exhibiting optical sensitivity 15.8 cm3 kJ−1 with 2 ms response-time.

Photorefractive hyper-structured molecular glasses constructed by calix[4]resorcinarene core and carbazole-based methine nonlinear optical chromophore

Seven C-methylcalix[4]resorcinarene (CRA)-based photorefractive (PR) hyper-structured molecular glasses (HSMGs) containing carbazole-based methine nonlinear optical (NLO) chromophores were designed and synthesized via esterification between carboxyl-containing asymmetric CRA core molecule and hydroxyl-containing functional compounds. When the feed ratio of hydroxyl-containing NLO functional compounds to carboxyl groups of CRA-COOH was kept at 1.5/1 by mole, the degree of introduction of NLO chromophore to the CRA core was above 70%. All the HSMGs show low glass transition temperatures (Tg), and good solubility in common low boiling point solvents such as THF, CHCl3, etc. Powder XRD and UV–vis absorption in films of HSMGs indicate that the aggregation, packing and crystallization of the NLO chromophores in these HSMGs had been effectively reduced. Doping with N-ethyl-carbazole (ECz) as a plasticizer and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as a photosensitizer, all the HSMGs composites showed good PR effects. All the coupling gain coefficients (Γ) of HSMGs-based composites are about twice to the corresponding small molecular NLO chromophores-based composites, thanks to the decreased NLO chromophores antiparallel packing caused by the asymmetric CRA core. Among them, CRA-CSN/ECz/PCBM (69:30:1, wt%) composite exhibited the best performance with the Γ value of 78.2 cm−1 at the external electric field of 12.5 V μm−1, which is one of the best performance in molecular glasses under the same test conditions. Coupled with their convenient synthesis, this work will provide a new simple design strategy for organic PR materials.

Enhanced photoconductivity and trapping rate through control of bulk state in organic triphenylamine-based photorefractive materials

In organic optical semiconductors, it is rather challenging to achieve precise control of photoconductivity and charge trapping, which determines the device performance. This paper reports on enhanced photorefractive response rate through control of the photoconductivity and trapping rate in organic triphenylamine-based photorefractive materials by means of bulk state tuning. The bulk state in organic triphenylamine-based photorefractive composites was controlled through a rapid cooling process from various melting temperatures during sample fabrication. The photoconductivity and trapping rate were determined from photocurrent measurements. Fabrication at lower melting temperatures enhanced the trapping rate for deep traps, whereas it reduced the trapping rate for shallow traps. As a result, a faster photorefractive response was obtained.

Optical properties of organic photorefractive polymers for phase retrieval and filtering

New polyphosphazene organic photorefractive materials with carbazole groups were synthesized by diazo-coupling reaction. They were characterized by 1H Nuclear magnetic resonance，infrared spectrum，Ultra-violet-visible，thermogravimetry and differential scanning calorimetry. The result showed that polymers had good heating stability. They started to decompose at 230 ℃ and decomposed completely at 450 ℃. UV-Vis indicated that the broad absorption peak ranging from 360 to 570 nm was derived from the increase in the conjugation length. Hence we can adjust the ratio of diazonium salt in the coupling reaction to control the access content of azo chromophore functional component. The photorefractive effects were investigated without electric field and polarization by two-beam coupling (2BC) and degenerated four-wave mixing(DFWM).The coupling coefficients of polymers P-2 and P-3 are measured to be about 38 cm-1 and 53.6 cm-1 respectively and the diffraction efficiencies of polymers P-2 and P-3 are measured to be about 2.7% and 8.1% respectively.

Coherence Gated Three-dimensional Imaging System using Organic Photorefractive Holography

This paper discusses a coherence-gated three-dimensional imaging system based on photorefractive holography, which was applied to imaging through turbid media with a view to developing biomedical instrumentation. A rapid response photorefractive device doped with 2,4,7-trinitro-9-fluorenylidene malononitrile was used to generate the hologram grating. The estimated depth resolution was 20 µm, which corresponds to the coherence length of the light source. In this coherence imaging system, tomographic imaging of a 3-dimensional object composed of a 50 µm thick cylindrical layer was achieved. The proposed coherence imaging system using an organic photorefractive material can be used as an optical tomography system for biological applications

Advances in Organic All‐Optical Photorefractive Materials

AbstractOrganic all‐optical photorefractive (PR) materials will soon become an interesting topic owing to the convenient signal‐writing without the application of external electrical field and poling, even without any photosensitizer. In this paper, organic all‐optical PR materials synthesized in our laboratory have been systematically summarized including organic monolithic PR molecular glasses with a linear or hyperbranched chemical structure such as organic carbazole/triphenylamine‐based amorphous compounds, and PR polymers with a non‐conjugated main chain (NCMC) such as polymethacrylates, polyphosphazenes and polynorbornenes. Much progress has been made not only in the synthesis strategy of organic all‐optical materials but also in understanding the primary mechanism of all‐optical PR effect in organic materials. It is believed that the outcoming of organic all‐optical PR materials will broaden the application fields of organic PR materials.

Organic Photorefractive Materials and Applications

AbstractThis review describes recent advances and applications in the field of organic photorefractive materials, an interesting area in the field of organic electronics and promising candidate for various aspects of photonic applications. We describe the current state of knowledge about the processes involved in the formation of photorefractive gratings in organic materials and focus on the chemical and photo‐physical aspects of the material structures employed in low glass‐transition temperature amorphous composites and organic photorefractive glasses. State‐of‐the art materials are highlighted and recent demonstrations of photonic applications relying on the reversible holographic nature of the photorefractive materials are discussed.

Four-wave mixing real-time intensity filtering with organic photorefractive materials

In this paper, we exploit the nonlinearity inherent in four-wave mixing in organic photorefractive materials and demonstrate edge enhancement, contrast conversion, and defect enhancement in a periodic structure. With the availability of these materials, which have large space-bandwidth products, edge enhancement, contrast conversion and defect enhancement are possible. Some simulation results also are provided.

The role of diffusion of excitons in organic photorefractive materials

A theory of photorefractive (PR) effect in organic PR materials based on exciton formation, diffusion, and dissociation in organic polymer semiconductors is presented. The exact numerical and approximate analytical solutions of the model indicate that an increase in the exciton diffusion constant reduces the amplitude of the resulting space-charge electric field. The model also shows that a space-charge field is produced even in the absence of traps, and particular combinations of the detrapping rates and the trap depths could be used to enhance the space-charge field. It is suggested that an organic semiconductor with a smaller exciton diffusivity and traps with optimal detrapping rates would be a better candidate for photorefractivity.

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).

Photorefractive Effect of Liquid Crystalline Materials

This paper reviews our recent work on the photorefractive effect in liquid crystalline materials. The photorefractive effect is defined as the optical modulation of the refractive index of a medium as a result of various processes. The interference of two laser beams in a photorefractive material establishes a refractive index grating. This phenomenon enables the creation of different types of photonic applications. Recently, the development of organic photorefractive materials has been attracting great interest. Liquid crystalline materials are a strong candidate for practical photorefractive materials and application. In this paper, the photorefractivity of liquid crystalline polymers and ferroelectric liquid crystals is described.

Effect of a trapping molecule on the monolithic organic photorefractive materials

A peculiar effect of a trapping molecule on two closely related monolithic organic photorefractive (PR) materials was described. It was found that the trapping molecule C affects the PR performances differently in two materials made from either A or B, although the two molecules have similar chemical structures. Detailed studies on the charge mobility with the time-of-flight technique and activation energies for charge transport revealed that the trapping molecule C plays the role of electron trapper in material A and bipolar trapping center for both hole and electron in material B.

The design, fabrication and property study for photorefractive applications of novel organic materials

Abstract The organic photorefractive (PR) materials and devices have attracted much interest due to the wide potential applications. In this talk, we report the recent study on PR materials and PR physical property in our group. To overcome the disadvantages (e.g. instability and critical environment requirement) of the multi-component PR composite, two novel three-component PR systems were fabricated based on a series of azo dyes and liquid crystals as both EO chromophore and plasticizer, respectively, and the excellent PR performances were obtained. More recent, mono-component PR sample based on a fully functional polymer, poly(9-(2-ethyl-hexyl)-3-[2-(4-methanesulfonyl-phenyl)vinyl]-9H-carbazole), was reported to exhibit two-beam coupling coefficient more than 100 cm−1 at 632.8 nm wavelength at applied electric field of 100 V/μm. The fast response time was obtained from composite doped with β,β-diacetyl-4-methoxylstyrene (DAMST). Besides, DAMST also is the more suitable selection to operate at short wavelength.

Synthesis and properties of glassy organic multifunctional photorefractive materials

Abstract We report the synthesis and characterization of several related amorphous multifunctional photorefractive (PR) materials with the charge transport agent covalently linked to a chromophore. DCTA (4,4′-di(carbazol-9-yl) triphenylamine) is used as the charge transport agent in conjunction with C60 to provide photoconductivity and DCST (an aminodicyanostyrene) is used as the birefringent and nonlinear optical chromophore. Chains with different lengths (2, 6 and 12 carbons) are used to covalently connect these two moieties. The linking units influence the glass transition temperature of the system, which in turn influences the PR performance. The longer linker allows faster orientation of the chromophore and results in higher PR speed but the response still appears to be limited by the molecular reorientation rather than photoconductivity in these systems.

Fine tuning of glass transition temperature in monolithic organic photorefractive material

Abstract In order to study the effect of Tg on the photorefractivity in organic monolithic system, we designed and synthesized photorefractive homopolymer (PCzBO) and organic glass (EHCzBO), consisting of the same monolithic chromophore. Photorefractive polymer composites were prepared by blending them. Tg of PCzBO and EHCzBO were 106 and 29 °C, respectively. Tg of polymer composite could be controlled precisely by the compositional ratio of PCzBO and EHCzBO in the range from 29 to 40 °C. Since both the polymer and the organic glass contained the same chromophore structure, all the samples had nearly the same chromophore concentration even though compositional ratios of them were different, as was confirmed by UV–visible spectroscopy. Frequency-dependent ellipsometric technique showed that the birefringence contribution through the orientation enhancement effect was dominant in the refractive index modulation of our low Tg composites. It was found that the reorientation of chromophore according to the modulated voltage was more limited for the higher Tg composite, which consequently reduced the birefringence contribution. The diffraction efficiency and two beam coupling coefficient were decreased with the increase of Tg.

Near-infrared sensitivity enhancement of photorefractive polymer composites by pre-illumination.

Among the various applications for reversible holographic storage media, a particularly interesting one is time-gated holographic imaging (TGHI). This technique could provide a noninvasive medical diagnosis tool, related to optical coherence tomography. In this technique, biological samples are illuminated within their transparency window with near-infrared light, and information about subsurface features is obtained by a detection method that distinguishes between reflected photons originating from a certain depth and those scattered from various depths. Such an application requires reversible holographic storage media with very high sensitivity in the near-infrared. Photorefractive materials, in particular certain amorphous organic systems, are in principle promising candidate media, but their sensitivity has so far been too low, mainly owing to their long response times in the near-infrared. Here we introduce an organic photorefractive material -- a composite based on the poly(arylene vinylene) copolymer TPD-PPV -- that exhibits favourable near-infrared characteristics. We show that pre-illumination of this material at a shorter wavelength before holographic recording improves the response time by a factor of 40. This process was found to be reversible. We demonstrate multiple holographic recording with this technique at video rate under practical conditions.

Lessons learned from research on photorefractive polymers and molecular materials

AbstractOrganic photorefractive (PR) materials are a new kind of electrooptic substance that emerged 10 years ago. These are very complicated materials that possess both electrooptic effects and photoconductivity. It is challenging to integrate these properties into a single polymer system that will exhibit this PR effect. This article highlights our efforts in the past nine years in synthesizing and characterizing fully functionalized organic PR materials. I emphasize the lessons we have learned from this research, which may be more valuable to those who are interested in this area. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2557–2564, 2001

Influence of the Charge Transport Characteristics on the Holographic Response of Organic Photorefractive Materials

Organic photorefractive materials have attracted a lot of interest recently. Their optical response times of a few milliseconds, however, are not yet adequate for the desired commercial applications. We present an investigation on the correlation between the optical response time and photoelectric quantities, such as the hole mobility and the dispersivity of charge carrier transport.