Photorefractive Polymer Composites Research Articles

Enhancement of the response time in organic photorefractive composites using alkoxy-substituted PDCST as a nonlinear optical chromophore.

The ease of the molecular orientation of a chromophore has an important effect on the electro-optical (EO) properties of polymeric photorefractive (PR) composites. A derivative of 4-piperidinobenzylidene-malononitrile (PDCST) with an alkoxy group added as a side branch was synthesized to improve the molecular orientation characteristics. Electrophoresis was performed on the polymeric PR composite to which the improved PDCST had been added. The optical properties and response times were examined to evaluate the effects of the substitution of the alkoxy group. PDCST substituted with the alkoxy group showed enhanced EO properties and a PR grating formation rate.

Electron spin resonance and photoelectron yield spectroscopic studies for photocarrier behavior in photorefractive polymeric composites

Abstract An electron spin resonance (ESR) spectroscopy as well as a photoelectron yield spectroscopy (PYS) is used to study the photocarrier behavior (generation and transport) for photorefractivity in poly(N-vinyl carbazole) (PVCz) and poly[bis(2,4,6-trimethylphenyl)amine] (PTAA) based photorefractive (PR) polymer composites. ESR spectra directly catch the radical species in PR composites on the laser illumination under applied electric field; the cation radicals and anion radicals induced under light illumination upon applying an electric field were evaluated. PYS was used to evaluate the energy levels and the width of the electron density of states (DOS) of the carrier transport manifolds. The narrower DOS width of the PTAA-based PR composite was significantly related to the faster response time and broader DOS width for the PVCz-based PR composite to the slower response time. Based on all these results, the comprehensive energy diagrams, including perspectives on the energy level and its distribution, the generated charge carriers with spins, and occupied trap density, are proposed.

Photorefractive polymeric composites with high two-beam coupling gain without external electric field

Herein, three photorefractive (PR) composite films were fabricated. They consist of poly (N-vinycarbazole) (PVK, 33 wt %), 2, 5-dimethyl-(4-p-nitrophenylazo) anisole (DMNPAA, 50 wt %), ethyl carbazole (ECZ, 16 wt %) and fullerene derivatives (1 wt %). The three fullerene derivatives [6, 6]-Phenyl-C61-butyric acid isopropyl ester (PCBIPE), [6, 6]-phenyl-C61-sec-butyl butyrate (PCSBB), and [6, 6]-phenyl-C61-phenethyl butyrate (PCPEB) were used as photosensitizer in the PR films. The PR composite films PVK/DMNPAA/ECZ/PCBIPE(33/50/16/1), PVK/DMNPAA/ECZ/PCSBB(33/50/16/1), and PVK/DMNPAA/ECZ/PCPEB(33/50/16/1) (in wt %) with a low glass transition temperature (21.3 ℃) can easily be fabricated into optically transparent films. The PR performances of the three PR composite films were investigated by two-beam coupling (TBC) experiment at 633 nm wavelength. The results illustrate that the TBC signals were observed under no external electric field or pre-poling condition. The maximum TBC gain of 43.90 cm−1 for PVK/DMNPAA/ECZ/PCPEB(33/50/16/1) PR composite films. The reason of TBC gain of PR materials at zero electric field was discussed.

Fast photorefractive response in polymeric composites enabled by the control of chromophore free volume.

The molecular orientation of a chromophore importantly affects the electro-optic characteristics of polymeric photorefractive composites. We designed methyl, ethyl, and isopropyl derivatives of 4-piperidinobenzylidene-malononitrile (PDCST) with the aim of enhancing molecular orientation properties, and investigated the effects of alkyl substitution on the electro-optic properties and response times of polymeric photorefractive composites. The three alkyl-substituted PDCSTs showed enhanced electro-optic responses and photorefractive grating buildup rates.

Sub-Millisecond Response Time in a Photorefractive Composite Operating under CW Conditions.

Extensive study of photorefractive polymeric composites photosensitized with semiconductor nanocrystals has yielded data indicating that the inclusion of such nanocrystals enhances the charge-carrier mobility, and subsequently leads to a reduction in the photorefractive response time. Unfortunately, the included nanocrystals may also act as a source of deep traps, resulting in diminished diffraction efficiencies as well as reduced two beam coupling gain coefficients. Nonetheless, previous studies indicate that this problem is mitigated through the inclusion of semiconductor nanocrystals possessing a relatively narrow band-gap. Here, we fully exploit this property by doping PbS nanocrystals into a newly formulated photorefractive composite based on molecular triphenyldiamine photosensitized with C60. Through this approach, response times of 399 μs are observed, opening the door for video and other high-speed applications. It is further demonstrated that this improvement in response time occurs with little sacrifice in photorefractive efficiency, with internal diffraction efficiencies of 72% and two-beam-coupling gain coefficients of 500 cm−1 being measured. A thorough analysis of the experimental data is presented, supporting the hypothesized mechanism of enhanced charge mobility without the accompaniment of superfluous traps. It is anticipated that this approach can play a significant role in the eventual commercialization of this class of materials.

Characterization of Carrier Transport and Trapping in Photorefractive Polymer Composites Using Photoemission Yield Spectroscopy in Air

Carrier trapping in organic photorefractive (PR) composites is affected by the bulk state (which can be varied by modifying the molecule structure and processing method used) and the presence of dipolar molecules. Such situations complicate the understanding of carrier trapping in PR composites. The density of states (DOS) provides direct information about carrier trapping in materials. Here, the first attempt to directly measure the DOS in PR composites is performed to investigate the effect of the change of the bulk state on carrier trapping. The distribution of the DOS is measured using photoemission yield spectroscopy in air. The bulk state of the PR composites is tuned based on the crystallization of dipolar nonlinear optical dyes included as a component. Thus, the bulk with a different net dipole (spatially averaged dipole) moment is prepared with the same composition as the PR materials. The edge part of the DOS profile clearly changes upon tuning the bulk state. The diminished net dipole moment achieved by the bulk state tuning causes the suppression of carrier trapping, which is well supported by the dipole trapping model. This result coincides with those obtained using photocurrent and PR grating measurements. image

Molecular design of photorefractive polymers

This review article describes the current state-of-the-art research on organic photorefractive polymer composites. A historical background on photorefractive materials is first introduced and is followed by a discussion on the opto-physical aspects and the mechanism of photorefractivity. The molecular design of photorefractive polymers and organic compounds is discussed, followed by a discussion on optical applications of the photorefractive polymers.

Photorefractivity of Perylene Bisimide‐Sensitized Poly(4‐(diphenylamino)benzyl acrylate)

A perylene bisimide derivative of N,N′‐diisopropylphenyl‐1,6,7,12‐tetrachloroperylene‐3,4,9,10‐tetracarboxyl bisimide (PBI) is synthesized and used as a sensitizer for a photorefractive (PR) polymer composite. Small amount (0.1 wt%) of PBI sensitizer gives the maximum PR performance of composite based on poly(4‐(diphenylamino)benzyl acrylate) (PDAA) (45 wt%), 2‐(4‐(azepan‐1‐yl)benzylidene)malononitrile (35 wt%), and (4‐(diphenylamino)phenyl)methanol (24.9 wt%). High external diffraction efficiency of 49%, response time of 47 ms, and sensitivity of 28 cm2 J−1 are obtained under an applied electric field of 40 V μm−1 at writing beam of 532 nm. PBI sensitizer effectively forms a charge‐transfer complex with PDAA, which favors the generation of the charge carriers followed by the space‐charge formation. Furthermore, low PBI concentration contributes to low absorption coefficient at 532 nm, which leads to better PR performance. image

Off-Resonance Photosensitization of a Photorefractive Polymer Composite Using PbS Nanocrystals

The photosensitization of photorefractive polymeric composites for operation at 633 nm is accomplished through the inclusion of narrow band gap semiconductor nanocrystals composed of PbS. Unlike previous studies involving photosensitization of photorefractive polymer composites with inorganic nanocrystals, we employ an off-resonance approach where the first excitonic transition associated with the PbS nanocrystals lies at ∼1220 nm and not the wavelength of operation. Using this methodology, internal diffraction efficiencies exceeding 82%, two-beam-coupling gain coefficients of 211 cm–1, and response times of 34 ms have been observed, representing some of the best figures of merit reported for this class of materials. These data demonstrate the ability of semiconductor nanocrystals to compete effectively with traditional organic photosensitizers. In addition to superior performance, this approach also offers an inexpensive and easy means by which to photosensitize composite materials. The photoconductive c...

Employing 532 nm Wavelength in a Laser Ultrasound Interferometer Based on Photorefractive Polymer Composites

Laser ultrasonic (LUS) receivers based on photorefractive (PR) materials are contactless and adaptive interferometers, which are used widely for materials characterization. Here, we present a simple LUS interferometer at 532 nm operating wavelength based on organic PR polymer composites (PVK/ECZ/C60) doped with the nonlinear chromophore 4-[4-(diethylamino)-2-hydroxyben-zylideneamino] benzonitrile (Dc). A picoseconds laser at 1064 nm wavelength is used to generate ultrasound pulses in aluminum plates and by using this LUS polymer interferometer, detection of these ultrasound waves is remotely performed at the surface of the specimens. The LUS sensor is used to determine the thickness of aluminum plates about 0.25 mm, 3 mm and 10 mm. We also show the potential of this polymer receiver for detection of an artificial defect in a metal sample.

Laser ultrasonic receivers based on organic photorefractive polymer composites

We present two laser ultrasonic receivers based on organic photorefractive polymer composites with 2- (4-bis(2-methoxyethyl)aminobenzylidene)malononitrile (AODCST) or 2-dicyanomethylen-3-cyano-5,5-dimethyl- 4-(4 0 -dihexylaminophenyl)-2,5-dihydrofuran nonlinear optical chromophores. Experimental results show sensi- tivities of the ultrasonic receivers of *9.5 9 10 -8 nm (W/Hz) 0.5 for both composites, and a faster response time (*60 ms) for the AODCST-based laser ultrasonic recei- ver. We show that such LUS detectors are highly suitable for contactless thickness measurements of aluminum, steel sheets and defect detection with an accuracy of 100 lm.

High-performance photorefractive polymer composites based on poly(9-vinyl-3-carbazolcarboxyaldehyde diphenylhydrazone)

Photorefractivity is based on the combined effects of photoconductivity and electro-optic property. Under the illumination of non-uniform light formed by the interference of two coherent laser beams, a spatially oscillating space charge field is formed arising from the generation and the redistribution of photo-induced charges. The refractive index of material is subsequently modulated via an electro-optic effect. The photorefractive materials have potentials applications in the fields such as dynamic holography, 3D display, 3D printer, reversible data storage and information processing. In 1994, remarkable improvement in the performance of PR polymeric material was obtained by using the photoconductive polymer poly(N-vinylcarbazole) (PVK). PVK/2,5DMNPAA/ECZ/TNF=34:50:15:1 wt% composite, showed a large gain coefficient of 220 cm and a diffraction efficiency of nearly 100% at the applied electric field of 90 V/ μm. Many studies have been done since Meerholz and coworkers proposed high-performance photorefractive (PR) polymeric composite. Despite this progress, the search for new photorefractive materials are still required for their widespread utilization in information technologies. In a previous paper, we reported on the synthesis and characterization of a photoconducting poly(siloxane) having pendant diphenylhydrazone. This composite shows a high diffraction efficiency of 81% at an electric field of 40 V/μm. Poly[4-(diphenyl-hydrazonomethyl)-phenyl]-[3-(methoxydimethyl-silanyl)-propyl]-methyl-amine (PSX-HZ) system have advantages including excellent optical quality, long shelf lifetime, the large birefringence, and high diffraction efficiency at low electric field. Despite the various advantages, the PSX-HZ system has suffered from relatively slow PR response time. To solve this problem, we applied the polyvinyl-backbone containing the hole transport moiety that is 9-vinyl-3-carbazolcarboxyaldehyde diphenylhydrazone (CPH). Hydrazone based on aromatic amines and arylaldehyde hydrazone are widely studied as organic hole transport materials due to enhanced hole mobility. Carbazole-containg hydrazone especially, exhibit relatively high hole mobility. We expect the realization of large photoconductivity of new polymer. The chemical structures investigated are shown in Figure 1, although the details of the synthesis and photophysical properties will be published elsewhere. The composition our samples was PVCPH/DB-IP-DC/ BBP/PCBM (54:30:15:1 wt%)-C1 and PVCPH/PDCST/BBP/ PCBM (54:30:15:1 wt%)-C2. Where poly(9-vinyl-3-carbazolcarboxyaldehyde diphenylhydrazone) (PVCPH) are photoconducting material, 2-{3-[(E)-2-(dibutylamino) ethen-1yl]-5,5-dimethylcyclohex-2-enylidene}malononitrile (DBIP-DC) and 2-(4-piperidinobenzylidene)malononitrile (PDCST) are nonlinear optical chromophore, butyl benzyl phthalate (BBP, Tokyo Kasei Kogyo Co.) is plasticizer, phenyl-C61butyric acid methyl ester (PCBM, Solenne Co.) is photosensitizer. The device was prepared by sandwiching the softened composite between two indium tin oxide (ITO) coated glass plates.

Quickly Updatable Hologram Images Using Poly(N-vinyl Carbazole) (PVCz) Photorefractive Polymer Composite

Quickly updatable hologram images using photorefractive (PR) polymer composite based on poly(N-vinyl carbazole) (PVCz) is presented. PVCz is one of the pioneer materials of photoconductive polymers. PR polymer composite consists of 44 wt % of PVCz, 35 wt % of 4-azacycloheptylbenzylidene-malonitrile (7-DCST) as a nonlinear optical dye, 20 wt % of carbazolylethylpropionate (CzEPA) as a photoconductive plasticizer and 1 wt % of 2,4,7-trinitro-9-fluorenone (TNF) as a sensitizer. PR composite gives high diffraction efficiency of 68% at E = 45 V μm−1. Response speed of optical diffraction is the key parameter for real-time 3D holographic display. The key parameter for obtaining quickly updatable holographic images is to control the glass transition temperature lower enough to enhance chromophore orientation. Object image of the reflected coin surface recorded with reference beam at 532 nm (green beam) in the PR polymer composite is simultaneously reconstructed using a red probe beam at 642 nm. Instead of using a coin object, an object image produced by a computer was displayed on a spatial light modulator (SLM) and used for the hologram. The reflected object beam from an SLM was interfered with a reference beam on PR polymer composite to record a hologram and simultaneously reconstructed by a red probe beam.

Formation speed and formation mechanism of self-written surface wave-based waveguides in photorefractive polymers

The formation speed of a self-written waveguide structure formed owing to the propagation of surface waves in a photorefractive polymer composite is measured. The formation speed linearly increases with the power of a laser beam. From the measurements of the dynamics of photorefractive grating and the photocurrent output of the polymer, it is revealed that the waveguide structure is formed by a single photorefractive grating, which is identical for the different power levels of the injected pump beam.

Interdigitated coplanar electrodes for enhanced sensitivity in a photorefractive polymer

Organic photorefractive polymer composites can be made to exhibit near 100% diffraction efficiency and fast writing times, though large external slants are needed to project the applied field onto the grating vector. We show here that the use of interdigitated electrodes on a single plane provides similar performance to these standard devices and geometries but without a external slant angle. This new device's structure also greatly improves the diffraction efficiency and sensitivity compared to less slanted standard devices necessary for some real applications, such as holographic displays, optical coherence imaging, and in-plane switching.

Nanoscale interlayer that raises response rate in photorefractive liquid crystal polymer composites

By depositing a nanoscale photoconductive layer on a stable photorefractive (PR) polymeric film, consisting of the polymer poly[N-vinylcarbazole] (PVK) doped with 4,4'-n-pentylcyanobiphenyl (5CB) and C60, both the response rate and beam coupling properties were improved greatly. Systematic measurements and observations unveiled the role played by the additive layer in preventing ion injection from the ITO layer into the PR film and hence in mitigating the charge compensation. A strong fanning effect and high diffraction orders at small angles have demonstrated the excellent PR property in the modified samples used. To demonstrate great potential of the PR composite in the updatable applications, real time double exposure interferometry was performed accordingly with good results.

Enhanced photorefractivity in a polymeric composite photosensitized with carbon nanotubes grafted to a photoconductive polymer

We report on the photosensitization of photorefractive (PR) polymeric composites through the inclusion of multiwalled and singlewalled carbon nanotubes (CNTs), respectively, having poly(N-vinyl carbazole) (PVK) grafted to their surfaces. The PR nature of the holographic gratings was confirmed via the asymmetric exchange of energy in a two-beam-coupling (TBC) geometry, yielding TBC gain coefficients approaching 80 cm−1. In addition, in degenerate-four-wave-mixing experiments the prepared composites exhibited diffraction efficiencies as high as 60% and overmodulation voltages as low as ∼40 V/μm. These notable figures of merit indicate that the grafting of the PVK polymer to the various CNTs results in enhanced PR performance. The mechanism responsible for this enhancement in PR performance is investigated using a variety of experimental techniques.

High-performance photorefractive polymer composites doped with 2-piperidino-5-thienylmalononitrile chromophore

We report photorefractive (PR) polymer composites doped with 2-piperidino-5-thienylmalononitrile chromophore. The PR polymer composites showed a glass transition temperature of 25∼27 °C, good transparency at 633 nm and long shelf lifetimes at room temperature. The PR polymer composites with a thickness of 100 μm showed a maximum diffraction efficiency of 77∼91% and a rapid response time of 110∼980 ms at the glass transition temperature. In addition, the composite C3 was observed at a video-rate response time at 35 °C.

(Invited) Phthalocyanines as Efficient Sensitizers in Photorefractive Polymer Composites

not Available.

Influence of the sensitizer reduction potential on the sensitivity of photorefractive polymer composites

We report on a series of near infrared (NIR)-sensitive photorefractive polymer composites (PPCs) based on the hole-conducting polymer PF6-TPD, which are sensitized by soluble fullerene-derivatives as electron-accepting agents. We demonstrate a direct correlation between the electron accepting capability of the sensitizer and the holographic response time. The holographic recording speed is found to improve by one order of magnitude when lowering the reduction potential of the sensitizer by approx. 400 mV, while all other physical parameters of the materials remain essentially identical. Furthermore, the lifetime of the mobile charge carriers is found to correlate linearly with the reduction potential, thus indicating a decrease in recombination rates for stronger accepting capability of the sensitizer. Finally, we found that pre-illumination enhanced the holographic sensitivity. The effect is found to be most pronounced for the strongest acceptor due to reduced recombination of the preformed carriers. Overall, the PPCs reported here feature the currently highest sensitivity in the NIR spectral region.