Photorefractive Performance Research Articles

Enhanced Photorefractive Performance by Controlling Ion‐Assisted Charge Injection and Accumulation with Random Biphenyl Co‐Polymerized Poly(Carbazolyl‐Amine)

AbstractIn this study, the synthesis of poly(carbazolyl‐amine)s with a random co‐polymerized biphenyl group through the Buchwald–Hartwig amination to enhance ion‐assisted space‐charge‐limited current (SCLC) and achieve efficient performance in photorefractive (PR) polymers, is reported. By modulating the polymer crystallinity, the efficient accumulation of ions near an electrode is achieved and enhanced ion‐assisted SCLC is realized, which results in a magnified electric field inside the polymers. The internal electric field is increased by the stacked charges injected from the electrode, and it is further enhanced by the washing out of ions. The polymer, blended with the electro–optical chromophore and sensitizer, exhibits a high PR sensitivity of 16.7 cm3 kJ−1 and a device stability of up to E = 80 V µm−1 at a wavelength of 633 nm, which is comparable to that of the inorganic crystals Bi12SiO20 and Sn2P2S6.

Graphitic nanosheets via two-dimensional polymerization enhancing organic all-optically controlled photorefractive performance

All-optically controlled devices are the attractive subjects due to their future applications in all-optically controlled networks and computers. In this work, two-dimensional (2D) graphite nanosheets are synthesized to improve organic photorefractive performance in the all-optical operation, i.e., the photorefractive performance under zero electric field. The synthetic process is performed by 2D-polymerizing electrophilic and nucleophilic carbon species under solvothermal effect, where [:C≡C:]2− in calcium carbide is applied as nucleophilic carbon species and [C(Cl)] in carbon tetrachloride (CCl4) as electrophilic carbon species. The polycarbonization reaction consists of 1D horizontal growth for nanoribbon and 2D vertical growth for single-layer graphene. Then, single-layer graphene nanosheets are aggregated via π–π stacking effect into 2D graphite nanosheets being composed of multilayer graphene nanosheets. Notably, the self-template characteristics induce the synchronous and alternate ongoing of three above processes. Interestingly, when these 2D graphite nanosheets are doped into organic photorefractive devices, the all-optically controlled photorefractive performance is effectively improved by enhancing the gain coefficient of two-beam coupling about 46.4% from 80.1 cm−1 of control device to 117.2 cm−1 of nanographite-doped device. This is attributed to the excellent electric conductance of 2D graphite nanosheets, which strengthens the charge separation under the nonuniform light field inside organic photorefractive devices. Then, the stronger built-in electric field will be induced so that the orientation enhancement effect is increased in organic photorefractive devices. This work provides an effective and facile approach to improve organic all-optically controlled photorefractive performance with 2D graphitic nanosheets.

Abnormal lattice occupation of Mo and related polarons in LiNbO3: Hybrid density functional theory investigation

Studies have reported that Mo-doped LiNbO3 exhibits excellent photorefractive performance, which may be attributed to the Mo abnormally occupation in Nb sites (MoNb) and the MoNb photorefractive centres. Therefore, we investigate the basic characteristics of Mo-doped LiNbO3, including the preferable substitutional sites, doping stability, lattice relaxation, and electronic structures using the spin-polarised density functional theory within semi-local and hybrid functionals. Particularly, the type and properties of the polarons in Mo-doped LiNbO3 are studied intensively, and the advantages of MoNb polarons with respect to the MoLi polarons are highlighted. The calculation results of both transfer energies and defect formation energies show that MoNb are preferable to be formed in both stoichiometric and congruent LiNbO3, while Mo substitutional Li (MoLi) is consumed in small amounts. MoNb could act as small bound polarons that exhibit fast response to light with respect to the MoLi bound bipolarons. The introduced double-centre by Mo doping is partly responsible for the excellent photorefractive properties of Mo-doped LiNbO3.

Optimal composition of the poly(triarylamine)-based polymer composite to maximize photorefractive performance

A holographic display system requires the external diffraction efficiency to be greater than 10% and four orders of magnitude of sensitivity for practical usage. To achieve such requirements, the photorefractive (PR) performance of PR composite based on poly[bis(2,4,6-trimethylpheneyl)amine] (PTAA) has been investigated. In the present report, the change of the content of PTAA as a photoconductive polymer, (2,4,6-trimethylphenyl)diphenylamine (TAA) as a photoconductive plasticizer, and second trap agent bathophenanthroline (BPhen) reasonably optimized the PR response time and external diffraction efficiency. High sensitivity of 1851 cm2 J−1 with response time of 494 μs and external diffraction efficiency of 23.9% were achieved at 532 nm and 60 V μm−1 by reducing the content of PTAA and increasing the contents of TAA and BPhen. Decreasing the amount of PTAA and increasing the contents of TAA and BPhen lowered the absorption coefficient, resulting in the high external diffraction efficiency. The narrower distribution of the electronic density of states (DOS) for PTAA/TAA (43.5/20 and 33.5/30) also contributed to the shorter PR response time of hundreds of microseconds.

Flexible All-Organic Photorefractive Devices

The objective of the present study is to demonstrate and evaluate the photorefractive (PR) performance of an all-organic PR device with a self-assembled monolayer (SAM)-modified organic conductive electrode of PEDOT:PSS coated on polyethylene terephthalate (PET). The PR composite consisted of a triphenylamine-based photoconductive polymer: poly(4-(diphenylamino)benzyl acrylate) (PDAA), a triphenylamine photoconductive plasticizer: (4-(diphenylamino)phenyl)methanol (TPAOH), a nonlinear optical dye based on aminocyanostyrene: (4-asacycloheptylbenzylidenemalononitrile) (7-DCST), and soluble fullerene: [6,6]-phenyl C61 butyric acid-methyl ester (PCBM). For comparison with the all-organic PR device, the PR performances using PET/ITO, glass/ITO, and glass/PEDOT:PSS substrates were also evaluated. At an applied electric field of 40 V μm–1, the diffraction efficiency and the response time of the PR device using the PET/PEDOT:PSS-SAM substrate were 21.9%, and 390 ms, respectively. As a result of repeating bending ...

A calix[4]resorcinarene-based hyper-structured molecule bearing disperse red 1 as the chromophore with enhanced photorefractive performance under non-electric field

A hyper-structured molecule (HSM) CRA-DR1 and a model molecule Ph-DR1 for photorefractive (PR) application under non-electric field were designed and synthesized via highly reactive “click” reaction between alkynyl derivatives of calix [4]resorcinarene (CRA) or resorcinol and an azide derivative of disperse red 1 (DR1) in satisfactory yields. XRD pattern and UV–Vis absorption spectra indicated that the CRA-DR1 with three-dimensional (3D) CRA core could effectively decrease the aggregation, packing and crystallization of the nonlinear optical (NLO) chromophores. AFM images showed a smooth and pinhole-free morphology of the CRA-DR1 film. Two-beam coupling (TBC) experiment was used to test the photorefractive performance in optically transparent films of Ph-DR1 and CRA-DR1 doped with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as a photosensitizer and N-ethyl-carbazole (ECz) as a plasticizer. Without poling and without external electric filed, the coupling gain coefficient obtained in the CRA-DR1-based composite is 147.4 cm−1, which is higher than that of Ph-DR1-based composite (132.6 cm−1) owing to its asymmetrical 3D CRA core. Coupled with its convenient synthesis and commercial DR1, CRA-DR1 could serve as a good candidate for PR application.

Enhanced photorefractive performance of bulk cu-doped KNSBN crystals through surface electrostatic modification

The remarkable photorefractivity enhancement was observed and investigated in copper-doped (K0.5Na0.5)0.2(Sr0.75Ba0.25)0.9Nb2O6 (Cu:KNSBN) crystals due to charge accumulation on the surface, stemming from electrostrictive effect. This electrostatic modification to the surface was studied with conventional two beam coupling experiment and over 17 high diffraction orders were observed. To estimate the amount of charge accumulation, the surface charge density was measured through direct current measurement using a close loop circuit. It was believed that a thin phase grating was responsible for the diffraction pattern, which was confirmed by the related reading experiment. Corresponding calculation and analysis were given to highlight the strong refractive index modulation of the gratings.

Enhancement in the photorefractive performance of organic composites photosensitized with functionalized CdSe quantum dots

Enhancement in the photorefractive (PR) performance of organic composites photosensitized by CdSe quantum dots (QCdSe) passivated with the charge-transport ligands, sulfonated triphenyldiamine (STPD), is reported. This enhancement is primarily attributed to the ability of the passivating ligand, STPD, to facilitate the charge-transfer process between the QCdSe and the triphenyldiamine (TPD) charge-transport matrix. The PR composites exhibited a maximum photocharge-generation efficiency of 0.9% and two-beam coupling gain coefficient of 110 cm−1. These figures of merit represent a significant improvement over similar composites photosensitized with more conventional trioctylphosphine oxide-passivated QCdSe (TQCdSe). Moreover, composites photosensitized with SQCdSe had a faster response time of τ = 128 ms at an electric field of 60 V/μm compared with τ = 982 ms for those containing TQCdSe. Because of the molecular similarity between the STPD passivating groups and the TPD-based charge-transport matrix, concentrations of up to 1.4 wt% of SQCdSe are achieved in PR composites without any detectable phase separation, a considerable improvement over the 0.7 wt% for TQCdSe.

Large exponential gain coefficient in polymer assisted asymmetric liquid crystal cells originating from surface effect

As large as 4607cm−1 gain coefficient in two beam coupling experiment was obtained by introducing PVK:C60 film to ZnSe assisted liquid crystal system. As short as 5.0ms holographic recording time was reached when probing the grating formation process, showing great potential in real time applications. Systematical two beam coupling and grating probing experiments were performed in studying the mechanism behind the high photorefractive (PR) performance. Unusual energy transfer direction change and gain coefficient fluctuation were observed when the voltage polarity and incidence side were altered in the related two wave coupling experiments.

Enhanced photorefractivity of a perylene bisimide-sensitized poly(4-(diphenylamino) benzyl acrylate) composite

Enhanced photorefractive (PR) performances are reported for a PR composite of perylene bisimide (PBI)–sensitized poly(4-(diphenylamino)benzyl acrylate) (PDAA), 2-(4-(azepan-1-yl)benzylidene)malononitrile (7-DCST), and (4-(diphenylamino)phenyl)methanol (TPAOH). The addition of a large amount of the TPAOH photoconductive plasticizer was found to produce a preferred hole manifold that reduces the disordered state. A photorefractive performance with a minimum response time of 11 ms, a maximum external diffraction efficiency of 41.6%, a maximum sensitivity of 117 cm2 J−1, and a maximum optical gain of 296 cm−1 were obtained with an external electric field of 55 V μm−1 for PDAA/7-DCST/TPAOH/PBI (30/30/39.9/0.1 by wt.).

Influence of recording angle on photorefractive properties in In:Fe:Cu:LiNbO3 crystal at 488 nm wavelength

Abstract Congruent LiNbO3 crystal doped with Fe2O3, CuO and In2O3, respectively, were grown by Czochralski method. The blue photorefractive performance (the change refractive index Δn, response time τw and recording sensitivity S, at different recording angle) was investigated by using the typical two-wave coupling experimental setup. The results show that Δn reduces, τw first decreases then increase, S first increase then decreases with the recording angle increasing within our measurement range. It should be compromised between the change refractive index and the recording sensitivity when we using In:Fe:Cu:LiNbO3 crystal to holographic storage. The compromising recording angle is near 50° in air under our experimental conditions. The experimental results also indicates that In:Fe:Cu:LiNbO3 crystal becomes a promising candidate for blue photorefractive holographic recording materials.

Enhanced photorefractive performance of the nematic liquid crystal cell containing photoconductive polymer film

Photorefractive (PR) nematic liquid crystals (NLCs) possess large optical non-linearity but coarse grating spacings (tens of microns) and slow response time (generally in the order of 1 s). Here we presented a great improvement in PR performance of NLC cells containing photoconductive films as aligning layers by varying the thickness and the sensitizer concentration of the photoconductive layer to enhance the surface electric field modulation. As a result of optimising, considerable diffraction efficiencies and the response time as fast as tens of milliseconds were obtained from steady-state PR gratings within a wide range of grating spacings from 30 to 0.7 μm. Besides, the grating response was enhanced significantly compared with the previous reports under similar experimental conditions.

Photorefractive performance of poly(triarylamine)‐Based polymer composites: An approach from the photoconductive properties

ABSTRACTPhotorefractive (PR) and photoconductive properties of methyl‐substituted poly(triarylamine) (PTAA) based PR composite is presented. PR composite consisted of PTAA, piperidinodicyanostyrene, (2,4,6‐trimethylphenyl)diphenylamine, and [6,6]‐phenyl‐C61‐butyric acid methyl ester. Photocurrent is simultaneously measured when a transient degenerate four wave mixing is recorded. Diffraction efficiency of 16.6%, response time of 5 ms, and sensitivity of 43 cm2 J−1 are measured under an applied field of 45 V μm−1 and 632.8 nm illumination with the intensity of 1.5 W cm−2. Response time of 10.2 ms with diffraction efficiency of 47.0% is obtained under a same field and 532 nm illumination with the intensity of 0.427 W cm−2. Higher diffraction and faster response is due to the large photocurrent in the order of hundreds μA measured. The resultant trap density is in the order of 1014 cm−3. Thus, space–charge field less than 1 V μm−1 is evaluated, which limits the PR response. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 502–508

Enhanced photorefractive performance of polymeric composites through surface plasmon effects of gold nanoparticles.

We investigated the photorefractivity enhancement of polymeric composites by introducing gold nanoparticles (NPs). The gold NPs enhance the photocharge generation rate of sensitizers through plasmon resonance coupling achieved between NPs and sensitizers. Systematic studies show that the presence of gold NPs has increased photocharge generation efficiency, photoconductivity, diffraction efficiency, refractive index modulation, and photorefractive (PR) grating formation rate. The gold-NP-doped PR sample exhibits 2 times larger photocharge generation efficiency and photoconductivity, and 2.5 times faster PR grating formation rate compared to the control sample without the NPs.

Enhanced Photorefractive and Third-Order Nonlinear Optical Properties of 5CB-Based Polymer-Dispersed Liquid Crystals by Graphene Doping

The enhanced photorefractive and third-order nonlinear optical properties of graphene/graphene oxide (GO)-doped polymer-dispersed liquid crystals (PDLC) were studied by two-beam coupling and z-scan methods. A higher interior electric field will be produced according to the photorefractive performance because of the high electrical conductivity of graphene. The doping of graphene also improved the uniformity of alignments of 4-cyano-4′-pentylbiphenyl (5CB) in PDLC because the rod-shaped liquid crystals tended to align along the surface of sheet structures and the alternate positions of the hexagons on the graphene surface matched well with the alkyl chains of 5CB molecules. Higher third-order nonlinear refraction and absorption coefficient can be attributed to the geometric relationship between graphene and 5CB. The GO-doped samples exhibited higher photorefractive efficiency, third-order nonlinear refraction, and absorption coefficient than undoped PDLC samples, but lower values than the graphene-doped sa...

Photorefractive effects in polymer dissolved liquid crystal composites dopes with fullerene derivatives

We investigated the photorefractive performance of the polymer dissolved liquid crystalline composite (PDLCC), in which liquid crystalline polymer and low-molar-mass liquid crystal are miscible without phase separation, doped with three kinds of fullerene derivatives with different length of alkyl groups. The photorefractive performance was improved for the photorefractive PDLCC doped with fullerene derivatives with long length of alkyl groups. The photorefractive grating formation originates in the cooperative reorientation of the liquid crystalline director and the space charge field was estimated using the elastic continuum theory and the field for the PDLCC doped with the functionalized fullerene with longer alkyl side groups was larger than that for the PDLCC doped with conventional fullerene C60.

Real-time dynamic hologram in photorefractive ferroelectric liquid crystal with two-beam coupling gain coefficient of over 800 cm–1 and response time of 8 ms

The photorefractive effect in photoconductive ferroelectric liquid crystals (FLCs) that contain photoconductive chiral compounds was investigated. Terthiophene compounds with chiral structure were chosen as the photoconductive chiral compounds and mixed with an achiral smectic C liquid crystal. The mixture exhibited the ferroelectric chiral smectic C phase and photoconductivity. The photorefractivity of the mixture was investigated by two-beam coupling experiment. It was found that the FLC containing the photoconductive chiral compound exhibits a large gain coefficient of over 800 cm−1 and a fast response time of 8 ms. The high photorefractive performance was considered to be due to enhanced charge mobility.

Dynamic holographic images using poly(N-vinylcarbazole)-based photorefractive composites

We present the optimization of poly(N-vinylcarbazole) (PVCz)-based photorefractive composite films for use in a dynamic holographic imaging system. The compositions of the composite films used in this study included PVCz/4-azacycloheptylbenzylidenemalononitrile (7-DCST)/carbazoylethylpropionate (CzEPA), N-ethylcarbazole (ECz), benzyl n-butyl phthalate (BBP)/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM), or 2,4,7-trinitro-9-fluorenone (TNF) (44/35/20/1 wt%). PVCz with molecular weights of 23 000, 100 000, 290 000, 370 000, and 810 000 gmol−1 were used. The PPC film (PVCz with Mw: 370000/7-DCST/CzEPA/TNF, 44/35/20/1 wt%) was observed to be the most well-balanced for photorefractive performance. To demonstrate the practical application of these films, dynamic holographic images were reflected from a spatial light modulator (SLM). The optimized PPC film was used in the dynamic holographic imaging system, and well-balanced dynamic holographic images were obtained.

Synthesis of organic phenothiazine-based molecular glasses and effect of racemic/homochiral aliphatic chain on near-infrared photorefractive property

Organic near-infrared photorefractive molecular glasses with a phenothiazine moiety are designed and synthesized through the introduction of linear, racemic/homochiral asymmetrically branched aliphatic chains into photorefractive chromophore as an auxiliary group. The compounds are characterized with 1H-NMR, IR, FAB-MS, UV–vis, TG, DSC, etc. The effect of different aliphatic chains on the absorption and thermal properties is investigated in detail. The molar absorption coefficiency at the absorption maximum wavelength showed that the homochiral asymmetrically branched aliphatic chain has a strong hypochromic effect in the dilute solution when it is introduced into photorefractive chromophore. The DSC measurement indicated that the introduction of asymmetrically branched aliphatic chain is the key issue to design organic molecular glasses whether it is racemic or homochiral. The effect of racemic/homochiral asymmetrically branched aliphatic groups on photorefractive property is investigated carefully with poly(N-vinylcarbazole) (PVK) as a photoconductor and with (2,4,7-trinitro-9-fluorenylidene) malononitrile (TNFM) as a photosensitizer. The results suggested that the racemic group is more beneficial to the improvement of photorefractive performance than the homochiral when the homochiral cannot induce rigid photorefractive chromophore to be much more ordered.

Innovative Sensitizer DiPBI Outperforms PCBM

A new photorefractive (PR) material sensitized with a novel functional unit DiPBI is prepared based on the guest/host approach. This unique mixture exhibits strong absorption in the whole range of visible light. Moreover, the photorefractive performance as well as PR speed is improved by far compared to the widely used sensitizer PCBM.