A photorefractive (PR) organic glass based on the chromophore IDOP-20 is reported. The electronic structure of this merocyanine dye was tuned close to the cyanine limit to optimize the anisotropy of the linear polarizability δα and to achieve a large PR figure-of-merit. The carefully chosen aliphatic side chains of the chromophore prevent dipolar aggregation even at very high concentrations in liquid solution and provide excellent compatibility with polymer matrices. IDOP-20 also forms a stable glass with glass transition temperature of Tg=58o C. Mixtures with the plasticizer DPP were prepared to adjust the glass transition temperature close to room temperature. Such mixtures showed PR response after sensitization with TNFM where sensitization occurred through a CT band between TNFM and IDOP-20 and charge transport was supported by the multifunctional by IDOP-20. A PR glass of composition IDOP-20:DPP:TNFM 69:30:1 wt% was studied by a complete set of holographic-optical experiments (two-beam coupling and four-wave mixing), (photo)conductivity measurements and ellipsometric measurements. The performance of the new PR glasses is discussed with respect to similar materials based on the ATOP chromophore. The (mono-exponential) response time of the material is strongly temperature dependent and limited by chromophore reorientation.
Derivatives of 2-dicyanomethylen-3-cyano-2,5-dihydrofuran (DCDHF) have been synthesized by different methods to be used as photorefractive (PR) chromophores. Structure modifications were performed on the donor, acceptor and conjugated π-system for improving properties such as glass formation. Structure-property relationships important for PR applications are discussed from the results of studies including UV-Vis, electrochemistry and DSC.
Since the first observation of the photorefractive (PR) effect in polymers, extensive efforts have been directed toward understanding the physics of the PR process in these systems, as well as optimizing polymer composites and glasses for various applications. Despite remarkable progress both in elucidating the mechanisms and processes contributing to the PR effect and in designing organic materials with high gain and diffraction efficiency, simultaneously attaining high refractive index modulation, fast dynamics, and good thermal properties in one material remains a challenge. Monolithic glasses represent an attractive class of PR organic materials since they possess large nonlinearities and minimal inert volume, which enhances the performance without stability problems. In this paper, we present a complete study of monolithic glasses based on a promising new class of chromophores (containing 2-dicyanomethylen-3-cyano-5,5-dimethyl-2,5-dihydrofuran, abbreviated as DCDHF-derivatives). We describe thermal, photoconductive, orientational, and photorefractive properties of these materials in both red and near infrared wavelength regions. By studying the temperature dependence of various parameters, we analyze the factors that affect photorefractivity in DCDHF-based materials.
Altering the sample temperature in a photorefractive material changes the rotational mobility of the chromophores. A change of three orders of magnitude in the response times over a temperature change of 12 K has been observed. In the photorefractive experiment, however, the chromophore orientation is induced by the non-instantaneous change of the space charge field. The finite speed of the latter causes the chromophore answer to be different from their normal relaxation behaviour to an instantaneous change. This effect is most pronounced when both time constants are in the same range.
Hole mobilities in substituted N, N'-bis-(m-tolyl)-N-N'-diphenyl-1,1'-biphenyl-4,4'-diamine (TPD) derivatives doped in polystyrene (PS), were analyzed by the time-of-flight technique to determine the effect of altering the geometric and electronic structure of TPD. Data were collected as a function of applied field and temperature to yield the energetic and positional disorder parameters defined in the disorder formalism. The impact of the molecular dipole moment on transport properties was also evaluated. The larger molecular dipole moments of the derivatives lead to an increase in the energetic disorder, which contributes to their lower mobilities. However, the dipolar disorder contribution was found to account only partially for the large differences in mobility.
This work describes how the sol-gel process can be a useful approach for the preparation of hybrid materials showing either holographic as well as photorefractive properties. The composition of the holographic and photorefractive materials prepared through the sol-gel approach are completely based on their respective multi-functional polymer analogues, but in the polymer acting as host matrix which is replaced by a nanoporous silica matrix. Such a replacement infers an overall improvement in both the optical and the mechanical properties of the material. The goodness of the properties found for either the holographic as well as the photorefractive materials demonstrate the validity of these new synthetic routes for the preparation of optical recording materials.
The photorefractive effect of a series of low-molecular-weight ferroelectric liquid crystals (FLCs) doped with photoconductive compounds was investigated using two-beam coupling experiments. Asymmetric energy exchange was observed only in the ferroelectric phase, and the refractive phase, and the refractive index formation time was found to be ~30 ms, which is faster than that of nematic photorefractive LCs. These results indicate that the mechanism of the photorefractive effect in FLCs is different from that in nematic LCs. The photorefractivity of a photoconductive polymer and FLC mixture was also investigated, and the effects of temperature, the strength of the applied electric field and spontaneous polarization on the diffraction efficiency were examined.
We have demonstrated the two types of applications in the field of the optical information processing by using the photorefractive mesogenic composite consisting of a mixture of three components including a functionalized copolymer, a low-molar-mass liquid crystal, and sensitizing dye. One of the applications is to amplify the two-dimensional optical image by means of the Fourier transform geometry with the two-beam coupling optical system. The other is to enhance the edge of the optical image using Fourier transform holographic geometry. We calculated the expected images on the basis of the Fourier transform optics and the photorefractive properties of the mesogenic composite, and obtained good agreement with the observed images and the theoretical expectation.
Two amorphous side-chain copolymethacrylates containing 60% azobenzene chromophores (PII, PIV) were irradiated homogeneously with a linear polarized Ar+ ion laser beam at 488 nm. From polarized UV/VIS spectra the maximum of the photoinduced dichroism was calculated (dPII=0.25, dPIV=0.35). No intensity dependence of this values was notified. PIV was irradiated under holographic conditions with a grating period of 30 μm. Two types of polarization gratings, circular orthogonal and linear orthogonal ones were inscribed. Using a microscope spectrometer polarized UV/VIS spectra were measured within the grating period with a lateral resolution of about 2 μm. The induced local dichroism was measured and the different modulation of the molecular orientation in the recorded gratings was confirmed by this direct experiment. PII was used to monitor the grows up stages of the surface relief grating, starting from the molecular photoorientation until the surface deformation. Parallel to the online observed diffraction efficiency, the offline surface profile measurements were done by AFM. The details of surface topography and the reversibility of the process are discussed and an interesting possibility of the surface structuring is demonstrated.
The relaxational response of a photorefractive polymer subjected to different poling voltages slightly above the glass transition temperature is measured using ellipsometry. We discuss our results using a new curve-fit function, which provides a good description of the experimental data. Using this new function we examine how the poling voltage influence the poling behavior of the chromophores. Furthermore, it is shown that the characteristic time for turn-off behavior is strongly influenced by the time span elapsed since turn-on.
Proc. SPIE 4802, Role of the polymer viscoelasticity on the orientational processes of chromophores and on the photorefractive performances in low-Tg-doped polymers, 0000 (5 November 2002); doi: 10.1117/12.452450
The dynamics of the photorefractive properties in low glass transition temperature (Tg) doped polymers essentially depend on the photoconductivity of the host and on the orientational dynamics of nonlinear optical chromophores imbedded in the matrix. A high rotational mobility of push-pull chromophores is required to observe the so-called orientational enhancement. The influence of Tg on the photorefractive performances of guest-host polymers has been previously pointed out. However, the effects of the viscoelastic properties of polymers on the orientational processes of chromophores are neglected in most of the studies devoted to the optimization of photorefractive dynamics. In the present work, the orientational dynamics of chromophores are investigated by dielectric spectroscopy and ellipsometric dynamical measurements in various low Tg doped polymers. The experimental results show the role of different physical parameters (temperature, applied electric field magnitude, amount of plasticizer, average molecular weight of polymer) on the rotational mobility of chromophores. These data underline the necessity to take into account the viscoelastic behavior to improve the dynamics of photorefractive polymers.
Quantum dots have significant potential as photosensitizers with a range of possible surface structures and a variety of materials available. Quantum dots composed of a cadmium selenide (CdSe) core and a cadmium sulfide (CdS) shell have been combined with poly(N-vinylcarbazole) and electro-optic dye molecules to form a new photorefractive composite. The photorefractive nature of the material was demonstrated with a gain of 4 cm-1 in s-polarized asymmetric two-beam coupling. The photosensitizing ability of CdS nanoparticles in a photorefractive polymer composite has been previously only evident for larger applied fields. The diffraction efficiency of the current composite is a vast improvement over the quantum dot composite reported previously, with degenerate four-wave mixing yielding diffraction of 1.3% at 70V/μm. Analysis of the field-dependence of the space-charge field rise time reveals a zero-field dissociation efficiency of (3.6+/-0.5)x10-5. Other example quantum dot sensitizers are also under investigation.
The polymeric photorefractive composite was prepared from the mixture of carbazole-substituted polysiloxane as a photoconducting medium, 2, 4, 7-trinitro-9-fluorenone as a photo-sensitizer, and 2-(3-((E)-2-(dibutylamino)-1-ethenyl)-5,5-dimethyl-2-cyclohexenyliden) malononitrile as an optically nonlinear chromophore. This polymeric composite with the thickness of 100 μm exhibited the high diffraction efficiency of 92% at the applied electric field of 30V/μm, which corresponds the refractive index modulation (Δn) of 3 x 10-3. The applications of this polymeric composite to pattern recognition are demonstrated. Character and fingerprint recognitions based on joint-transform optical correlation are successfully demonstrated.
In this work we report for the first time observation of AC photocurrents generated by photo-emf effect in high quality crystals of the conjugated polymer 2,4-hexadiyne-1,6-diol-bis-(para-toluene sulfonate) or polydiacetylene PTS (usually referred simply as PTS). We used illumination at λ=633nm where the absorption spectra has its peak. In principle, photocurrent at this wavelength should be small due to the large exciton binding energy, however we show that relative large photocurrents can be observed at modest electric fields (up 2000 V/cm). When the sample is illuminated with a periodically oscillating interference pattern it leads to spatial redistribution of charge carriers among traps. Spatial overlap of the resulting space-charge field with the oscillating photoconductivity pattern gives rise to an oscillating photocurrent (photo-emf signal). From photo-emf measurements, a Maxwell relaxation time of 0.17 sec at 6 W/cm2 and lifetime of carriers of 6 ms were estimated. The later was confirmed from photoconductivity measurements.
The optical responses and orientational photorefractives of nematic liquid crystals doped with fullerene were investigated with layer structures in cells. Nematic liquid crystals injected at isotropic phase suffer optical losses due to undesirable light scattering and the switching disorder of liquid crystals under applied voltages, which can lower the photorefractive effects. However, when nematic liquid crystals are injected at a nematic phase, the better cell texture can be obtained due to the flow-induced orientation effect. Sample with this method showed much higher photorefractive effects than that injected at an isotropic phase, because the molecular ordering of liquid crystals highly influences the orientational photorefractive effects. As the temperature at injection state changes, the molecular ordering of liquid crystals may also change. Therefore, through the quantitative analysis of molecular ordering, it was believed that the temperature at an injection state could determine the molecular ordering and in fact, the photorefractive effect.
We report the characteristics of the diffraction behavior of the photorefractive (PR) molecular materials that contains the phenothiazine derivatives. Diphenylhydrazine and malononitrile were reacted with N-alkyl substituted phenothiazinyl aldehyde to provide the charge transporting and nonlinear optical (NLO) chromophore, respectively. The resultant phenothiazine derivatives were employed to fabricate the efficient photorefractive media either without or with 2,4,7-trinitrofluorenone (TNF). The prepared molecular samples can contain the very high NLO chromophore (50-100 wt%). We showed very unusual complementary grating to reduce the hole transport/trap effect from pure photorefractivity and how to control the counter-effect from the electron transport/trap phenomena. Adding the specific molecules to the host PR materials reduced the electron transport effect that is responsible for the complementary grating, thereby causing a dominant contribution of hole transport effect to the grating formation and erasing processes.