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.     

High-resolution, high-speed photorefractive incoherent-to-coherent optical converter

We demonstrate a photorefractive incoherent-to-coherent optical converter driven by ultraviolet light that provides a 35-mus response time and an optical resolution of 124 line pairs/mm. The device, implemented in KNbO(3) , operates with a modulating intensity of 85 mW/cm(2) , which corresponds to an optical switching energy per bit of 0.5 pJ. A conversion rate of the order of 90 Gbits/(s cm(2)) is achieved. The conversion between the ultraviolet light and the visible laser beam at lambda=532 nm occurs through anisotropic Bragg diffraction at a modulated interband photorefractive grating. Our device has a better optical resolution and conversion rate than optically addressed solid-state spatial light modulators based on the photorefractive effect and multiple quantum wells, and it is also faster than devices based on liquid crystals.     

Fast photorefractive response in strongly reduced near-stoichiometric LiNbO(3) crystals

The photorefractive response time of LiNbO(3) crystal is of the order of minutes, and such a long response time limits the crystal's practical applications. We report the photorefractive properties of nominally pure near-stoichiometric LiNbO(3) crystal that is strongly reduced in vacuum. A short photorefractive response time of the order of 100 ms is measured at a wavelength of 514.5 nm, with incident light intensity of 1.6 W/cm (2) , and possible corresponding mechanisms are discussed. To our knowledge this is the first experimental evidence of a subsecond photorefractive response in pure LiNbO(3) crystals. The diffraction efficiency of a holographic grating written in this reduced crystal is low but can be enhanced by an externally applied electric field.     

Light scattering in single crystals of nonlinear-optical photorefractive LiNbO<Subscript>3</Subscript>:Cu and LiNbO<Subscript>3</Subscript>:Zn

Comparative studies are made of the photorefractive scattering of light in nonlinear single crystals of lithium niobate with congruent compositions (LiNbO₃) doped with "photorefractive" Cu [0.015 mass %] and "nonphotorefractive" Zn [0.5 mass %] cations. For the first time it is found that single crystals doped with "photorefractive" and "nonphotorefractive" cations have different indicatrices for photorefractive light scattering. The aperture angle for photorefractive scattering reaches its steady state value more rapidly with high laser powers than with low. However, at high powers laser induced heating of the crystal is greater, and this leads to a narrowing of the scattering indicatrix. It is also found that photorefractive scattering in these single crystals depends on the region of the boule from which a sample has been cut. This indicates that there is a nonuniform distribution over the boule of the imperfections with localized electrons which determine the magnitude of the photorefractive effect.     

Photorefractive two-step recording in a piezoelectric La3Ga5SiO14 crystal doped with praseodymium

The first experimental verification to the authors' knowledge of efficient two-step recording in a photorefractive and piezoelectric crystal of La(3)Ga(5)SiO(14):Pr(3+) with cw laser light (I<20W/cm(2)) is reported. We found nonlinear behavior of the photorefractive sensitivity and photovoltaic current, indicating that the excitation process is connected to excitation of 4f(2) electrons of the Pr(3+) dopant through an intermediate level to the conduction band. This two-step nature of the photorefractive effect makes the crystal an exciting new candidate for applications in holographic storage.     

Multiple incoherent gray photorefractive spatial solitons

We show that an even-number sequence of multiple incoherent gray photorefractive spatial screening solitons is possible. When the external voltage is small, only an incoherent Y-junction photorefractive spatial screening soliton is generated, corresponding to the lowest order in the even-number incoherent soliton sequence. When the external voltage is increased, the incoherent gray soliton beam splits into an even-number sequence of multiple incoherent gray photorefractive spatial screening solitons. For a given physical system such an even-number sequence of multiple incoherent gray photorefractive spatial screening solitons is obtained by either increasing the external voltage and decreasing the full width half maximum (FWHM) of the beam’s intensity or increasing the external voltage and decreasing the beam’s grayness.     

Detection of small phase modulation using two-wave mixing in photorefractive crystals

Two-wave mixing of phase-modulated beams in photorefractive crystals under a dc electric field is studied both theoretically and experimentally. Based on the vectorial theory of light diffraction in crystals of cubic symmetry, we derived an analytical expression that describes the phase demodulation in crystals with an arbitrarily oriented electric field. The phase-demodulation technique was used for estimation of the space-charge field created in photorefractive crystals. It is shown that the space-charge-field grating created in GaP and Bi₁₂TiO₂₀ crystals is much smaller than that predicted from the one-level band-transport model of the photorefractive effect. Received: 12 December 2000 / Revised version: 8 February 2001 / Published online: 27 April 2001     

Major improvements of the photorefractive and photovoltaic properties in potassium niobate

Optical and electrical measurements have been made on a new codoped potassium niobate crystal (KNbO3:Fe,Ag) that yields a significant enhancement of the photorefractive and photovoltaic effects when compared with the published results for singly doped potassium niobate crystals. The codoped Ag impurity enters the K site, rather than the typical Nb site, thus changing the local field in the lattice. It is believed that Fe perturbed by the Ag in the K site is responsible for an enhancement of the linear absorption and photocurrent, as well as a probable increase in the effective trap density. An enhanced trap density is likely the cause of the increased photorefractive counterpropagating two-beam coupling efficiency.     

Orientational photorefractive effects observed in polymer-dispersed liquid crystals

We present a novel type of photorefractive polymer-dispersed liquid crystal (PDLC) that contains a polymer, low-molecular liquid crystals (LC's), and a photoconductive sensitizer. Photorefractivity is demonstrated by two-beam-coupling experiments with an external dc field. This material shows highly sensitive photorefractivity, and a purely refractive-index grating (Dn >/= 3.0 x 10(-3)) is generated by irradiation of interference light (lambda=632.8 nm) . The grating in the photorefractive PDLC's shows strong anisotropy, and the dynamics depends strongly on the composition. In the case of a 40:60 weight ratio of polymer:LC's, the photorefractive PDLC's present a stable memory effect.     

Low-temperature thermal fixing of holograms in photorefractive La(3)Ga(5)SiO(14):Pr(3+) crystal

Thermal fixing in a La(3)Ga (5)SiO (14):Pr (3+) photorefractive crystal is demonstrated all the way down to room temperature. This, to our knowledge, is the first report of such an effect in any photorefractive material. From the temperature dependence of the process the activation energy of the carriers involved in the fixing process is measured to be E(A)=0.89 eV . Further, an effective photorefractive charge density of (1.4+/-0.2)x10(16) cm (-3) and Debye screening length of (6.8+/-0.7)x10(-6) cm is measured.     

High photorefractive sensitivity at 860 nm in reduced rhodium-doped KNbO(3)

We show that high-temperature reduction in a CO-CO(2) atmosphere increases the photorefractive sensitivity of KNbO(3):Rh at 860nm by 4 orders of magnitude compared with that of the as-grown crystal. The effective trap density is increased by a factor of 3, and the photoconductivity by a factor of 30, and the photorefractive response at a grating spacing of 0.15 mu;m is accelerated by a factor of 400. The grating buildup time at a grating spacing of 0.7 microm and an intensity of 1Wcm(-2) is 0.5 s, a value comparable with that of as-grown KNbO(3):Fe at visible wavelengths. The optical and photorefractive parameters of Rh-doped KNbO(3) subjected to reduction treatment are characterized for wavelengths of 0.48-1.064 microm .     

High diffraction efficiency and a quasi-permanent grating in photorefractive nematic liquid crystal at low temperature

At a temperature lower than the melting point of pure pentyl-cyanobiphenyl liquid crystal (5CB) we studied the photorefractive effect of 5CB heavily doped with fullerene (C60) and obtained a diffraction efficiency of the photorefractive grating of more than 40%, which is more than twice as high as the best data reported to our knowledge. A quasi-permanent grating was observed with a diffraction efficiency of 11.2% after the grating was kept in the dark for a month.     

Photorefractive properties of iron-doped stoichiometric lithium niobate

The photorefractive properties of stoichiometric LiNbO(3) crystals with a small number of defect densities grown by the double-crucible Czochralski method are investigated and compared with the defect densities of commercially available congruent Fe-doped LiNbO(3) crystals. Two-wave-mixing experiments show that novel stoichiometric crystals exhibit larger photorefractive gain and considerably faster response times than congruent ones. The results indicate that the nonstoichiometry defect control of photorefractive crystals is of key importance for the improvement of their properties.     

有偏压中心对称光折变晶体中的屏蔽孤子

给出了中心对称光折变晶体中屏蔽孤子的高阶空间电荷场.当高阶项可以忽略时,这个电荷场就变为早前在中心对称光折变晶体中研究的屏蔽孤子的空间电荷场.研究了中心对称光折变晶体中屏蔽孤子的高阶非线性波动方程.在适当的条件下,这个非线性波动方程能够展示明暗空间光孤子.这类晶体不同于非中心对称晶体,其非线性折射率的改变来源于二次电光效应,而不是一般的线性电光效应.应用光束传播的方法,对这些孤子的稳定性进行了讨论.表明在小的微扰下这类孤子是稳定的,不会发生分裂.     

Polymeric photorefractive surface waves

We theoretically studied photorefractive surface waves supported by polymers with orientationally enhanced photorefractive effect. We find that the orientational enhancement and the dependency of the quantum efficiency of generating mobile holes on the electric field make the diffusion nonlinearity in photorefractive polymers dependent not on only the variation of light intensity but also on external electric field, which is very different from the external electric field independent diffusion nonlinearity in inorganic photorefractive crystals. Consequently the profile, frequency and especially the penetration depth of polymeric photorefractive surface waves strongly depend on external electric field, which makes it more controllable than that in photorefractive inorganic crystals. The stability of polymeric photorefractive surface waves, the effects of birefringence component and electro-optic component of orientationally enhanced photorefractive nonlinearity on the formation of x or y-polarized polymeric photorefractive surface waves are also discussed.     

光折变聚合物材料在光电子技术中的应用

光折变聚合物材料与光折变晶体相比，其非线性光学系数大，响应时间快，成本低廉，制备灵活，有可能成为制作光子学集成器件的材料。简要介绍了光折变聚合物材料在图像识别、光信息存储及光学相干层析技术等方面的应用情况，指出光折变聚合物材料中存在光子带隙。最后预测了光折变聚合物材料作为光子晶体在光子学器件中的应用前景。     

Simple real-time 3-D displacement measurement by fringe analysis using a photorefractive interferometer

A simple 3-D displacement measurement method using a photorefractive interferometer is proposed and experimentally demonstrated. A photorefractive crystal attached to the sample under test is used to record an original spherical wave that provides a shifted reconstructed wave after displacement. The shifted wave interferes with direct wave to form an interferogram. By analyzing the fringe pattern the in-place and out-of-place displacements can be determined at the same time. This method works in real-time and is easy to operate with adequate accuracy comparable with or even better than that of other techniques.     

Raman spectra of photorefractive lithium niobate single crystals

We have conducted comparative studies of the Raman spectra of lithium niobate (LiNbO₃) crystals of different compositions for excitation in the visible and near IR regions. We have observed that the photorefractive effect is one of the factors leading to line broadening. For this reason, the linewidths may be greater upon Raman excitation in the visible region than for excitation in the near IR region. This may be explained by formation in the crystal, when illuminated by laser radiation in the visible region, of a three-dimensional sublattice of nanostructures and microstructures (with refractive index and other physical parameters different from the parameters of the host crystal) from which photorefractive light scattering occurs. Formation of nanostructures and microstructures makes an additional contribution (besides the contribution due to random and dynamic disorder in the arrangement of the structural units) to the broadening of the Raman lines in the visible region of the spectrum. Illumination of the crystal by radiation in the near IR region does not induce a sublattice of nanostructures and microstructures, due to a significantly smaller photorefractive effect.     

Optical properties of LiNbO3:Mg(5.21 mol %) and LiNbO3:Fe(0.009 mol %):Mg(5.04 mol %) crystals

Using methods of electronic spectroscopy, laser conoscopy, photoinduced (photoreactive) light scattering, and Raman light-scattering spectroscopy, we have studied the optical homogeneity, optical transmission, and photorefractive properties of single crystals LiNbO₃:Mg(5.21 mol %) and LiNbO₃:Fe(0.009 mol %):Mg(5.04 mol %) that were grown from congruent melts. We have ascertained that doping with “nonphotorefractive” Mg²⁺ cations causes suppression of the photorefractive effect in a lithium-niobate crystal. Upon double doping (Fe:Mg), if the concentration of Mg²⁺ cations exceeds the threshold concentration, the photorefractive effect is almost not observed and the presence of “photorefractive” Fe cations does not affect the photorefractive effect as strongly as in congruent crystals doped with Fe.     

双光子光折变介质中非相干耦合亮-暗混合屏蔽孤子族

 根据光波耦合方程及亮-暗孤子对解,对稳态情况下多束互不相干的光束在有外加电场的双光子非光伏光折变晶体中的传播进行分析,给出产生亮-暗双光子光折变屏蔽光伏孤子族需满足的条件。证明有外加电场双光子非光伏光折变晶体中存在非相干耦合亮-暗混合屏蔽孤子族,孤子族由偏振态和波长都相同的多束互不相干的光形成。当外加电场方向和晶体光轴方向相同时,双光子光折变晶体可支持亮孤子族总峰值光强稍大于暗孤子族总峰值光强的的非相干耦合亮-暗混合孤子族,当外加电场方向与晶体光轴方向相反时,双光子光折变晶体可支持亮孤子族总峰值光强稍小于暗孤子族总峰值光强的非相干耦合亮-暗混合孤子族。