High longitudinal selectivity of shifting multiplexing in volume holograms

High longitudinal selectivity of the shifting multiplexing with spherical reference wave is proposed and demonstrated. A simplified method based on wave optics is used for calculating the selectivity, and the result fits well the experimental measurement. Under the paraxial condition, a simple formula for the longitudinal selectivity is introduced. With use of an object lens with effective NA=0.817, we obtain that an FWHM of selectivity is as small as 1 μm.     

Photorefractive shearing interferometer

A novel method of obtaining shearing interferogram by slightly moving the crystal in a photorefractive interferometer is proposed. This method can measure the phase of an object itself instead of its diffraction field, and it is easy to realize continuously changeable shearing distance in any lateral direction and introduce carrier fringes at the same time. Both the theoretical analysis and experimental verification are given.     

Photorefractivity of hyperbranched polyisophthalate endcapped with pendant carbazolyl and azobenzene groups

A novel hyperbranched polyisophthalate containing pendant carbazolyl and azobenzene groups at the terminals was synthesized, and its photorefractivity has also been evaluated by the effective electro-optic (EO) coefficient, two-beam couple (TBC) coefficient, diffraction efficiency and photorefractive (PR) response time. This PR polymer has a low glass transition temperature of 21 °C without any extra plasticizer added, which provides a necessary free volume for EO chromophore reorientation during the recording process at room temperature. The obtained PR film exhibited an effective EO coefficient of 30 pm V⁻¹, a TBC coefficient of 59 cm⁻¹, a diffraction efficiency of 17%, and a modulation index of 1.67 × 10⁻³ at the wavelength of 632.8 nm with an applied electric field of 80 V μm⁻¹ at room temperature. The TBC coefficient was found to be beam-intensity dependent, compared with what is found in inorganic crystals. Moreover, a fast PR response time of 10 ms was deduced at an erasing beam intensity of 1.0 W cm⁻².     

Influence of surroundings on photorefractive effect in lithium niobate crystals

In the paper results of the investigation of the influence of electric properties of the environment surrounding LiNbO₃ crystals on photorefractive effect induced in these crystals by Gaussian Ar⁺ laser beam with various intensities are presented. We show spatial and temporal dependences of changes of the refractive index obtained experimentally in LiNbO₃: Fe and LiNbO₃: Fe:Mn samples surrounded by media with different electric conductivities and different permittivities (water, air, water solution of CaCl₂). The space and time dependences of the refractive index changes induced by the Ar⁺ laser beam are observed by means of the Mach–Zehnder interferometer using light from HeNe laser. The experimentally obtained results are in a good agreement with those following from numerical calculations using the manifold mirroring method. The agreement between calculated and experimental results indicates that the polarization charge at the photorefractive crystal/surrounding medium boundary significantly influences the photorefractive process in the crystal. The experimentally observed slow spontaneous decrease of the refractive index change in a sample placed into a slightly conducting medium (air) after switching off the beam also indicates that the polarization charge in the sample's surroundings affects the photorefraction.     

Optofluidic droplet router

This contribution presents an optofluidic droplet router which is able to route and steer microdroplets using optically induced forces created solely by the bulk photovoltaic effect on a nonlinear substrate. The combination of microfluidic tools with the properties of a photorefractive crystal allows for the generation of dielectrophoretic forces that can be either repulsive, leading to virtual barriers, or attractive, creating virtual rails. The sign of these forces is solely determined by the electrical properties of the liquid medium under investigation. Moreover, the induced structures on the bottom of the microfluidic channel are optically reconfigurable, so that the same device can easily be adopted for different purposes. Appropriate droplet‐generating devices are fabricated by UV illumination of SU‐8 and polydimethylsiloxane replica molding of the master structures. The bottom of the channels is formed by an iron‐doped lithium niobate crystal, whose internal electric fields are induced by structured illumination patterns and exert dielectrophoretic forces on droplets in the microfluidic section.

     

Orientationally polarized dependence of image contrast in doped lithium niobate crystals

In lithium niobate crystals doped with photorefractive iron and copper admixtures when lightening objects with broadband incoherent radiation one can observe typical orientation and polarized dependences of image contrast. The record does not realize at orientation of light bands in the image parallel to the polar axis of the crystal.     

Imaging anisotropic elastic properties of an orthotropic paper sheet using photorefractive dynamic holography

An important material property in the paper industry is the anisotropic stiffness distribution due to the fibrous microstructure of paper and to processing procedures. Ultrasonic methods offer a means of determining the stiffness of sheets of paper from the anisotropic propagation characteristics of elastic Lamb waves along the machine direction and the cross direction. That is, along and perpendicular to the direction of paper production. Currently, piezoelectric ultrasonic methods are employed in the industry to measure the elastic polar diagram of paper through multiple contacting measurements made in all directions. This paper describes a new approach utilizing the INEEL Laser Ultrasonic Camera to provide a complete image of the elastic waves traveling in all directions in the plane of the paper sheet. This approach is based on optical dynamic holographic methods that record the out of plane ultrasonic motion over the entire paper surface simultaneously without scanning. The full-field imaging technique offers great potential for increasing the speed of the measurement and it ultimately provides a substantial amount of information concerning local property variations and flaws in the paper. This report shows the success of the method and the manner in which it yields the elastic polar diagram for the paper from the dispersive flexural or antisymmetric Lamb wave.     

掺杂偶氮化合物的PMMA光折变聚合物的拉曼光谱分析

光折变聚合物具有良好的光折变效应,采用以惰性聚合物PMMA为基体,掺杂偶氮化合物染料的光折变聚合物具有较好的光学非线性和稳定性。文章主要介绍使用激光拉曼光谱仪对合成聚合物的各单体和聚合物薄膜进行拉曼光谱扫描,对比分析各个单体成分的特征谱峰与聚合物薄膜特征谱峰,肯定了实验中得到的光折变聚合物薄膜与理论上要求得到的一致性。     

Separate spatial soliton pairs in a biased series centrosymmetric photorefractive crystal circuit

It is shown theoretically that three types of separate soliton pairs: bright-bright, bright-dark, and dark-dark, can be established in a biased series circuit consisting of two centrosymmetric photorefractive crystals and the two solitons in a separate soliton pair can affect each other by the light-induced current. The effects of the intensities of soliton beams and background lights on the normalized profiles of solitons are discussed.     

Calculation of the gain in optically active photorefractive crystals considering the self-diffraction in a two-wave mixing process

We present a physical and computational procedure to calculate the gain in a two-wave mixing process in a thick optically active photorefractive BSO crystal under diffusion regime. The procedure is based on the integration of the amplitude of the signal beam through the sample. In the model, the effects of self-diffraction are taken into account by using the more general expression of the diffraction efficiency for the case of dynamic gratings. It allows also including the effects of the optical activity on the coupling factors and not only on the rotation of the optical field vectors when the waves are propagated along the crystal. The <001> crystallographic configuration is used. The model is used to predict the behavior of the gain as a function of the beam polarization angle. Numerical and experimental results using crystals with thickness of 5 mm and 10 mm are presented. A good agreement between the proposed model and the experimental measurements is observed.