Polarization holographic gratings in an azobenzene side-chain liquid-crystalline polymer

Polarization holographic recording was investigated in a liquid-crystalline azobenzene side-chain polymer. A highly stable polarization holographic grating with the diffraction efficiency of 12.6% was written in the polymer film by two orthogonally linearly polarized 532 nm beams with equal intensities of 150 mW/cm². The polarization grating was studied with a 633 nm probe beam by surveying the conversion of the polarization state of the probe beam and measuring the dependence of diffraction efficiency on the polarization direction of the probe beam. It was found that the ± first-order diffraction efficiency and the polarization of the diffracted wave depended strongly on the polarization of the readout wave. The unusual diffraction properties of the recorded grating were attributed to the presence of the circular photoinduced birefringence. PACS 42.40.Ht; 42.40.Lx; 78.20.Fm     

Volume polarization holographic recording in thick phenanthrenequinone-doped poly(methyl methacrylate) photopolymer

Volume polarization holographic recording in phenanthrenequinone-doped poly (methyl methacrylate) photopolymer is obtained. Photoinduced birefringence in a 2 mm thick sample is measured by a phase-modulated ellipsometry. The birefringence induced in this material by linearly polarized beam at 514 nm reaches 1.2×10(-5). In addition, ability for recording volume polarization grating using two different polarization configurations is demonstrated and compared. The experimental results show that the diffraction efficiency of the hologram reaches to ～40% by using two orthogonal circularly polarized beams.     

Polarization holographic and surface-relief gratings at 257 nm in an amorphous azobenzene polyester

Polarization holographic and surface-relief gratings have been recorded in an amorphous azobenzene polyester by use of a frequency-doubled argon-ion laser beam at 257 nm. Higher excited states of azobenzene in the trans and cis configurations contribute to the formation of a diffraction grating in this experiment. A combination of right and left circularly polarized writing beams has been found to give the highest diffraction efficiency. The contributions to the total phase difference that arise from anisotropy and surface relief have been separated experimentally, and it is shown that the surface-relief grating contributes a larger phase difference than that which is due to anisotropy.     

Inscription of holographic gratings using circularly polarized light: influence of the optical set-up on the birefringence and surface relief grating properties

The polarization properties of the optical set-up used for holographic recording of diffraction gratings on azopolymer thin films are analyzed. The state of polarization of circularly polarized light is fully analyzed after reflection on a mirror at various incidences (Lloyd-mirror set-up). The Stokes analysis is performed using a photopolarimeter and the phase shift, the ellipticity and the azimuth orientation are compared with those calculated from Fresnel formulae. At large angles of incidence, an initially right circularly polarized (RCP) beam becomes elliptically polarized with an azimuth of nearly +45°. From these results, holographic diffraction gratings are recorded on an azobenzene-containing polymer thin film using (i) co- and contra-circularly polarized beams and (ii) a right circularly polarized beam interfering with a +45° linearly polarized light beam. Using Jones-matrix formalism, the polarization states of the diffracted orders from the birefringence (Δn) and the surface-relief (2Δd) gratings are derived and compared with experimental measurements. Finally, the induced local birefringences and surface-relief amplitudes are discussed in connection with atomic force microscopy measurements. The diffraction efficiencies obtained under the (+45°+RCP) and (LCP + RCP) (where LCP = left circularly polarized) configurations are thus compared and discussed. Received: 5 October 2001 / Revised version: 26 November 2001 / Published online: 17 January 2002     

Effect of polarization on efficiency of volume Bragg grating filter

<正>Diffraction efficiency of volume Bragg grating,whose period is in the same order as the incident wavelength, is related to the polarization direction of the incident linear polarized beam.When two linearly polarized recording beams with the same polarization direction are used for recording volume Bragg gratings in a photopolymer with diffusion amplification,the azimuth of polarization of the reconstruction beam influences the diffraction efficiency of the grating.When the probe beam is linearly polarized and oriented orthogonally to the grating vectors,the±1-order diffraction beams are also linearly polarized with polarization direction parallel to that of the probe beam.According to the results,a two-dimensional nonspatial optical filter consisting of the volume Bragg gratings would achieve significantly higher efficiency.     

菌紫质同线偏振全息记录时再现光偏振方向对衍射效率的调制

当用相同偏振方向的物光和参考光在菌紫质薄膜上记录光栅时,再现光的偏振方向会影响其衍射效率.采用琼斯矩阵方法对此进行了理论分析,结果显示,再现光的偏振方向对衍射光的光强产生余弦调制;当再现光的偏振方向平行或垂直于记录光的偏振方向时,衍射光仍为线偏振光,其他情况下衍射光均变成椭圆偏振光.再现光偏振方向对衍射效率峰值的调制为正向余弦调制,对衍射效率稳定值的调制为反向余弦调制(与前者在相位上相差π).加入辅助紫光可抑制光栅的饱和,从而使得再现光偏振对衍射效率稳定值的调制由反向余弦调制变为正向余弦调制,并且提高了衍 关键词： 菌紫质 光致各向异性 衍射效率 琼斯矩阵     

Polarization holographic gratings with high diffraction efficiency recorded in azopolymer PAZO

Azopolymers are one of the most efficient materials able to record the polarization state of light. They have numerous applications, such as data storage and diffractive optical elements with unique polarization properties. An essential parameter for each diffractive element is its diffraction efficiency η. In order to optimize the recording conditions and obtain high-efficient polarization holographic gratings, in the present work we study the dependence of the diffraction efficiency on the recording angle and thickness of a series of azopolymer layers. Three recording angles are used ??10°, 20° and 30° and three series of thin films with thicknesses 470, 850 and 2400 nm from the water-soluble azopolymer PAZO. The gratings are inscribed by two plain waves with left and right circular polarization from a He-Cd gas laser (442 nm). The diffraction efficiency of the gratings is probed with a right hand circularly polarized beam from a probe laser with wavelength 635 nm. The kinetics of diffraction efficiency η(t) in the +?1 diffraction order are presented and compared. Our experimental results indicate that highest diffraction efficiency (more than 40%) is obtained for the sample with thickness 2400 nm and for recording angle 10°. As the holographic recording in azopolymers is usually accompanied by formation of surface relief gratings, the surface topography of the recorded samples is also investigated by atomic force microscopy.     

Diffraction efficiency increase by corona discharge in photoinduced surface-relief gratings on an azo polymer film

The diffraction efficiency of a surface-relief grating (SRG) was dramatically increased by corona discharge. We fabricated this SRG by applying surface modulation to azo polymer films, using laser interference fringes. The electric charge was deposited upon the SRG by a corona discharge in an oven. The first-order diffraction efficiency measured before and after corona discharge above the glass-transition temperature increased from ~2% to ~40% in a SRG written with a circularly polarized beam. The relief depth of the SRG as measured with an atomic-force microscope also increased, from ~130 nm to ~450 nm. The increase in diffraction efficiency was independent of the writing polarization.     

Improvement of holographic recording property of a Ce: KNSBN crystal by the moving grating technique

The moving grating technique is applied to improve the holographic recording property of the Ce: KNSBN crystal. In the case of extraordinarily polarized recording, the diffraction efficiency at large fringe modulations is enhanced by a factor of up to 35% at the optimum fringe velocity and the fringe modulation dynamic range is improved from m = 0.2 to m = 0.6. In the case of ordinarily polarized recording, a linear holographic reconstruction in the range of m ≤ 0.8 can be achieved by the moving grating at a fringe velocity of 9.6 nm/s. These results are significant in holography where a grey-level object needs to be recorded and reconstructed with high fidelity.     

Polarization-controlled switching between diffraction orders in transverse-periodically aligned nematic liquid crystals

Transverse-periodic-oriented nematic liquid crystals (LCs) are a special type of optical axis grating that are capable of very high efficiency diffraction (theoretically, 100%) in thin layers of materials with thickness comparable to the radiation wavelength. In particular, they fully diffract linearly polarized input beam into circularly polarized +1st and -1st diffraction orders. We experimentally demonstrate switching between diffraction orders of such gratings when the polarization of the incident beam changes from right-circular to left-circular and vice versa with the aid of an electrically controlled LC phase retarder. Such a setup in which the diffraction efficiency and direction are controlled externally, without application of an electric field to the transverse-periodic grating, provides additional control opportunities and does not compromise the quality of the grating. The grating used in the experiment was 1.5 microm thick and had a period of 4 microm. The contrast ratio of switching between the +1st and -1st orders was as high as 267:1 for a He-Ne laser beam with a switching time of 6.6 ms.     

线偏光闪耀反射光栅衍射特性研究

通过数值方法对线偏光在Littrow装置中的理想导体闪耀光栅的偏振特性进行研究.用时域有限差分程序计算出任意偏振方向线偏光经过闪耀光栅衍射后的电磁场分布,提取衍射波的S偏振和P偏振分量,讨论了与光栅闪耀角和入射光波偏振角的关系.分析了在1530～1570nm光谱范围内,入射光波为线偏振波和园偏振波时,衍射光波的偏振色散特性与光栅闪耀角的关系,并给出了存在偏振色散的光栅闪耀角范围.这些分析和结果对工程设计具有一定的指导意义.     

Anisotropic gratings recorded from two circularly polarized coherent waves

A constant intensity, rectilinear light vibration, with spatially varying orientation, is obtained from superposition of two orthogonal circularly polarized waves propagating, in slightly different directions. Such an exposure on a photoanisotropic medium, prints a grating characterized only by spatial variations in the orientation of the induced optic axis. The diffraction properties of such a grating are investigated. It behaves as a uniformly transparent isotropic medium when illuminated by the same waves as during the recording process. Illumination by a circular wave, generates a transmitted and a single diffracted beam, while an incident elliptically polarized one would produce, apart from the central order, two diffracted beams with orthogonal circular polarizations.     

Lyot型单色光退偏器设计与研究

为了实现单色线偏振光的退偏,在Lyot型退偏器的基础上,设计了一种新型退偏器。该器件由2个慢轴夹角为45的1/4波片组成,通过对透射光的叠加分析,发现该退偏器可将任意振动方位角的线偏振光转化为强度稳定的圆偏振光,并得到其退偏度表达式。针对633 nm波段,精选2个1/4波片制作样品,并测试其退偏度。测试结果表明:对不同方位角的线偏振光,退偏器退偏度超过94%,当微调入射角时,退偏度则达到97%以上。     

Study on the rewritability of bisazobenzene-containing films in optical storage based on two-photon process

In this paper, we use a two-photon technique to record data points on bisazobenzene-containing films. The recording data points are evidently polarization dependent. It is found that the recording data points can be erased by the linearly polarized light whose polarization direction is perpendicular to that of the recording beam and then, in the same region, new data can be rewritten after erasing. In addition, the recorded data can also be erased completely by the circularly polarized light. Compared with linearly polarized beam, the circularly polarized light is a better choice for erasure as no main orientation is regained after erasing rather than reorientation.     

偶氮液晶聚合物薄膜的二维偏振全息记录

利用偏振全息记录的方法在一种含偶氮侧链的液晶聚合物薄膜中写入了二维偏振光栅.实验采用两束正交偏振的532 nm线偏光作为写入光,在样品同一点上分别记录了相互垂直的两个一维偏振光栅,继而对所构成的二维偏振光栅的特性进行了研究.实验结果表明：二维光栅的衍射效率比一维光栅低,其偏振特性是两个一维光栅元特性的叠加;二维光栅衍射效率对入射光偏振态的依赖性和光栅的偏振转换性质来源于材料中线双折射和圆双折射的共同作用. 关键词： 偏振全息 二维光栅 偶氮液晶聚合物     

Polarization and Color Separator Using Binary Phase Grating with Subwavelength Period

A data projector using three liquid crystal display panels has a complex optical system. The illuminating optics separate the beam from a light source into three primary colors and separate those into opposite polarizations using multi-layer films and prisms. A reflection grating with the period of subwavelength has high diffraction efficiency for p polarized light and high regularly reflectance for s polarized light. The diffraction angle of a grating largely depends on the wavelength, because a diffractive optical element (DOE) has large chromatic dispersion. The grating with the period of subwavelength can separate the unpolarized light into polarization components effectively using its polarization dependency and can separate white light into color components using its chromatic dispersion simultaneously. The grating makes the optical system simpler and smaller than those with conventional devices. In this paper the efficiency of polarization separation for the grating is calculated by a rigorous analytical method. Next, the condition for color separation is calculated by Snell#x0027;s law, and an optical system using a grating that performs polarization and color separation is proposed. Experimental results of the DOE fabricated are well matched with those of this simulation.     

亚波长光栅的偏振闪耀特性

 使用琼斯矩阵的方法推导了连续结构亚波长光栅的衍射方程,给出了光栅衍射效率表达式,对偏振特性与衍射特性进行了研究。发现连续结构亚波长光栅仅存在３个衍射级,总衍射效率为100%,且衍射效率可在衍射级间任意分配,0级的偏振态与入射光的偏振态相同,±1级衍射光偏振态与入射光无关,－1级为左旋圆偏振光,＋1级为右旋圆偏振光。通过设置入射光偏振态与光栅相位延迟等参数,可使光栅具有闪耀特性,据此可用于设计高效偏振光分束器和偏振光开关。     

偏振敏感材料中具有圆各向异性的偏振全息光栅

在向列相液晶中掺杂质量分数约0.2%的偶氮侧链型聚合物制成偏振敏感材料,使用正交圆偏振光记录了具有圆各向异性的一维偏振全息光栅,用线偏振光探测偏振全息光栅,得到正,负一级衍射光也为线偏振光,且偏振态与探测光和零级偏振态相互正交,可以运用琼斯矩阵理论加以解释.偏振全息光栅可由外加电场控制衍射级的变化,即在外加电场作用下,正、负一级衍射产生了90°的相位差.在频率6.05 Hz、电压2.2 V的外场作用下,衍射级的极值振荡变化.     

偶氮苯聚合物全息光栅衍射效率和偏振特性研究

研究了不同偏振全息模式下基于交联偶氮苯聚合物薄膜的相位光栅的形成机理、衍射效率和偏振特性.利用琼斯矢量表征了正交线偏振(SP)和半行线偏振(SS)干涉模式下的偏振干涉场.基于SP和SS两种模式,制作了无明显表成起伏的纯折射率光栅和有表面起伏的浮雕光栅.偏光显微镜(POM)观测说明纯折射率光栅具有周期性的折射率分布;近场光学显微镜(SNOM)探测说明纯折射率光栅和浮雕光栅的表而起伏在5 nm以下和 85.23 nm.实验表明.当探测光为水平偏振时,纯折射率光栅的衍射效率达到22.2%.浮雕光栅只能达到1.65%.纯折射率光栅的0、 1级衍射光分别为水平和竖直线偏振光,偏振度达到0.9969和0.9963;浮雕光栅的0, 1级衍射光均为水平线偏振光.     

Femtosecond pulse laser-induced self-organized nanostructures on the surface of ZnO crystal

This paper reports self-organized nanostructures observed on the surface of ZnO crystal after irradiation by a focused beam of a femtosecond Ti：sapphire laser with a repetition rate of 250kHz. For a linearly polarized femtosecond laser, the periodic nanograting structure on the ablation crater surface was promoted. The period of self-organization structures is about 180 nm. The grating orientation is adjusted by the laser polarization direction. A long range Bragglike grating is formed by moving the sample at a speed of 10μm/s. For a circularly polarized laser beam, uniform spherical nanoparticles were formed as a result of Coulomb explosion during the interaction of near-infrared laser with ZnO crystal.