光束轨道角动量谱的测量技术研究进展（特邀）

自Allen等证明具有螺旋相位波面的激光束携带有轨道角动量以来,对光束轨道角动量调控技术的研究取得了跨越式的发展,获得了包括相位涡旋光束、矢量涡旋光束、激光束阵列等多种新型结构光场,在超大容量光通信、遥感探测、激光加工、高分辨率成像等领域展现出广阔的应用前景。准确测量光束的轨道角动量是其应用的重要基础,早期人们更多地关注对待测光束所包含的轨道角动量成分分布的测量,后来逐步拓展至对各个轨道角动量成分的强度比重即轨道角动量谱的测量。文中系统地回顾并总结近年来光束轨道角动量谱测量技术的发展,主要介绍了包括基于衍射、模式分束等方法的新型光束轨道角动量谱测量技术。     

矢量涡旋光束经光阑-透镜系统的OAM与偏振特性

与标量涡旋光束不同,矢量涡旋光束同时具有各项异性的空间偏振分布和螺旋相位分布,并携带与相位分布有关的轨道角动量(Orbital angular momentum,OAM)。根据柯林斯衍射积分理论,得到了傍轴近似条件下矢量涡旋光束的OAM密度,实验采集了矢量涡旋光束通过光阑-透镜系统后的光场,详细讨论了光阑截断参数以及光阑-透镜间距等参数对矢量涡旋光场及其OAM密度的影响。结果表明:相比标量涡旋光束,矢量涡旋光束OAM通过光阑系统后随传输距离的衰减更慢,受光阑截断参数影响更小。矢量涡旋光束偏振态不受光阑-透镜系统影响,截断参数大于4.2时,轨道角动量密度和光强不受截断参数影响。在透镜焦点位置处,OAM密度达到最大值。研究结果为矢量涡旋光束OAM特性的应用提供理论依据。     

斜程湍流大气中矢量涡旋光束的OAM特性研究

基于螺旋谱理论推导出矢量涡旋光束在斜程大气湍流传输中的轨道角动量(orbital angularmomentum,OAM)谱,讨论了修正Kolmogorov大气湍流对不同阶矢量涡旋光束光场分布及光束OAM谱的影响。利用旋转毛玻璃作为随机相位屏,实验采集了矢量涡旋光束通过旋转毛玻璃后的光强图像与归一化强度。结果表明:经大气湍流斜程传输后,矢量涡旋光束的光强分布产生畸变且OAM谱发生弥散,OAM谱弥散程度随传输距离的增大而增大。传输距离不变时,天顶角越大,主OAM模相对功率越小,湍流内尺度减小与折射率结构常数增大均会导致主OAM模相对功率减小。同一传输路径下,随着湍流强度的增大,矢量涡旋光束中心相位奇点的强度逐渐增大,并且拓扑荷不变时,偏振阶数高的矢量涡旋光束受到的湍流影响较小。     

涡旋光场相干调控及拓扑荷测量北大核心CSCD

涡旋光场是一类具有螺旋型波前的特殊结构光场,因其携带相位奇点、轨道角动量以及拥有中央暗核结构等物理特性,被广泛应用于光学微操纵、大容量光通信、超分辨成像等领域。通过对涡旋光场传统的物理维度(振幅、偏振、频率)进行调控,可以得到模式更加丰富、应用领域更广泛的新型涡旋光场。此外,涡旋光场还有一个非常重要的调控维度,即相干性。近年来,研究人员通过对涡旋光场的相干性进行调控得到了一类新型涡旋光场,即部分相干涡旋光场。相比于完全相干涡旋光场,部分相干涡旋光场在某些领域更具优势,如具有较高的抗湍流大气干扰性、更丰富的光束整形、更高的自修复能力和更强的微粒捕获能力等。本综述介绍了近年来整数阶和分数阶涡旋光场相干性调控方面的研究进展,重点对部分相干涡旋光场的理论模型、产生方法、传输特性、拓扑荷测量、应用等方面进行了详细阐述。     

太赫兹特殊光束的研究与应用

太赫兹特殊光束具有独特的光场分布和衍射特性,在粒子操控、光学成像和光通信等众多领域都有着明显的发展优势。综述了近年来本课题组和其他科研团队在太赫兹特殊光束方面展开的部分研究,包括太赫兹贝塞尔光束、涡旋光束、艾里光束、瓶子光束和径向偏振光束,系统地阐述了这些光束的光场分布特征、衍射特性及其潜在应用前景,旨在探讨太赫兹特殊光束对未来太赫兹技术的推动作用。     

复杂涡旋结构光场的产生方法

涡旋光是一种携带轨道角动量、相位面呈螺旋状分布的新型结构光场,在量子纠缠、量子通信、光学微操控等领域已经获得了广泛应用。随着研究的深入,具有比传统涡旋光更复杂的拓扑结构、相位奇点、轨道角动量和偏振奇点的结构光场的产生,吸引了众多研究人员的兴趣。从固体激光腔内直接激发产生空间结构光和腔外调控得到空间结构光出发,分别介绍了离轴泵浦加像散转换、调制元件调制波形、泵浦整形三种腔内方法,以及空间光调制器光场定制、模式叠加、超构表面微结构设计的三种腔外方法,并分析比较了几种方法的优缺点,展望了未来空间涡旋结构光场的发展趋势。     

涡旋光束和光学涡旋的研究

近年来,涡旋光束由于在囚禁和操控原子及其他微粒中的应用而引起了不少关注和研究.涡旋光束在光束的传播方向上有一个位相项e(ilθ),而且它拥有一个光束轨道角动量,如何从一个高斯基模变换到涡旋光束,已经提出了许多方法,比如在腔内放螺旋位相片直接产生,用计算机得到的位相片产生,用柱面镜或楔形镜产生光学涡旋.此外,在光纤中涡旋也能产生,比如可以使用光子晶体光纤作为一个非线性的两维光子晶体来产生光涡旋孤子.而在螺旋光纤中,纯的光学涡旋或者光学涡旋和TE及TM模式一起以高阶模形式出现,纯的模式在螺旋光纤传导中,会加上一个和内禀角动量以及螺旋立体角成比例的拓扑位相,而且HE和EH模式的工和y分量的分布模式在传输时也有旋转.1997年,E Abramochkin用一个图像旋转腔来产生一个涡旋激光束,在钝角三角形腔中用一个Dove棱镜来旋转光束,随着不同的棱镜旋转角度,可以得到不同的螺旋类型的激光光束.2003,Arlee V Smith在一个图像旋转腔的纳秒级光学参量振荡器里获得了涡旋光束,它的种子光和腔轴失调,在腔中形成了4个有着固定位相差的稳定模式,从而输出耦合成为一个涡旋光束.文中介绍用一种失调的多模光纤来做一个旋转腔,而把高斯基模光转换为一个涡旋光束的方法.在实验中,光源采用He-Ne的基模高斯光束,当激光束倾斜入射在光纤耦合器时,出射光场的模式会随输入位置和输入角而改变,在实验中用的是多模光纤,分别得到了顺时针和逆时针的涡旋光束, '还的到了环状空心光束.用光强分布仪记录了光束的分布,得到了不同尺寸光纤纤芯的光强分布图;测量了光束转换耦合效率,结果是涡旋光束的耦合输出效率达到了80%以上;空心光束的耦合输出效率达到了50%以上.另外,对于失调耦合器下多模光纤产生光学涡旋的理论分析和研究也将进一步研究.由于这种方法产生的光束稳定性很好,不仅可以产生实验所需要的光束,还可以把这种光束传输到所需要的地方,因此,应用前景很广,如操控原子、微粒和细胞等,也是一种特殊的光镊.     

高功率涡旋拉曼光纤激光器EI北大核心CSCD

近年来,携带轨道角动量的涡旋光束在激光加工、光学微粒操纵、超分辨成像、大容量光通信等领域应用广泛,目前已在稀土掺杂光纤激光器中得到了广泛研究。基于声致光纤光栅,实验搭建了全光纤结构拉曼光纤激光器,实现LP_(01)模与LP_(11)模的有效调控,并进一步通过偏振控制实现环形径向偏振光和拓扑荷数l=±1的涡旋光束输出,最高输出功率~70 W,中心波长为1134 nm。文中提出的激光器有利于拓宽涡旋光束输出波段,在多维光通信、光场和物质相互作用等领域具备较大研究价值和应用潜力。     

基于S波片和双延迟器的矢量光场偏振调控方法

基于S波片和双延迟器的偏振特性,提出了一种矢量光场的生成及偏振调控方法,该方法能够将标量高斯光束转换为偏振态分布可调谐的矢量光束。结合Stokes-Mueller矩阵算法,建立了矢量光场偏振调控的数学模型,并仿真计算出标量高斯光束经过S波片和双延迟器（包括双1/4波片和双1/2波片两种情况）后的偏振态空间分布。讨论了双延迟器相对旋转时光场偏振分布的演化规律及机理,实验结果与数值仿真结果相互吻合,表明该方法可以实现对标量相干光场的复杂偏振调控,验证了理论分析的正确性和该方法的可行性。     

完美涡旋光场的研究进展北大核心CSCD

完美涡旋光场具有亮环半径不随拓扑荷值改变而改变的优点,在微粒操纵及量子通信等领域具有重要的应用价值,已成为近年来光场调控领域的研究热点。介绍了产生完美涡旋光场的三种典型方法,综述了完美涡旋光场的调控技术及表征方法,并对其应用进行了总结。     

Development of the readout system for waveguide multilayer optical card

Waveguide multilayer optical card （WMOC） is a novel storage device of three-dimensional optical information. An advanced readout system fitting for the WMOC is introduced in this paper. The hardware mainly consists of the light source for reading, WMOC, motorized stages addressing unit, microscope imaging unit, CCD detecting unit and PC controlling ＆ processing unit. The movement of the precision motorized stage is controlled by the computer through Visual Basic （VB） language in software. A control panel is also designed to get the layer address and the page address through which the position of the motorized stages can be changed. The WMOC readout system is easy to manage and the readout result is directly displayed on computer monitor.     

Nanoimprint Lithography -A Next Generation High Volume Lithography Technique

IntroductionNanoimprint Lithography is a well-acknowl-edged low cost, high resolution, large area pattern-ing process. It includes the most promising methods,high-pressure hot embossing lithography (HEL) [2],UV-cured imprinting (UV-NIL) [3] and micro contactprinting (m-CP, MCP) [4]. Curing of the imprintedstructures is either done by subsequent UV-lightexposure in the case of UV-NIL or by cooling downbelow the glass transition temperature of the ther-moplastic material in case of HEL…     

Collinear phase-matching study of terahertz-wave generation via difference frequency mixed in GaAs and Inp

The collinearly phase-matching condition of terahertz-wave generation via difference frequency mixed in GaAs and InP is theoretically studied. In collinear phase-matching, the optimum phase-matching wave hands of these two crystals are calculated. The optimum phase-matching wave bands in GaAs and lnP are 0.95-1.38μm and 0.7-0.96μm respectively. The influence of the wavelength choice of the pump wave on the coherent length in THz-wave tuning is also discussed. The influence of the temperature alteration on the phase-matching and the temperature tuning properties in GaAs crystal are calculated and analyzed. It can serve for the following experiments as a theoretical evidence and a reference as well.     

Effects of the alloy compositions on the phonon-polaritons in ternary mixed crystals

Composition dependence of bulk and surface phonon-polaritons in ternary mixed crystals are studied in the framework of the modified random-element-isodisplacement model and the Bom-Huang approximation. The numerical results for Several Ⅱ - Ⅵ and Ⅲ- Ⅴ compound systems are performed, and the polariton frequencies as functions of the compositions for ternary mixed crystals AlxGa1-xAs, GaPxAS1-x, ZnSxSe1-x, GaAsxSb1-x, GaxIn1-xP, and ZnxCd1-xS as examples are given and discussed. The results show that the dependence of the energies of two branches of bulk phonon-polaritons which have phonon-like characteristics, and surface phonon-polaritons on the compositions of ternary mixed crystals are nonlinear and different from those of the corresponding binary systems.     

Influence of PVK ： NPB hole transporting layer on the characteristics of organic light-emitting devices

A doping system consisting of NPB and PVK is employed as a composite hole transporting layer （CHTL）. By adjusting the component ratio of the doping system, a series of devices with different concentration proportion of PVK ： NPB are constracted. The result shows that doping concentration of NPB enhances the competence of hole transporting ability, and modifies the recombination region of charge as well as affects the surface morphology of doped film. Optimum device with a maximum brightness of 7852 cd/m^2 and a power efficiency of 1.75 lm/W has been obtained by choosing a concentration proportion of PVK ： NPB at 1：3.     

New insert layer structure OLEDs

An insert layer structure organic electroluminescent device（OLED） based on a new luminescent material （Zn（salen）） is fabricated. The configuration of the device is ITO/CuPc/NPD/Zn（salen）/Liq/LiF/A1/CuPc/NPD/Zn（salen）/Liq/LiF/A1. Effective insert electrode layers comprising LiF（1nm）/Al（5 nm） are used as a single semitransparent mirror, and bilayer cathode LiF（1 nm）/A1（100 nm） is used as a reflecting mirror. The two mirrors form a Fabry-Perot microcavity and two emissive units. The maximum brightness and luminous efficiency reach 674 cd/m^2 and 2.652 cd/A, respectively, which are 2.1 and 3.7 times higher than the conventional device, respectively. The superior brightness and luminous efficiency over conventional single-unit devices are attributed to microcavity effect.     

Analysis of error performance on Turbo coded FDPIM

Due to variable symbol length of digital pulse interval modulation（DPIM）, it is difficult to analyze the error performances of Turbo coded DPIM. To solve this problem, a fixed-length digital pulse interval modulation（FDPIM） method is provided. The FDPIM modulation structure is introduced. The packet error rates of uncoded FDPIM are analyzed and compared with that of DPIM. Bit error rates of Turbo coded FDPIM are simulated based on three kinds of analytical models under weak turbulence channel. The results show that packet error rate of uncoded FDPIM is inferior to that of uncoded DPIM. However, FDPIM is easy to be implemented and easy to be combined, with Turbo code for soft-decision because of its fixed length. Besides, the introduction of Turbo code in this modulation can decrease the average power about 10 dBm, which means that it can improve the error performance of the system effectively.     

A method for calculating the center position of collimated light

It is a key problem to accurately calculate beam spots＇ center of measuring the warp by using a collimated laser. A new method, named double geometrical center method （DGCM）, is put forward for the first time. In this method, a plane wave perpendicularly irradiates an aperture stop, and a charge couple device （CCD） is employed to receive the diffraction-beam spots, then the geometrical centers of the fast and the second diffraction-beam spots are calculated respectively, and their mean value is regarded as the center of datum beam. In face of such adverse instances as laser intension distributing defectively, part of the image being saturated, this method can still work well. What＇s more, this method can detect whether an unacceptable error exits in the courses of image receiving, processing and calculating. The experimental results indicate the precision of this method is high.     

Preparation and Characterization of TaN ALD Precursors

High purity organic-tantalum precursors for thin film ALD TaN were synthesized and characterized.Vapor pressure and thermal stability of these precursors were studied.From the vapor pressure analysis,it was found that TBTEMT has a higher vapor pressure than any other published liquid TaN precursor,including TBTDET,TAITMATA,and IPTDET.Thermal stability of the alkyl groups on the precursors was investigated using a 1H NMR technique.The results indicated that the tertbutylimino group is the most stable group on TBTDET and TBTEMT as compared to the dialkylamido groups.Thermal stability of TaN precursors decreased in the following order:TBTDET ＞ PDMAT ＞ TBTEMT.In conclusion,precursor vapor pressure and thermal stability were tuned by making slight variations in the ligand sphere around the metal center.     

Curved crystal spectrometer for the measurement of X-ray lines from laser-produced plasmas

In order to diagnose the laser-produced plasmas, a focusing curved crystal spectrometer has been developed for measuring the X-ray lines radiated from a laser-produced plasmas. The design is based on the fact that the ray emitted from a source located at one focus of an ellipse will converge on the other focus by the reflection of the elliptical surface. The focal length and the eccentricity of the ellipse are 1350 mm and 0.9586, respectively. The spectrometer can be used to measure the X- ray lines in the wavelength range of 0.2-0.37 nm, and a LiF crystal （200） （2d = 0.4027 nm） is used as dispersive element covering Bragg angle from 30° to 67.5°. The spectrometer was tested on Shengnang- Ⅱ which can deliver laser energy of 60-80 J/pulse and the laser wavelength is 0.35 μm. Photographs of spectra including the 1 s2p ^1P1-1s^2 ^1S0 resonance line（w）, the 1s2p ^3P2-1s^2 1S0 magnetic quadrupole line（x）, the 1s2p ^3P1-1 s^2 ^1S0 intercombination lines（y）, the 1 s2p ^3S~1-1 s^2 ^1S0 forbidden line（z） in helium-like Ti Ⅹ Ⅺ and the 1 s2s2p ^2P3/2-1 s622s ^2S1/2 line（q） in lithium-like Ti Ⅹ Ⅹhave been recorded with a X-ray CCD camera. The experimental result shows that the wavelength resolution（λ/△ 2） is above 1000 and the elliptical crystal spectrometer is suitable for X-ray spectroscopy.