激光二极管泵浦Nd:NaY(WO4)2晶体的激光性能

采用提拉法生长了平均尺寸为φ25 mm×50 mm的掺钕钨酸钇钠[Nd:NaY(WO4)2,Nd:NYW]晶体,测量了Nd:NYW晶体的吸收光谱.吸收光谱表明:该晶体在804,752 nm和586 nm附近的吸收峰较强、较宽,有利于用激光二极管(laser diode,LD)泵浦.分别以氙灯和LD作为泵浦源,对晶体的激光特性进行了研究.结果表明:当氙灯输入能量为11.7 J时,该激光棒获得了36.9 mJ的输出;当LD泵浦功率为1 w时,输出273 mW的1.06 μm波长激光,激光阈值为160 mW,光-光转换效率为27.3%,斜效率为34%.     

Holographic polymer dispersed liquid crystal reflection gratings formed by visible light initiated thiol-ene photopolymerization

Visible laser light of wavelengths 442, 488 and 532 nm was used to write holographic reflection gratings with notches into the infra-red spectral region in polymer dispersed liquid crystals (H-PDLC). The gratings were formed by the spatially periodic phase separation of nematic liquid crystal during thiol-ene photopolymerization. An organic titanocene based complex, Rhodamine 6G (Rh6G) and Pyrromethene 597 (Pym) dyes in combination with benzoyl peroxide, were used as initiators. Using visible laser light and a single prism, electrically switchable reflection gratings with notches in the blue, green, yellow, red, and near infra-red (NIR) spectral regions were fabricated using angle tuning. The photophysics of the initiator dyes was studied by absorption and fluorescence spectroscopy. Morphology studies by TEM showed a periodic distribution of discrete nematic droplets of nearly spherical shape. The development of visible photoinitiator systems broadens the range of fabrication wavelengths of dynamic reflection filters using inexpensive visible lasers.     

分子结构对乙基氰乙基纤维素/丙烯酸胆甾型液晶性的影响

乙基氰乙基纤维素 [(E -CE)C]可溶于丙烯酸 (AA)形成胆甾型溶致液晶体系 ,液晶溶液的临界浓度 (C1 )与 (E -CE)C的分子量 (MW)有关 ,MW 越高 ,C1 越低。同时 ,C1 还受 (E -CE)C氰乙基取代度 (DS)的影响 ,DS越大 ,C1 越低。相同DS时 ,胆甾相的螺距 (P)随MW 增加而增大 ;在MW 相近时 ,随DS的升高 ,P先减小 ,后增大 ,在DS 0 .2～ 0 .3之间有最小值。P随 (E -CE)C/AA溶液浓度的增大而降低。 (E -CE)C/AA溶液中的AA快速聚合后 ,体系胆甾相选择性反射光的能力和选择性均有所降低 ,并且最大选择性反射光的波长 (λmax)蓝移。聚合前溶液的浓度越小 ,聚合后λmax的变化值越小     

Extreme UV single crystal diamond photodetectors by chemical vapor deposition

The detection properties of a UV photodetector realized on a 150 μm thick CVD single crystal diamond film, grown at Roma “Tor Vergata” University on a low cost HPHT diamond substrate, are reported. The device was tested in the 210–2400 nm spectral range using pulsed laser irradiation and in the 20–250 nm range in continuous mode by both a deuterium lamp and a helium DC gas source irradiation.The detector shows more than five orders of magnitude of visible/UV rejection ratio, a very sharp signal drop of about 10⁴ being observed in correspondence of the diamond energy gap. In the extreme UV range, the He II 25.6 and 30.4 nm as well as the He I 58.4 nm emission lines are clearly detected. The diamond time response is demonstrated to be considerably lower than 5 ns and 0.2 s in pulse and continuous mode, respectively.The extremely good signal to noise ratio, stability and reproducibility of the device response obtained, indicate that no persistent photoconductivity nor undesirable pumping effects are present, which represented so far the main problems preventing the use of diamond based detectors for UV applications.     

Er^3＋掺杂量对Er^3＋：BaY2F8晶体上转换发光性能的影响

利用温度梯度法生长了Er3＋：BaY2F8晶体。测量了样品的吸收谱、上转换荧光发射谱和上转换发光强度与激光泵浦功率的对数关系;分析了Er3＋的上转换可见发光机制和Er3＋掺量对Er3＋：BaY2F8晶体发光强度的影响。结果表明,在相同泵浦功率的980 nm光激发下的Er3＋：BaY2F8晶体的上转换发射谱中,552 n...     

Interfacial conversion for poly(para‐phenylenevinylene) at a cholesteric liquid crystal surface

A new approach for the preparation of poly (para‐phenylenevinylene) (PPV) with an ordered structure is presented. PPV is synthesized by a precursor method using a cholesteric liquid crystal medium. The resultant PPV film is observed to have a cholesteric fingerprint‐like texture, indicating replication of the cholesteric liquid crystal medium. Infrared absorption measurements confirm that the PPV has essentially the same molecular structure as normal PPV film. Ultraviolet–visible absorption measurements show that the absorption band due to π?π* transition of the conjugated main‐chain is located at 402 nm. The conversion is found to occur at the interface with the cholesteric liquid crystal medium as a mechanical process. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Optical properties of (acetoxypropyl)cellulose mesophases: factors influencing the cholesteric pitch

(Acetoxypropyl)cellulose (APC) forms a thermotropic cholesteric liquid crystalline phase, and with dibutyl phthalate (DBP) it also forms a lyotropic cholesteric phase. The reflection bands for the mesophases occur in the visible region, at wavelengths which depend on concentration and temperatures. The pitch of the cholesteric helicoidal structure is derived from measurements of the mean refractive indices and of the reflection band wavelengths for mesophase samples containing from 0 to 30% diluent at temperatures from ambient to 170°C. The pitch of the thermotropic mesophase increases with increasing temperature and with decreasing molar mass. The pitch of the lyotropic mesophase increases with increasing temperature and diluent content. Pitch values approach infinity at temperatures close to the clearing temperature of the mesophase, and no reversal in the sense of the pitch with temperature or diluent content was detected. The experimentally observed changes in pitch with composition and temperature are in reasonable agreement with the predictions of a recent theory for cholesteric mesophases composed of helical rod-like species. The average distance between chains in the mesophase is estimated from X-ray diffraction measurements, and hence the average angle of twist between neighbouring APC molecules may be found. The angle decreased from 2.2° for pure APC to 0.9° for a volume fraction of 0.73 APC in DBP.     

光聚合对乙基氰乙基纤维素/丙烯酸胆甾相结构的影响

乙基氰乙基纤维素 [(E -CE)C]能够很好地溶解在丙烯酸 (AA)中并形成胆甾型液晶溶液。在 (E -CE)C/AA溶液中加入适当的光引发剂 ,采用紫外光引发聚合抑制聚合过程中相分离的发生 ,可以制得保持胆甾相结构的复合物。聚合反应会对胆甾相的结构产生一定的影响。文章研究了溶液浓度、聚合温度和 (E -CE)C分子量等因素对聚合以后胆甾相选择反射光波长及结构的影响。     

界面致稳型柔性胆甾相液晶显示器件的制备与性能

以镀有氧化锡铟的聚对苯二甲酸乙二酯（ITO-PET）薄膜为柔性电极,对电极导电表面进行处理,以间隔子控制器件厚度,采用灌注法制备了界面致稳型可挠性双稳态胆甾相液晶显示器件。利用接触角测试仪、显微镜和紫外-可见分光光度等分析胆甾相液晶显示器件的微观结构及光电特性。结果表明,表面处理涂层可与液晶分子作用并产生界面致稳效应;采用大小为6 μm的间隔子,当改性层溶液中间隔子质量分数为3%时,制备了有效面积为10×15 cm²的柔性双稳态液晶显示器件,测得饱和驱动电压为40 V,在550 nm的透过率为90%;最后通过刻蚀技术处理器件电极制备了字符显示器件。     

丙烯酸铅对乙基纤维素胆甾型液晶的影响

本文利用紫外-可见分光光度计研究了丙烯酸铅(PbA)对乙基纤维素(EC)/丙烯酸(AA)胆甾型液晶的影响,研究发现:在EC/AA胆甾型液晶中,其最大选择性反射波长和螺距随溶液浓度增加而降低;PbA的加入也使反射波长和螺距降低,并且溶液中PbA浓度越大,反射波长和螺距降低越多,这可能是由于Pb2+与纤维素分子的络合作用,拉近了相邻EC分子链距离,导致胆甾型液晶溶液中的层间分子距离减小所致。利用这种性质调节纤维素胆甾型液晶的光学反射性能。     

Thermotropic cellulose derivatives with flexible substituents

Summary Three kinds of fully substituted cellulose derivatives, cellulose triheptanoate (CTH), tri-O-heptyl cellulose (THC) and tri-O-2-butoxyethyl cellulose (TBC) were prepared. These polymers, having a side chain of similar length but of differing chemical structure, were found to show significantly different therraotropic behavior: TBC forms a cholesteric liquid crystalline phase at temperatures between 50 °C and 165 °C, the cholesteric pitch covering the region of visible lights. THC also displays a cholestric structure at temperatures between 80 °C and 130 °C with pitches on the order of several micrometers. CTH appears to show liquid crystallinity only in a very limited temperature range around 100 °C.     

Chiral polymer networks with a broad reflection band achieved with varying temperature

In this study, we demonstrate a novel method for fabricating a single layer cholesteric liquid crystal (CLC) film with non-uniform pitch distribution by utilizing the chiral polynetworks. A chiral monomer, the helical twist power (HTP) of which decreases with the increase of temperature, was synthesized. The temperature dependence of the pitch of the mixtures before and after polymerization was investigated. To broaden the reflection band, the experimental processes of thermally induced pitch variation were presented. The results suggest that the chirality of the polymer network plays an integral role in the reflection spectra. Additionally, scanning electron microscopy (SEM) was used to examine the role of the polymer network induced by polymerization temperature. These wide-band reflection optical properties make the CLC gels interesting for their potential applications in many fields, such as colour filters, brightness enhancement films, smart switchable reflective windows etc.     

Mechano-Optical Response of Novel Polymer Composites Based on Elastic Polyurethane Matrix Filled with Low-Molar-Mass Cholesteric Droplets

Among the various types of liquid crystalline (LC) structures, the cholesteric mesophase attracts special attention in view of helical supramolecular structure determining their specific optical properties. The helix pitch of cholesteric liquid crystals is very sensitive to mechanical stress, resulting in change of their optical properties and position of selective light reflection peak. Here, films of polymer-dispersed cholesteric liquid crystals based on polyurethane elastomer are prepared and studied. Liquid crystal mixtures based on chiral cholesterol derivatives are uniformly dispersed in the form of micrometer-sized droplets in an elastomeric matrix. A significant mechano-optical response of the composites under the action of deformation is demonstrated, and the change in the position of the selective light reflection band during mechanical stress is studied in detail. A shift in the selective light reflection peak by 100 nm or more is found. This process is accompanied by color transformations of brightly colored films. Such materials can be promising for creating stress and deformation sensors, ripple and bending sensors, and other technical devices.     

Synthesis and characterization of novel monomers and polymers containing chiral (−)-menthyl groups

To investigate the steric effects of chiral menthyl groups on the induction of cholesteric liquid crystals and the sensitivity of the photoisomerizable azobenzene derivatives, a series of chiral monomers and a photoisomerizable chiral azobenzene derivative with various spacers end-capped with (−)-menthyl group were synthesized. The structures of the novel chiral compounds synthesized in this investigation were identified using ¹³C NMR, FTIR, and elemental analysis. The phase transition temperatures of the chiral compounds were investigated using X-ray diffraction, differential scanning calorimetry, and polarizing optical microscopy. The thermogravimetric characteristics, the glass-transition temperatures (T_g) and the weight-average molecular weights (M_w) of the homopolymers were also evaluated. Polymers containing chiral menthyl groups with a biphenyl segment were found to reveal high thermal resistance. However, the existence of the steric hindered menthyl group disturbed the arrangement of chiral monomers leading to the disappearance of liquid crystal phases. The specific optical rotation of the synthesized monomers and polymers were also evaluated. The effect of the synthesized chiral compounds, monomers and photoisomerizable azobenzene derivative on the induction of the cholesteric liquid crystal films was investigated. The morphological network structure of the polymer matrix inside a liquid crystal cell was studied using a scanning electron microscope (SEM). The phototuning ability of the AzoM on the cholesteric liquid crystals was also established.     

Nd:GGG晶体生长、光谱性能及激光性能

采用提拉法生长掺钕钆镓石榴石(neodymium-doped gadolinium gallium garnet, Nd: GGG)激光晶体,选择最佳工艺参数:提拉速率为2～4mm/h:转速为20～40r/min;冷却速率为20℃/h.测试了晶体的吸收和荧光光谱,结果表明:主吸收峰位于808nm,主发射峰位于9430cm-1,对应于Nd3 的4F3/1-4I11/2跃迁.对晶体样品进行了激光性能测试.结果表明:当泵浦功率为900mW时,对泵浦光的吸收效率为85%,激光输出波长约为1μm,激光输出功率为305mW,激光阈值功率为380mW,光-光转换效率达57.8%,斜效率达57.6%.     

Structural chirality of cholesteric liquid crystal produces atropisomerism: Chiroptical polyisocyanides from achiral monomer in cholesteric liquid crystal matrix

1-Naphthyl isocyanide was polymerized with Ni(II) catalyst in a cholesteric matrix at the liquid crystal (LC) temperature range. The resultant polymers showed optical activity. In this reaction, the structural chirality of cholesteric LC effectively functions to impart one-handed helicity on the corresponding polymers as an optically active atropisomer.     

高效酰化催化法合成新型胆甾醇酯液晶

采用高效酰化催化法和酰氯化法分别合成了燕尾型胆甾醇酯和对称脂肪族二羧酸胆甾醇酯两类衍生物样品。采用元素分析、红外光谱、DSC、热台偏光显微镜对样品的化学结构和液晶性能进行了表征。3个对称脂肪族二羧酸胆甾醇酯样品均为胆甾型液晶,而燕尾型的支链结构对胆甾醇酯液晶的形成具有不利的影响。两种合成胆甾醇酯的酰化方法经过对比表明,高效酰化法具有反应条件温和、副反应小、不破坏反应物、产物易分离、产率高等优点,可作为胆甾醇酯液晶合成的首选方法。     

Nanoconfinement-Induced Structures in Chiral Liquid Crystals

We employ Monte Carlo simulations in a specialized isothermal-isobaric and in the grand canonical ensemble to study structure formation in chiral liquid crystals as a function of molecular chirality. Our model potential consists of a simple Lennard-Jones potential, where the attractive contribution has been modified to represent the orientation dependence of the interaction between a pair of chiral liquid-crystal molecules. The liquid crystal is confined between a pair of planar and atomically smooth substrates onto which molecules are anchored in a hybrid fashion. Hybrid anchoring allows for the formation of helical structures in the direction perpendicular to the substrate plane without exposing the helix to spurious strains. At low chirality, we observe a cholesteric phase, which is transformed into a blue phase at higher chirality. More specifically, by studying the unit cell and the spatial arrangement of disclination lines, this blue phase can be established as blue phase II. If the distance between the confining substrates and molecular chirality are chosen properly, we see a third structure, which may be thought of as a hybrid, exhibiting mixed features of a cholesteric and a blue phase.     

Frequency-converted dilute nitride laser diodes for mobile display applications

We demonstrate a 1240-nm GaInNAs multi-quantum well laser diode with an integrated saturable electro-absorber whose wavelength is converted to 620 nm. For conversion, we used a MgO:LN nonlinear waveguide crystal with an integrated Bragg grating in direct coupling configuration. Broadened visible spectral width and reduced speckle as well as a high extinction ratio between the below and above threshold powers were observed in passively triggered pulsed operation with smooth direct current modulation characteristics. The demonstration opens a new avenue for developing 620-nm semiconductor lasers required for emerging projection applications.     

掺钕钨酸钆钠晶体生长、结构及光谱特性

采用提拉法生长了尺寸为φ(30～35)mm×80mm的掺钕钨酸钆钠[Nd:NaGd(WO4)2,Nd:NGW]晶体。生长Nd:NGW晶体的最佳工艺参数为:晶体的提拉速率为1～2mm/h,晶体转速为15～18r/min,冷却速率为10℃/h,液面上轴向温度梯度为0.7～1℃/mm。通过热重-差热分析(thermogravimetry-differential thermal analysis,TG-DTA),X射线衍射(X-ray diffraction,XRD)对晶体进行表征。测试了晶体的红外及Raman光谱,分析了晶体的振动模式,并将晶体振动光谱进行归属。由TG-DTA曲线得到晶体熔点为1251.7℃。XRD分析表明:晶体属于四方晶系、白钨矿结构、I41/a空间群,晶胞参数a=0.53213nm,c=1.13070nm。吸收光谱表明:Nd:NGW晶体在805nm附近有较强、较宽的吸收峰,吸收截面积为3.581×10-20cm2,适合于激光二极管泵浦。