Random lasing in dye-doped polymer dispersed liquid crystal film

A dye-doped polymer-dispersed liquid crystal film was designed and fabricated,and random lasing action was studied.A mixture of laser dye,nematic liquid crystal,chiral dopant,and PVA was used to prepare the dye-doped polymer-dispersed liquid crystal film by means of microcapsules.Scanning electron microscopy analysis showed that most liquid crystal droplets in the polymer matrix ranged from 30 μm to 40 μm,the size of the liquid crystal droplets was small.Under frequency doubled 532 nm Nd:YAG laser-pumped optical excitation,a plurality of discrete and sharp random laser radiation peaks could be measured in the range of 575–590 nm.The line-width of the lasing peak was 0.2 nm and the threshold of the random lasing was 9 m J.Under heating,the emission peaks of random lasing disappeared.By detecting the emission light spot energy distribution,the mechanism of radiation was found to be random lasing.The random lasing radiation mechanism was then analyzed and discussed.Experimental results indicated that the size of the liquid crystal droplets is the decisive factor that influences the lasing mechanism.The surface anchor role can be ignored when the size of the liquid crystal droplets in the polymer matrix is small,which is beneficial to form multiple scattering.The transmission path of photons is similar to that in a ring cavity,providing feedback to obtain random lasing output.     

基于全息液晶/聚合物光栅的分布反馈式激光器的波长调谐特性研究

首先制备了不同周期的染料掺杂全息液晶/聚合物光栅并进行激光抽运实验, 得到了激光器的调谐曲线,确定了激光器在574 nm到685 nm的谱带里均可以实现激光输出, 即激光器具有110 nm左右的可调谐范围. 之后, 通过温控仪控制样品的温度, 对周期为610 nm的染料掺杂全息液晶/聚合物光栅进行激光抽运, 探测不同温度下的输出激光光谱, 观察到随着温度由20℃升高到65℃, 激光器的中心波长由627.9 nm减小到623 nm, 产生了4.9 nm的波长蓝移.     

基于透射式液晶/聚合物光栅的分布反馈式激光器的研究

本文研究了染料掺杂透射式液晶/聚合物光栅的制备以及基于透射式液晶/聚合物光栅的分布反馈式激光器的激光特性.实验选取DCM作为激光染料,制备了周期为586 nm的掺杂DCM的透射式液晶/聚合物光栅;使用532 nm输出的Nd ∶YAG倍频脉冲激光器作为抽运光源对染料掺杂液晶/聚合物光栅进行侧面抽运,得到了中心波长为603 nm的窄线宽、低阈值激光输出.激光线宽为1.4 nm、阈值能量约为17.3 μJ,与之前国外的报道相比,阈值能量有了很大幅度的降低. 关键词： 液晶/聚合物光栅 分布反馈式激光器 阈值 线宽     

纳米ZnO粉末中随机激光现象

实验采用三倍频Nd:YAG(波长355 nm，脉宽8ns，频率30Hz) 脉冲激光器作为抽运光源，在ZnO纳米粉末(直径～100 nm)中发现了类似激光现象.并用环形腔理论模拟了ZnO的颗粒密度对平均自由程的影响，从理论上证明在纳秒级激光器的抽运下，ZnO纳米粉末也可以发射激光. 关键词： 类似激光 ZnO纳米粉末 平均自由程     

Bichromatic coherent random lasing from dye-doped polymer stabilized blue phase liquid crystals controlled by pump light polarization

In this paper, we investigate the bichromatic coherent random lasing actions from the dye-doped polymer stabilized blue phase liquid crystals. Two groups of lasing peaks, of which the full widith at half maximum is about 0.3 nm, are clearly observed. The shorter- and longer-wavelength modes are associated with the excitation of the single laser dye (DCM) monomers and dimers respectively. The experimental results show that the competition between the two groups of the lasing peaks can be controlled by varying the polarization of the pump light. When the polarization of the pump light is rotated from 0° to 90°, the intensity of the shorter-wavelength lasing peak group reduces while the intensity of the longer-wavelength lasing peak group increases. In addition, a red shift of the longer-wavelength modes is also observed and the physical mechanisms behind the red-shift phenomenon are discussed.     

Random lasing from dye-doped chiral nematic liquid crystals in oriented and non-oriented cells

These are the experimental results describing random lasing in dye-doped chiral nematic liquid crystals. A novel random lasing emission is studied in this article based on the helical domains of dye-doped chiral nematic liquid crystals in oriented and non-oriented cells. Under frequency doubled 532 nm Nd:YAG (yttrium aluminum garnet) laser-pumped optical excitation, we carefully observed and analysed random lasing from dye-doped chiral nematic liquid crystals with wavelength ranges from 600 nm to 620 nm. In addition, the line-width of multi-mode peaks is less than 0.2 nm. The difference between the two random lasing behaviours in the oriented and non-oriented cells arises from the fact that random lasing appearing in the oriented cell results from stronger multiple scattering of light generated by the spiral domains of the liquid crystal molecules. Furthermore, chiral nematic liquid crystal micro-domains with different orientations can induce variation of the diffusion constant, thereby resulting in a decrease or increase in the lasing intensity of the random lasers, and an increase or decrease in their energy thresholds. In addition, a detailed comparison of the two experimental results is also presented in the article, showing the dependence of the lasing threshold and the number of lasing modes on the transport mean free path, the excited area, and the sample size. This process allows us to obtain a random laser by changing the structure of the sample, realising tunable random lasers at low cost.     

向列相液晶染料可调谐激光器的研究

对向列相液晶染料的可调谐激光器进行了光学特性研究. 以650 nm为中 心波长设计了SiO₂和TiO₂多层膜的一维光子晶体, 以激光染料与向列相液晶的混合物作为增益介质层, 制备了波长可调谐激光器.用Nd: YAG倍频脉冲激光器输出的532 nm激光抽运所制备的激光器样品得出如下光学特性: 激光发射波长随温度调谐范围为605.5---639.8 nm, 达到34.3 nm, 随电压调谐范围为634.5---619.5 nm, 达到15 nm. 发射激光每脉冲的阈值能量为12.3 μJ, 激光线宽小于1 nm.     

Flexible electrically pumped random lasing from ZnO nanowires based on metal–insulator–semiconductor structure

Flexible electrically pumped random laser(RL) based on ZnO nanowires is demonstrated for the first time to our knowledge. The ZnO nanowires each with a length of 5 μm and an average diameter of 180 nm are synthesized on flexible substrate(ITO/PET) by a simple hydrothermal method. No obvious visible defect-related-emission band is observed in the photoluminescence(PL) spectrum, indicating that the ZnO nanowires grown on the flexible ITO/PET substrate have few defects. In order to achieve electrically pumped random lasing with a lower threshold, the metal–insulator–semiconductor(MIS) structure of Au/SiO_2/ZnO on ITO/PET substrate is fabricated by low temperature process. With sufficient forward bias, the as-fabricated flexible device exhibits random lasing, and a low threshold current of ～ 11.5 m A and high luminous intensity are obtained from the ZnO-based random laser. It is believed that this work offers a case study for developing the flexible electrically pumped random lasing from ZnO nanowires.     

胆甾相液晶激光器的调谐特性研究

基于胆甾相液晶螺距及折射率随温度与电场变化的特性, 研究了温度与电场对液晶染料可调谐激光器发射特性的影响. 首先探讨了手性剂浓度、温度与液晶螺距的关系, 制作了液晶染料可调谐激光器, 在温度23–35℃变化时, 其发射波长调谐范围为618.90–594.76 nm, 达到24.14 nm, 在电压0–9 V 变化时, 其发射波长调谐范围为617.40–608.11 nm, 共9.29 nm. 关键词： 可调谐激光器 胆甾相液晶 激光染料     

脉冲激光沉积法制备的ZnO薄膜的低阈值电抽运紫外随机激射

分别采用直流反应溅射法和脉冲激光沉积法在硅衬底上沉积ZnO薄膜, 用X射线衍射、扫描电镜、光致发光谱等手段对两种方法沉积的ZnO薄膜的结晶状态、 表面形貌和光致发光等进行了表征. 进一步对比研究了以上述两种方法制备的ZnO薄膜作为发光层的金属-绝缘体-半导体结构器件的电抽运紫外随机激射. 结果表明, 与以溅射法制备的ZnO薄膜作为发光层的器件相比, 以脉冲激光沉积法制备的ZnO薄膜为发光层的器件具有更低的紫外光随机激射阈值电流和更高的输出光功率. 这是由于脉冲激光沉积法制备的ZnO薄膜中的缺陷更少, 从而显著地减少了紫外光在光散射过程中的光损耗. 关键词： 随机激射 ZnO薄膜 脉冲激光沉积 溅射     

基于全息聚合物分散液晶的有机二维光子晶体激光器的研究

采用一次性曝光光路制备了基于全息聚合物分散液晶的亚微米周期染料掺杂二维光子晶体,晶格常数为582nm.选取4-(二氰亚甲基)-2-甲基-6-(4-二甲氨基苯乙烯基)-4H-吡喃作为激光染料,利用输出波长为532nm的Nd:YAG倍频脉冲激光器作为抽运光源对染料掺杂二维光子晶体进行抽运,得到了中心波长为627.4nm的窄线宽、低阈值的输出激光,激光线宽为0.4nm,阈值能量约为22.7 μJ.与目前国外的报道相比,激光线宽和阈值能量都有了很大幅度的降低.将其与基于一维光栅的分布反馈式激光器相比,线宽从1.4 关键词： 全息聚合物分散液晶 二维光子晶体激光器 带隙结构 阈值能量     

楔形盒染料掺杂胆甾相液晶激光器研究

设计制作了楔形盒掺杂激光染料PM580的胆甾相液晶器件, 研究了激光辐射行为. 在楔形液晶盒中出现了一系列与楔棱平行的向错线和不同规则形状的晶畴, 胆甾相液晶形成了平面态排列. 采用固体Nd:YAG倍频532 nm 波长激光作为抽运光, 获得调谐精度约1 nm, 调谐范围约17 nm的一维波长可调谐激光器. 楔形盒中, 液晶扭曲力与取向膜表面锚定力相互平衡的过程中胆甾相液晶螺距伸张, 光子禁带位置移动, 从而调谐光子禁带边沿出射激光波长. 关键词： 胆甾相液晶 楔形盒 激光辐射     

Biphotonic effect-induced phase transition in dye-doped cholesteric liquid crystals and their applications

This work studies the biphotonic effect in samples that are cholesteric liquid crystals (CLCs) doped with azo-C5. The experimental results show that the photo-isomerization of azo-C5 not only changes the clearing point of the sample, but also shifts the reflection band that is associated with the planar texture. Additionally, azo dye-doped CLCs (DDCLCs) have bi-stable or tri-stable states, as determined by the ambient temperature. Photo-switching between these bi-stable/tri-stable states is systematically studied. The result indicates that photo-addressing one of these states (planar, focal conic, and isotropic states) using a low- or high-intensity Ar⁺ laser beam is feasible. The results thus obtained are used to fabricate a photo-rewritable DDCLC display.     

Brillouin scattering study on elastic and piezoelectric properties of laser irradiated ZnO single crystals

Brillouin light scattering technique can be successfully used to determine the whole set of elastic and piezoelectric constants of a ZnO single crystal irradiated by different laser energy densities,into a micron range (radiation layer thickness).It is found that the scattering intensity,the linewidth and the Brillouin scattering shift of acoustic phonons are all strongly dependent on laser energy density.Based on the sound propagation equations and these results,the directional dependences of the compressional and shear moduli of the irradiated ZnO sample in the (001) plane are investigated.It is found that under an appropriate laser condition,248 nm KrF excimer laser irradiation can significantly improve the surface quality and increase the elastic properties of ZnO single crystal.This procedure has potential applications in the fabrication of ZnO-based surface acoustic wave and optic-electronic devices.     

Influence of silica nanoparticles on dielectric spectroscopy and polarization switching responses of novel ferroelectric liquid crystals

Silica (SiO₂) nanoparticles in different wt/wt% ratios were dispersed in a novel room temperature ferroelectric liquid crystal mixture. Dielectric relaxation spectroscopy (50 Hz–1 MHz) and electro-optic responses were investigated. Dielectric permittivity gradually decreases by about 40% up to 1 kHz with increasing SiO₂ concentration in the chiral smectic C phase. However, an increase of up to 1.5% has been observed for all the silica-doped samples above 2 kHz. We noticed an increase in optical transmission with increasing SiO₂ concentration from 0.01 to 0.1 wt/wt%.     

Random lasing in an organic light‐emitting crystal and its interplay with vertical cavity feedback

The simultaneous vertical‐cavity and random lasing emission properties of a blue‐emitting molecular crystal are investigated. The 1,1,4,4‐tetraphenyl‐1,3‐butadiene samples, grown by physical vapour transport, feature room‐temperature stimulated emission peaked at about 430 nm. Fabry‐Pérot and random resonances are primed by the interfaces of the crystal with external media and by defect scatterers, respectively. The analysis of the resulting lasing spectra evidences the existence of narrow peaks due to both the built‐in vertical Fabry‐Pérot cavity and random lasing in a novel, surface‐emitting configuration and threshold around 500 μJ cm⁻². The anti‐correlation between different modes is also highlighted, due to competition for gain. Molecular crystals with optical gain candidate as promising photonic media inherently supporting multiple lasing mechanisms.     

Influences of nanoparticles and chain length on thermodynamic and electrical behavior of fluorine liquid crystals

Hydrogen-bonded polar nematic liquid crystal series with the general formula nOBAF (n = 7—12) is studied. The mesomorphic characterization is demonstrated through differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The complexes with short alkyl chains (n=7, 8) present a wide nematic range and monotropic smectic F mesophase, whereas the longer alkyl chain (n=10—12) analogues show high melting and low clearing mesomorphic liquid crystals. The thermal range of the mesophase and the birefringence increase with chain length decreasing. Furthermore, the effect of the nanoparticles (LiNbO₃) on the thermal and the electrical behavior of 8OBAF are investigated. The presence of LiNbO₃ nanoparticles increases the conductivity and reduces the resistivity of the complex.     

Plasmon enhanced CdS-quantum dot sensitized solar cell using ZnO nanorods array deposited with Ag nanoparticles as photoanode

CdS-quantum dot sensitized solar cell using ZnO nanorods (ZnO NRs) array deposited with Ag nanoparticles (Ag NPs) as photoanode was fabricated. Light absorption effect of Ag NPs on improvement of the cell performance was investigated. Performance improvement of metal nanoparticles (MNPs) was controlled by the structure design and architecture. Different decorations and densities of Ag NPs were utilized on the photoanode. Results showed that using 5% Ag NPs in the photoanode results in the increased efficiency, fill factor, and circuit current density from 0.28% to 0.60%, 0.22 to 0.29, and 2.18 mA/cm² to 3.25 mA/cm², respectively. Also, incident photon-to-current efficiencies (IPCE) results showed that cell performance improvement is related to enhanced absorption in the photoanode, which is because of the surface plasmonic resonance and light scattering of Ag NPs in the photoanode. Measurements of electrochemical impedance spectroscopy revealed that hole transfer kinetics increases with introduction of Ag NPs into photoanode. Also, it is shown that chemical capacitance increases with introduction of Ag NPs. Such increase can be attributed to the surface palsmonic resonance of Ag NPs which leads to absorption of more light in the photoanode and generation of more photoelectron in the photoanode.     

Size-dependent visible photoluminescence from ZnO nanoparticles prepared via SiO2 network

This study reports a simple method for the synthesis of different size of wurtzite ZnO nanoparticles in assistance of tetraethyl orthosilicate (TEOS). With the increase of the amount of TEOS added, the average size of ZnO nanoparticles was found decreased from ∼14.6 to ∼1.9 nm by characterization of X-ray diffraction (XRD) and high-resolution electron microscopy (HRTEM). The growth of ZnO nanoparticles is proposed to be controlled by the density of the SiO₂ chain mesh which is determined by TEOS amount in precursor. Ultraviolet–visible (UV–VIS) absorption and photoluminescence (PL) spectra show both shift to higher energy in cut-off edge and in visible emission bands respectively. The electron transition process in the mechanism of the visible emission shift was described and related to quantum size effect in ZnO nanoparticles.