无序介质激光的方向效应

无序介质激光是由该介质的非周期-类结构与相匹配的泵浦光场相互作用时其整体多重散射的干涉效应产生的.因此其局域模不仅与无序介质被泵浦的功率、面积有关,而且与方向有关.本文以ZnO粉末激光为例,运用传输矩阵的方法计算模拟了其局域模谱的方向效应.     

ZnO薄膜的光抽运紫外激射

采用等离子体增强MOCVD方法生长出高质量的ZnO薄膜,并观察到了ZnO薄膜的光抽运紫外激射现象。在不同激发强度下进行了光荧光谱测量,发现紫外发光强度随着激发光强度的增加呈直线增强,证明此紫外发光峰来源于带边自由激子辐射复合。激发的激光器为3倍频YAG激光器,脉宽15ps,每秒10个脉冲。抽运光达到样品的光斑直径约为25μm,激射阈值为0．28μJ,利用光纤连接到CCD来探测接收激射光。在385～390nm之间的激射峰,其半峰全宽为0．03nm。所观察到的激射没有固定的方向,也就是说是往各个方向发射的。对于ZnO薄膜,由于我们并没有制作通常激光器的谐振腔,激射是通过晶粒强烈的散射导致的自形成谐振腔所产生的。     

ZnO粉末激光的研究

从ZnO粉末激光的实验现象出发,将激光泵浦下的ZnO粉末介质相互作用系统作为一个整体,并建立相应的物理模型。运用传输矩阵的方法数值模拟了该激光与ZnO粉末被泵浦的面积、功率和方向之间的关系。模拟结果在定性上与实验吻合。     

ZnO纳米柱无序介质中泵浦面积对局域模发射的影响

基于ZnO纳米柱制备及发光实验,建立了ZnO纳米柱的位置和大小都是无序的二维介质结构模型.通过构建增益模型,用时域有限差分法数值模拟了无序介质中频谱特性以及ZnO增益频谱范围内的某一个共振峰对应的波源在无序介质中的光场分布情况,发现了局域模的存在.分四种情况讨论了此局域模的受激辐射与泵浦面积的关系:改变泵浦功率,从左到右依次增加两层ZnO纳米柱泵浦和单独泵浦一个局域区域;泵浦功率一定时,增加泵浦局域区域和非局域区域中ZnO纳米柱个数.结果表明:存在一个临界泵浦功率,当泵浦功率小于临界泵浦功率时,无论泵浦面积多大都不能激发局域模;当泵浦功率大于临界泵浦功率时,对于不同的泵浦功率,局域模被激发所需的临界泵浦面积不同;随着泵浦功率的增加,当泵浦面积一定时,光场相对强度呈递增趋势,当泵浦功率超过临界功率时,光场相对强度急剧上升.     

ZnO纳米柱无序介质中泵浦面积对局域模发射的影响

基于ZnO纳米柱制备及发光实验,建立了ZnO纳米柱的位置和大小都是无序的二维介质结构模型.通过构建增益模型,用时域有限差分法数值模拟了无序介质中频谱特性以及ZnO增益频谱范围内的某一个共振峰对应的波源在无序介质中的光场分布情况,发现了局域模的存在.分四种情况讨论了此局域模的受激辐射与泵浦面积的关系:改变泵浦功率,从左到...     

含金属散射体的中红外无序介质的光子定域化理论研究

基于Mie散射理论发现,各种金属在中红外区的各个频率点上的散射行为均极为相近,是一种在中红外区反照率高但散射效率较低的散射体.在浓度为10%时,介质的定域化参量最小只能达到6.6,并且与体系的常用基质材料无关.数值研究揭示了这种金属散射体系统与低吸收、高折射率散射体系统间的内在联系 关键词： 光子定域化 无序介质 中红外 Mie散射 金属     

无序对二维声子晶体平板负折射成像的影响

利用多重散射理论分析了位置无序对钢/水声子晶体负折射成像的影响.发现声子晶体负折射成像与周期结构中的方向性通路有关;该方向性通路不同于一般意义上的位置波导,它是声子晶体周期散射的结果;通路中的障碍对成像有较大影响;随着散射体位置无序程度的增加,周期性散射减弱,方向性通路被破坏,成像也随之减弱,甚至消失.     

本征GaAs量子阱中电子自旋扩散输运的时-空分辨吸收光谱研究

发展了一种时-空分辨圆偏振光抽运-探测光谱及其理论,并用于本征GaAs量子阱中电子自旋扩散输运的实验研究.获得室温下本征GaAs量子阱中的“自旋双极扩散系数”为D_as=37.5±15 cm²/s.此结果比用自旋光栅法测量到的掺杂GaAs量子阱中电子自旋扩散系数小.解释为是由于“空穴库仑拖曳”效应减慢了电子自旋波包的扩散输运. 关键词： 时-空分辨抽运-探测光谱 电子自旋扩散 GaAs量子阱     

时间分辨技术测量高散射介质光学参量

采用曲线拟合反演算法来恢复时间分辨系统中的真实光脉冲信号,从中可提取出介质光学参量.在模拟人体组织光学参量的模型介质试验中,利用时间相关的单光子计数系统测量光子的逸出曲线,拟合的结果验证了算法的可行性.     

随机介质ZnO激光的一种整体效应模型

从实验现象出发建立了无序介质激光的一种整体散射效应物理模型，认为无序介质激光是无序介质中局域-非周期-类结构与相匹配的抽运光相互作用的结果.并以ZnO粉末激光参数为例进行数值模拟，结果与实验定性符合. 关键词： 无序增益介质 局域-非周期-类结构     

Multiple scattering in random media

The paper is an application of a general microscopic approach to the theory of the average scattering matrix for a particle interacting with random scatterers. We present a detailed treatment for the case of uncorrelated positions of the scatterers. First, the general two-body additive approximation is used to truncate the hierarchy of correlation functions for fluctuations. It is shown that the self-energy is accurate through the fourth power of the individual scattering amplitude. Second, the hierarchy is terminated at the next stage. The self-energy is correct to the sixth power of the scattering amplitude.Work supported in part by the National Science Foundation under Contract No. NSF DMR 79-23213.     

Multiple scattering in random media I. Restricted two-body additive approximation

We present a microscopic theory of the problem of finding the properties of a particle interacting with potentials located at random sites. The sites are governed by a general probability distribution. The starting point is the multiple scattering equations for the amplitude k ₁|T |k ₂ in terms of the individual scattering amplitudes k ₁|T |k ₂. We work with quantitiesA defined by k ₁|T |k ₂=k ₁|T |k ₂exp[i(k ₁–k ₂)R ]. The theory is based on a splitting of the fundamental equation forA into equations for the mean A and the fluctuationsAA . Neglect of the fluctuations yields the quasicrystalline approximation. We rearrange the equation forAA to isolate the collective part of the fluctuations. We then make the simplest microscopic truncation which is thatAA is a restricted two-body additive function of the site positions. With the contribution of the collective fluctuations, this yields results forA that are accurate to ordert ⁴.Work supported in part by the National Science Foundation under Contract No. NSF DMRWork supported in part by the National Science Foundation under Contract No. NSF DMR     

A method to control the polarization of random terahertz lasing in two-dimensional disordered ruby medium

Terahertz (THz) random lasing is studied numerically for two-dimensional disordered media made of ruby grains with a three-level atomic system. A method via the adjustment of the pumping area to control the polarization of the THz wave is proposed. Computed results reveal that transverse electric THz lasing modes could occur if pumping is supplied on the whole medium, while transverse magnetic THz lasing modes could occur if pumping is appropriately supplied on a partial area of the medium.     

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.     

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.     

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     

Scaling relationships for anisotropic random walks

Aspects of transport in a highly multiple-scattering environment are investigated by examining random walkers moving in media having anisotropic angular scattering cross sections (turn-angle distributions). A general expression is obtained for the mean square displacement x² of a random walker executing ann-step walk in an infinite homogeneous material, and results are used to predict scaling relations for the probability() that a walker returns to the planar surface of a semi-infinite medium at a distance from the point of its insertion.     

二维无序介质中横磁模的谱线宽度随抽运强度的变化特性

基于随机激光时域理论,数值研究了二维随机激光器中横磁模式的输出光谱线宽度,获得了谱线宽度与抽运强度的关系曲线,由此曲线可以计算横磁模式的很多特征参数.从谱线宽度的角度来看,横磁模式的输出特性结果与已有的横电模式结果相比,横磁模式具有较好的输出特性. 关键词： 随机激光器 偏振 谱线宽度 无序介质中的光学效应     

超短THz脉冲在随机散射介质中的传播

用Monte-Carlo方法模拟了超短THz脉冲在随机散射介质中的传播.根据Mie理论计算出随机散射介质的散射系数和各向异性因子，研究了随机散射颗粒及介质厚度的大小对透射脉冲的影响.结果表明在Mie散射范围内，在相同的浓度下，颗粒尺寸越小，散射介质越厚，THz散射越严重，对透射脉冲的影响越大.散射同时降低了THz脉冲在随机散射介质中的成像分辨率. 关键词： 超短THz脉冲 随机散射介质 Mie理论 Monte-Carlo方法