二维随机激光器的模式选择及阈值与饱和特性

基于将Maxwell方程与四能级原子系统速率方程相结合而建立起随机激光时域理论，并利用有限时域差分法，研究了二维随机介质中激光模式的输出特性与介质尺寸、外形及抽运速率等参数的关系.结果表明，与传统激光模式相似，随机激光模式的强度随抽运速率的变化不仅具有阈值特性，而且具有饱和特性.基于模式特性对介质及抽运参数的依赖关系，提出了二维随机激光器的选模方式，在很大程度上不同于传统激光器的选模方式. 关键词： 随机激光器 模式选择 无序介质中的光学特性     

一维随机激光器中准态模的饱和特性研究

针对随机激光器的半经典理论是应用电磁场时域有限差分法数值求解半经典随机激光模型方程组，基于半经典理论数值研究了一维随机激光器中准态模式的饱和特性，获得了不同准态模的空间分布和频谱分布图。计算结果分析显示：准态模式的某些特征参数如局域程度将会影响系统的饱和特性。利用这一结果，可以通过单频抽运方法选择具有低阈值、高饱和强度的准态模式作为整个激光系统的稳态输出模式。     

同材质颗粒不同填充密度的随机介质中光场的空间分布

用时域有限差分法研究了同一材质的颗粒在不同填充密度下的随机介质中光局域化问题. 依据随机介质激光的实验参数,模拟了颗粒填充密度不同的随机介质中光场的空间分布. 结果表明：当散射颗粒的散射平均自由程与波长相当时,随机介质中的光场分布呈现局域化的特征,而且随着颗粒填充密度增大,光场的局域化程度增强. 因此,在同样的抽运激励下,颗粒填充密度越大的随机增益介质越容易产生激光辐射. 数值模拟结果与实验定性符合. 关键词： 随机激光 时域有限差分法 颗粒填充密度 局域化     

随机激光器中准态腔的阈值与其局域化程度的关系

基于光波在有限随机介质中的局域化理论，利用有限时域差分法数值求解Maxwell方程 组，研究了随机介质中的激光现象，分析了准态模的放大与其 空间局域性的关系. 通过研究二维非增益随机介质中光波的局域化，确定了准态模的空间分 布和频谱特征. 通过引入增益，研究了准态模的放大过程和阈值特性. 结果表明空间局域化 强的准态模在增益介质中被优先放大，且有较低的阈值. 关键词： 随机激光器 准态模 随机介质中的光学特性     

飞秒抽运随机激光输出波形的可控性研究

本文基于随机激光的时域理论,研究了飞秒脉冲抽运下二维随机激光的辐射特性,并着重讨论了抽运脉冲的峰值强度、脉宽和脉冲波形对辐射光时域波形的影响.结果表明, 辐射光的时域波形强烈依赖于抽运光脉冲的参数,通过调整抽运方式可以控制辐射光的输出波形.数值模拟结果为研究随机激光输出波形的可控性技术提供了理论依据. 关键词： 激光物理 随机激光器 飞秒抽运 脉冲波形     

随机介质的相干背散射与局域化研究

研究了不同尺度参数和粒子浓度下的ZnO随机介质相干背散射强度的分布规律,采用时域有限差分法分析了不同浓度随机介质的光场能量空间分布,预测了随机激光器阈值的高低。结果表明:同一折射率的介质随着介质尺寸的增大,相干背散射的带宽变窄,局域化参量kl值相应增大,使得局域化程度呈较大幅度减弱趋势;并且随着介质浓度的增加,相干背散射的带宽变宽,局域化程度增强,阈值降低。相干背散射有着光子局域化的先期特征,现在已成为研究光子局域化出现与否的基本判断依据,对研究光子局域化以及随机激光器具有重要意义。     

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     

单负材料异质结构中损耗诱导的场局域增强和光学双稳态

光学双稳态的阈值取决于非线性材料中的场局域程度, 场局域越强阈值越低. 而材料的损耗是影响场局域强弱的重要因素. 之前, 人们普遍认为, 增加损耗会削弱场局域, 不利于降低阈值. 本文研究了由磁单负材料和电单负材料组成的异质结构中光学双稳态现象, 发现随着损耗的增大, 其阈值可以呈现先降后升的非单调变化. 进一步研究表明, 异质结构界面处的电磁场强度随着损耗增大呈现先降后升的非单调变化, 即增加损耗也有可能增强场局域. 研究结果揭示了场局域程度与材料损耗之间的非单调依赖关系, 为设计开发非线性功能器件提供了新的思路.     

二维随机介质中偏振模式的竞争和阈值特性

基于随机激光的时域理论,建立了横磁与横电光波模式共享反转粒子数情况下的理论模型,并利用时域有限差分法(FDTD)数值求解麦克斯韦方程组和速率方程组,从而获得了二维随机介质内横磁与横电光波模式的阈值特性.结果表明,如果横磁与横电偏振模式之间没有共享反转粒子数,则横磁光波模式拥有较低的激发阈值;反之,则横电光波模式的激发阈值更低.在共享反转粒子数的情况下,改变样品的表面填充率对两种偏振模式的阈值并没有很大的影响,而增加样品面积则可以明显的降低横磁模式的阈值.     

随机性对部分随机介质激光器阈值的影响

基于速率方程和Maxwell方程相结合的模型，采用时域有限差分法(FDTD)研究了介质的随机性和层数对部分随机介质激光器阈值的影响.模拟结果显示，当抽运速率超过阈值时，出现一个或者多个振荡模；随机性或者系统的尺度增加时，振荡模数量也增加；部分随机介质激光器的阈值在一定的随机强度和层数下将达到最小值，它与完全随机情况下的结论有所不同.对所得到的结论给出了物理解释.这些结果对于制作随机激光器和光集成潜在应用价值. 关键词： 激光物理 随机激光器 阈值 时域有限差分法     

Theoretical investigation on temporal properties of random lasers pumped by femtosecond-lasing pulses

Pulsing random lasing property has been investigated in both one- and two-dimensional random medium by numerically solving Maxwell’s equations and rate equations in which the pumping rate is described by a time function with duration of 10s or 100s of femtoseconds. The peak intensity, width and delay time of a random lasing pulse are traced with the variation of the peak intensity, duration, shape and numbers of a pumping pulse. Results show that the behavior of random lasing depends strongly on the pumping process, some of which are in agreement with previously reported experiments pumped by femtosecond-lasing pulses. The present work enriches the knowledge about random lasers, especially in temporal regime, and could offer more guidance for relevant experiments.     

Numerical study of random lasing in three dimensional amplifying disordered media

In this paper, the lasing action in three-dimensional active random systems has been numerically investigated. Here, random systems of spherical dielectric particles imbedded in an active medium are considered. The quasi steady state approximation for the population inversion of the active medium is applied to solve three dimensional governing equations. Results show that when the density of particles increases to an upper limit, the intensity of lasing modes is enhanced. Also, the effects of pumping rate and particle size on the number of lasing modes and their intensity are studied. Lasing threshold of laser modes in different disordered systems is calculated and it is shown that by an appropriate selection of the central frequency of gain line-shape, the output power intensity of random lasers increases. These results are in agreement with the experimental results observed by others.     

Theoretical Investigation on the Saturation Effects in Two-photon Pumping Random Lasers

A numerical analysis of the saturation effects of emitted light intensity in two-photon pumping random lasers is presented by adding the saturation item to modify the gain in the diffusive equation. Numerical results show that the emitted intensity reaches its saturation value in a short time interval with sufficient pumping. From the analysis of normalized emitted intensity and saturation intensity, one can obtain a method to control steady output intensity of two-photon pumping random lasing by adjusting some parameters of the random media     

Time dependent theory for random lasers

A model to simulate the phenomenon of random lasing is presented. It couples Maxwell's equations with the rate equations of electronic population in a disordered system. Finite difference time domain methods are used to obtain the field pattern and the spectra of localized lasing modes inside the system. A critical pumping rate P(c)(r) exists for the appearance of the lasing peaks. The number of lasing modes increases with the pumping rate and the length of the system. There is a lasing mode repulsion. This property leads to a saturation of the number of modes for a given size system and a relation between the localization length xi and average mode length L(m).     

Temporal properties of random lasers under ultrashort pulse excitation

Temporal properties of random lasing under ultrashort pulse excitation are investigated in a two-dimensional disordered medium. The pumping light is described individually and coupled into the rate equations. The Maxwell equations and rate equations are numerically solved by using the finite-difference time domain method. The time evolution of the emission pulse is studied with the variation of the surface-filling fraction, refractive index,and scatterer radius. Results show that the behavior of random lasing depends strongly on the sample parameters. Our work enriches the knowledge about random lasers in the ultrashort pulse pumping regime and offers some guidance for relevant experiments.     

Lasing in random media

A random laser is a non-conventional laser whose feedback mechanism is based on disorder-induced light scattering. Depending on whether the feedback supplied by scattering is intensity feedback or amplitude feedback, random lasers are classified into two categories: random lasers with incoherent feedback and random lasers with coherent feedback. A brief survey of random lasers with incoherent feedback is presented. It is followed by a review of our recent experimental work on random lasers with coherent feedback, including measurement of the lasing threshold, lasing spectra, emission pattern, dynamical response, photon statistics, speckle pattern and the investigation of relevant length scales. Large disorder leads to spatial confinement of the lasing modes, that is the foundation for the micro random laser. Some theoretical models of random lasers with coherent feedback are briefly introduced. The study of random lasers improves our understanding of the interplay between light localization and coherent amplification.     

Effective control over the dual-color emitted light waves from one-dimensional random media pumped by Femtosecond-Lasing Pulses

The spectral property of dual-color random laser is investigated by simultaneously solving Maxwell’s equations and rate equations of electronic population in which the pumping rate is described by a time function with duration of hundreds of femtoseconds. Results show that the delay time and peak intensity of the emitted light depends strongly on the pumping process. An effective optimization method is proposed to exercise control over the dual-color laser emission from the dye solution with scattering nano-particles. It is demonstrated numerically that the controllable single mode operation of a dual-color random laser by choosing the proper pump profile and pumping region. The present work enriches the knowledge about random lasers and offers more guidance for relevant experiments.     

The research on polarization states with local pumping in two-dimensional active random medium

The polarization-dependent competition on inversed population with local pumping in two-dimensional (2D) active random media is investigated by simultaneously solving Maxwell’s equations and rate equations of electronic population for both transverse magnetic (TM) and transverse electric (TE) modes. The threshold property of the two polarization states with local pumping is analyzed. Results show that the lasing threshold of TM modes is lower than that of TE ones when the radius of the local pumping region is suitably selected, and the case would be contrary when the selected radius of the local pumping region is excessive.