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.     

Lasing modes in a spiral-shaped dielectric microcavity

The resonance patterns and lasing modes in a spiral-shaped dielectric microcavity are investigated through passive and active medium calculations. We find that the high-Q resonance modes are whispering-gallery-like modes, and these resonance modes can be easily excited as lasing modes. We also find that the quasi-scarred resonance mode, which shows strong directional emission beams from the cavity boundary, can be excited with selectively applied external pumping. Through a spectral analysis of the time evolution of the light field, the competition between these lasing modes is discussed.     

Lasing with guided modes in ZnO nanorods and nanowires

A vertically arranged nearly parallel array of ZnO nanorods and randomly oriented nanowires has been grown by low pressure chemical vapor deposition (CVD) on silica substrates and on stainless steel gauze woven from a wire with a diameter of 40 μm, respectively. The quality of the produced material is high enough to act as a gain medium for stimulated emission in the ultraviolet spectral region. Multiple sharp lasing peaks were realized from single hexagonal nanorods and arrays of hexagonal ZnO nanorods. The lasing peaks display successive onset and saturation with increasing excitation power density and fit well the expected resonance spectrum of guided modes in hexagonal nanorods. The behavior of lasing spectra from shot to shot of pumping in randomly oriented nanowires along with the independence of the lasing threshold on the excitation spot area suggest that the emission spectrum results from the superposition of lasing modes in individual nanowires, rather than from random lasing due to photon coherent scattering.     

Self-modulation lasing regimes in a fast-flow CO<Subscript>2</Subscript> laser

Numerical simulation of self-modulation lasing regimes has been carried out for a CO₂ laser with transverse flow of the active medium through an unstable resonator with inhomogeneous internal pumping. Two types of steady-state self-modulation oscillation differing in the feedback mechanism were observed. The type of lasing regime and the conditions of its realization depend both upon the pumping profile and upon the composition and pressure of the working mixture, which makes it generally possible to control temporal characteristics of lasing.     

Effect of Pump Area on Lasing Modes in Active Random Media

We investigate the effect of pump area on lasing modes in an active random medium. Considering the structure characteristics in a real experimental system, the random medium is divided into two regions, i.e. pump and non-pump areas. The dependence of lasing modes on the pump area is qualitatively explained by means of the model in which the lasing is ascribed to the interaction of the complex localized modes in the active random medium with local aperiodic quasi-structure with appropriate pump light. There exist different pump sizes for lasing with different modes. As the pump size decreases in this random system, the pump threshold of the lasing modes increases. There are different lasing modes in different excitation regions in this random system. This gives us some information about the dependence of lasing modes on pump areas in active random media.     

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     

一维随机介质中准态模式的局域化程度对其阈值的影响

基于将Maxweil方程与四能级原子系统速率方程相结合而建立起的随机激光时域理论,并利用有限时域差分法,研究了一维随机激活介质中激光模式的输出强度与抽运速率之间的关系.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.用模式的空间区域来表征模式的局域化程度,发现局域化程度越强的模式,阈值越低.不过,这种关系不是一一对应的.这表明模式的局域化程度是决定阈值大小的重要因素,但不是唯一的因素.     

Terahertz beat frequency generation from two-mode lasing operation of coupled microdisk laser

We propose a coupled microdisk laser as a compact and tunable laser source for the generation of a coherent continuous-wave terahertz radiation by photomixing. Using the Schr?dinger-Bloch model including the nonlinear effect of active medium, we find single-mode and two-mode lasings depending on the pumping strength. We explain the transitions of lasing modes in terms of resonant modes that are the solutions of the Schr?dinger-Bloch model without active medium and nonlinear interaction. In particular, a two-mode lasing is shown to generate a terahertz oscillating frequency originating from the light beating of two nearly degenerated resonant modes with different symmetries.     

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.     

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.     

Linearly polarised three-colour lasing emission from an evanescent wave pumped and gain coupled fibre laser

A novel Whispering--Gallery--Mode (WGM) fibre laser, emitting linearly polarised three-colour light, is demonstrated by pumping and gain coupling with evanescent waves. The pump light is longitudinally coupled into a bare optical fibre immersed in a dye solution of lower refractive index. The dye molecules around the bare fibre are excited by the evanescent waves of pump light when they propagate along the fibre in a total internal reflection. When the pump beam within the fibre is a meridian beam, the WGM lasing emission from the fibre laser is a linearly polarised transverse electric wave, while it is a mixed wave of the linearly polarised transverse electric and magnetic waves if the pump beam is a skew beam within the fibre. Because the excited molecules are located within the evanescent field of WGM, a good spatial overlap between the dye gain and the evanescent field leads to a high pumping efficiency and a longer gain distance along the fibre. Once the bare fibre is inserted into three glass capillaries filled with Rhodamine 6G, 610 and 640 dye solutions, respectively, WGM laser oscillations at the wavelengths of 567--575, 605--614 and 656--666~nm occur simultaneously, and a linearly polarised three-colour lasing emission is achieved in a single optical fibre.     

Absorption-induced confinement of lasing modes in diffusive random media

We present a numerical study of lasing modes in diffusive random media with local pumping. The reabsorption of emitted light suppresses the feedback from the unpumped part of the sample and effectively reduces the system size. The lasing modes are dramatically different from the quasi modes of the passive system (without gain or absorption). Even if all the quasi modes of a passive diffusive system are extended across the entire sample, the lasing modes are still confined in the pumped volume with an exponential tail outside it. The reduction of effective system volume by absorption broadens the distribution of decay rates of quasi modes and facilitates the occurrence of discrete lasing peaks.     

光子晶体晶粒尺寸和排列结构对随机激光辐射特性的影响

研究了光子晶体对随机激光器的输出特性的调控,并分析了光子晶体的晶粒尺寸和排列结构等对调控作用的影响.对于同一随机介质而言,上下表面用同种材料不同晶粒尺寸的光子晶体覆盖,结果显示:晶粒尺寸合适的光子晶体会使光在系统中来回振荡从而得到很好的放大,使系统中的光与随机增益介质的相互作用加强,激光阈值降低;同时还对激光模式有较强的调制能力,能在一定程度上抑制自发辐射,使之向所需要的频率内辐射.但如果光子晶体的晶粒尺寸与随机增益介质结构匹配不当,则光子晶体对激光模式调制能力较弱,光场能量不能有效地被局域在系统中,系统 关键词： 有限时域差分法 光子晶体 随机激光 辐射特性     

管板式大功率横流CO2激光横模测量及数值模拟

为了提高激光稳定性和激光加工质量,实验测量了大功率横流CO2激光横模,并理论分析了横模形成机理。针对大功率横流CO2激光器管板式电极结构,由麦克斯韦方程给出电场的横向分布,并通过数值计算得到了激光强度的横向分布。实验结果与数值模拟得到的激光能量分布基本一致。结果表明,在给定放电电压条件下,电极结构和气体流动速率决定了激光横模峰值大小,激光峰值的横向位置取决于气体流动的速率。     

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).     

The competition between two polarization states in two-dimensional random medium

In a two-dimensional random medium, when the transverse magnetic (TM) polarization state and the transverse electric (TE) polarization state share the same population inversion, there is a competition in the population inversion between the two states. Such competition is analyzed by using the finite difference time domain method to numerically solve Maxwell equations with a set of system parameters. The results indicate that TM state has a larger lasing threshold than TE state does so that TM state is strongly suppressed by TE state in the competition. The larger the size of the medium, the easier the lasing modes in TM state appears.     

二维随机介质中模式的空间局域化分布

基于随机激光的时域理论,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,利用分区域计算法,分别计算了整体和局部泵浦二维随机介质时,不同泵浦强度下,不同区域的辐射谱,得出了随机介质中模式的空间局域化分布。计算结果显示,同样条件下不同区域的辐射谱是不同的,且随着激励源泵浦强度的变化,随机激光辐射始终集中在介质中某几个固定区域,但各个区域的随机激射效率不同,在区域之间存在模式的空间范围重叠。利用该方法,可以分析介质的随机分布对随机激射的影响,理论上可以为高激射效率的伪随机介质制备提供一定指导。     

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.     

基于光放大的长脉冲抽运太赫兹激光

为了获得较窄增益带宽、较大光强的太赫兹辐射,尝试了利用TEA CO₂激光器对6 W可调谐连续波CO₂种子激光实行双程光放大实验,以期望获得长脉冲、高峰值功率的抽运光.通过实验,获得了大约30倍的放大系数,放大光输出功率随种子光注入功率的增加而增加,其中10 P （20）支线表现更加突出,并且被放大的激光支线半峰全宽在14 μs左右.基于这种长脉冲抽运源,建立了一种产生脉冲太赫兹的动力学模型,给出了产生太赫兹辐射的饱和抽运光强表达式,确定了抽运光的光强范围.另外, 关键词： 激光物理 长脉冲 动力学模型 太赫兹激光     

Low-threshold lasing eigenmodes of an infinite periodic chain of quantum wires

We study the lasing eigenvalue problems for a periodic open optical resonator made of an infinite grating of circular dielectric cylinders standing in free space, in the E- and H-polarization modes. If possessing a "negative-absorption" refractive index, such cylinders model a chain of quantum wires made of the gain material under pumping. The initial-guess values for the lasing frequencies are provided by the plane-wave scattering problems. We demonstrate a new effect: the existence of specific grating eigenmodes that have a low threshold of lasing even if the wires are optically very thin.