增益随机散射介质中的非相干辐射

用蒙特卡罗方法仿真了增益随机散射体中的非相干辐射,观察了非相干随机激光的特性。当抽运能量超过一定阈值时,散射体的整体辐射谱突然变窄;随着抽运能量继续增大,在光滑谱背景上会出现分离尖峰;散射体内空间某位置处频率组成不是单一的;辐射谱中某单个频率的空间方向分布和位置分布比较广。增益随机散射体中产生的非相干随机激光本质上既不同于无反馈的普通放大自发辐射,又不同于相干反馈形成的常规激光。解释了非相干随机激光辐射谱上出现分离尖峰的原因,出现这种现象是由于少数光子在增益散射体中经历较多次数散射后得到了相对充分的放大。     

Random lasing in vertical ZnO nanorods

Random lasing with coherent feedback is detected in the array of vertical ZnO nanorods synthesized on a Si(100) substrate. Under high-intensity optical pumping at the wavelength of 337.1 nm at room temperature, several narrow laser peaks with the width of 0.1 nm are observed in the wavelength region around 392 nm. The laser radiation is emitted in a direction orthogonal to the axis of the nanorods. Study of the behavior of the laser spectrum with the excited area shows good agreement of the data with the theory of random lasers. The mechanism of laser emission is related to the formation of closed paths of light due to multiple coherent scattering at the vertically oriented ZnO nanorods.     

Multiphoton stimulated Compton laser

When Compton backscattering (head to head collisions) arises between a relativistic electron beam and one of two intense counterpropagating laser beams, electrons are mainly scattered by the laser beam from the reverse direction and can hardly interact with the other laser beam propagating in the same direction. Thus, the interaction of electrons with the latter laser beam, which induces stimulated Compton scattering, can be treated as a perturbation. With perturbation theory, we calculate the probability of stimulated emission and absorption of the photons by electrons in two intense counterpropagating laser beams. The results demonstrate that stimulated emission or absorption is significant only if the momenta of photons and the energy of the electron satisfy certain condition. We also formulate the gain coefficient of the multiphoton stimulated Compton laser, which is sensitive to the intensity of the pumping laser and the energy spread of the electron beam     

石英光纤中受激喇曼散射光谱形成及其能量转换

将泵浦光产生的受激喇曼散射（ Ｓ Ｒ Ｓ） 光谱看作新的独立的单色光源,在光纤内传播的过程中,每个光源按喇曼增益曲线各自独立地激发新的斯托克光谱, 因此 Ｓ Ｒ Ｓ 光谱曲线是光谱内无数个单色光源之间相互吸收相互放大的结果。据此,建立耦合波方程,利用 Ｒｕｎｇｅ － － Ｋｕｔｔａ方法可以计算 Ｓ Ｒ Ｓ 光谱。文中讨论了一阶斯托克光谱的形成,同阶斯托克光谱内的能量红移现象,高阶斯托克光之间以频移４４０ ｃｍ － １ 传递能量以及频带展宽等问题     

无序激光器激光出射的空间分布

讨论一种散射,导出相应的分布函数。分析增益区几何形状,运用蒙特卡罗法模拟光子在无序介质中的随机行走,得到激光出射的空间分布。结果表明：在界面法线方向,激光强度最强,随角度增大强度逐渐减弱,并且光强关于法线旋转对称。激光出射的空间分布与无序介质中光子平均自由程有关,据此可通过测量激光出射的空间分布来反推光子平均自由程。出射的激光光子绝大多数（一般95%以上）都是在离界面3个平均自由程距离内散射出来的。     

一维随机介质散射颗粒对激光阈值的影响

基于光波在随机介质中的局域化理论，利用有限时域差分法数值求解麦克斯韦方程组，研究了在一维无序系统中准态模式的输出强度与激励水平之间的关系．通过考察具有不同颗粒数目的随机介质中的准态模阈值特性，分析了介质中散射颗粒个数的变化对随机激光器阈值的影响．     

飞秒激光与固体靶相互作用中背表面的渡越辐射

在飞秒激光与固体靶相互作用中,利用光学CCD相机和光学多道分析仪(OMA),分别在固体薄膜靶背表面法线方向测量了渡越辐射(TR)积分成像图案和渡越辐射光谱.测量结果显示,渡越辐射光斑呈现圆盘状结构,在圆盘中明亮而强的光信号呈局部化分布,并且有分离的光斑出现.该现象表明,超热电子在输运过程中存在成丝效应,引起严重的不稳定性;渡越辐射光谱在800 nm附近出现了尖峰,是基频波(ω0),并且光谱向红光方向移动;基频波产生的原因归结于超热电子束在传输过程中产生的微束团而引起的相干渡越辐射(CTR);光谱红移的原因是由于等离子体临界面的迅速膨胀所致;渡越辐射光强随靶厚度的增加而减小.     

二维非球形粒子随机介质中光场的传输特性

研究了散射粒子形状改变对光波在二维随机介质系统中的传输情况的影响。基于整体散射效应模型,建立了非球形粒子作为散射粒子的二维随机介质的模型。构建了模型的Maxwell方程,采用非均匀网格划分的时域有限差分(FDTD)方法解Maxwell方程,得到了TM模在非球形粒子二维随机介质模型中的传输及空间分布。采用快速傅里叶变换(FFT)对仿真获得的数据进行频谱变换,得到光波在频域上的发射谱。与以往的研究相比较,仿真结果表明,在非球形粒子系统中,光波的电场强度与球形粒子系统中电场强度随着散射粒子浓度的增加而增加不同,而是出现振荡的现象;发射谱显示,非球形粒子系统的模式竞争强于球形粒子系统,更易于实现模式选择。     

Induced versus parametric scattering processes in potassium

Stimulated and enhanced two-photon emissions from excited states in potassium have been observed. This emission is primed by stimulated electronic Raman radiation generated by scattering of a giant pulse ruby radiation. Four-photon parametric processes also take place. The two processes can be distinguished by tuning the laser frequency. Most of the emitted radiations are plane-polarized. A pumping model is proposed explaining the required population inversions and the polarization properties of the radiations.     

Direct observation of the differences between Raman scatering and population inversion transition for their emission frequencies and pulse waveforms

In FIR laser emissions optically by a CO₂ laser, it has been reported that there are two different process namely “induced Raman scattering” and “population inversion transition”. In this paper, we show that the two different process have clearly different characteristics in emission frequensies and pulse waveforms in the case of a TEA-CO₂ laser excitation. We used in this work an intra-cavity etalon for the study of the emission frequensies and also we used a MOM point contact diode for the detection of the wavaformes. Especially, we show that the FIR pulse waveformes are able to analyze numerically on the concept of rate equations. In conclusion, we indicate that molecules would be classified into two categories with regard to optically pumping by a TEA-CO₂ laser. One of them produce mainly Raman scattered emissions, and other produce only the FIR emissions from population inversion transition in spite of high power intensity excitation.     

Rb+(Ar,N2)混合蒸气中5P3/2能级有效辐射率的计算和测量

计算和测量了Rb (Ar,N2)混合蒸气中Rb(5P3/2)共振能级的有效辐射率。使用单模半导体激光器(抽运激光)将Rb原子激发至5P3/2态,另一调谐到5P3/2→7S1/2的单模激光束(检测激光)与抽运光束反平行通过样品池,并在池的径向平行移动,通过检测激光束的吸收测定了激发态原子密度及其空间分布。由于辐射陷获存在,有效辐射率为自然辐射率与透射因子(发射的光子在探测区域内没有被吸收的平均概率)的乘积。5P3/2原子密度及其空间分布结合5P3/2←5S1/2跃迁线的碰撞增宽计算了透射因子,从而得到了不同Ar或N2气压下,RbD2线的有效辐射率。对5P3/2-Ar系统,在不同气压下测得的D2线强度比值与有效辐射率计算值的比值相符。对于5P3/2-N2系统,研究了电子态向振动态的碰撞转移,得到了转移截面。     

光子晶体光纤载体中液晶随机激光辐射行为

将向列相液晶TEB30A、手性剂S-811、激光染料PM597的混合物填充空芯光子晶体光纤中,以Nd: YAG倍频532 nm激光作为泵浦光源,测量激光辐射谱,研究了光子晶体光纤载体中的随机激光辐射行为。泵浦激光侧面入射,侧面出射随机激光波长范围为590~605 nm,半高全宽约为0.3 nm;辐射方向较广。泵浦光端面入射,端面出射随机激光波长范围为580~605 nm,半高全宽约为0.3 nm。加热样品至各向同性温度时,端面和侧面激光辐射被关断。由实验结果得出,光子晶体中随机激光辐射源于微孔中填充的染料掺杂液晶混合物。手性向列相液晶中光子传输平均自由程和液晶分子介电张量的涨落随温度的变化,是影响出射激光强度的主要因素。     

圆偏振周期量级激光脉冲作用下电子辐射的空间分布

在电子汤普孙散射的经典理论的基础上,通过理论分析和计算机模拟,研究圆偏振周期量级激光脉冲作用下电子辐射的空间分布特性,提出可能的应用方向。研究发现,在相对论光强下,随着激光强度的增强,电子辐射逐渐增强,能量不断的向辐射中心集中,辐射能量分布的方向性逐渐突出;当改变激光的初相位时,电子辐射的总体方向发生偏移,表现为辐射分布图形的对称平面发生旋转,且旋转角度与激光初相位角改变大小相同。本文所进行的相关探讨基于电子辐射的三维空间立体图,所得到的有关电子谐波辐射的结论,可以为实验和工程上对电子辐射和激光参数测量的相关应用提供参考。     

Observation of Amplified Stimulated Terahertz Emission from Optically Pumped Heteroepitaxial Graphene-on-Silicon Materials

We experimentally observed the fast relaxation and relatively slow recombination dynamics of photogenerated electrons/holes in a heteroepitaxial graphene-on-Si material under pumping with a 1550-nm, 80-fs pulsed fiber laser and probing with the corresponding terahertz beam generated by and synchronized with the pumping laser. The time-resolved electric-nearfield intensity originating from the coherent terahertz photon emission is electrooptically sampled in total-reflection geometry. The Fourier spectrum fairly agrees the product of the negative dynamic conductivity and the expected THz photon spectrum reflecting the pumping photon spectrum. This phenomenon is interpreted as an amplified stimulated terahertz emission.     

Analysis of spatial modes in half-disk lasers based on AlGaAsSb/InGaAsSb quantum well nanoheterostructures

Spectral and spatial distributions of the emission of whispering-gallery-mode (WGM) lasers with a half-disk cavity have been studied. Up to 13 broad lobes with different intensities, subdivided into order-of-magnitude narrower lobes, are distinguished in the spatial distribution. The emission spectrum is nearly single-mode. A mathematical model of the electromagnetic field distribution in a WGM laser is suggested, and the radiation pattern of the laser is calculated. It is shown that each lobe corresponds to its own stable radial spatial mode. The highest emission intensity is provided not by the peripheral mode itself, but by the next-closest mode, which presumably exhibits the lowest optical loss. The predominance of a single spatial mode results in a nearly single-mode emission spectrum being observed.     

小型光泵远红外激光的放大过程

利用三能级系统的密度矩阵方程，通过迭代法计算了小型光泵ＮＨ３分子远红外激光器（ｍｉ－ｎｉ－ＮＨ３－ＯＰＦＩＲＬ）的介质增益Ｇｓ沿激光管的变化规律以及远红外信号光强Ｉｓ沿激光管的放大过程。计算表明激光管有激活区和饱和区，对一定管长的ｍｉｎｉ－ＮＨ３－ＯＰＦＩＲＬ，在一定范围内工作气压的提高，主要使Ｇｓ的峰值增大，同时使激活区缩短，考虑散射损耗和自吸收效应后，Ｇｓ峰值的提高对总的输出光强贡献不大。而泵浦光强Ｉｐ０的增大，有利于激光管内的介质沿光轴方向均匀激活，从而提高总的输出光强。另外，信号频偏对ｍｉｎｉ－ＮＨ３－ＯＰＦＩＲＬ的激光放大过程有较大的影响     

激光诱导Al等离子体中Al原子能量吸收

分析了Ａｌ等离子体连续辐射特征。根据连续辐射强度的时间分布,简要讨论了激光诱导等离子体连续辐射的产生机理。根据连续辐射强度的波长分布,提出了原了对激光诱导等离子体连续辐射共振吸收理论。我们认为：Ａｌ原子吸收能量的主要机制是原子实吸收连续辐射光子,原子实吸收光子的方式是与价电子构成极性振子对连续辐射共振吸收。     

热氧化法与PLD方法制备ZnO薄膜的特性比较

用热氧化与脉冲激光沉积(PLD)两种方法在Si(111)上制备了ZnO薄膜。对两种方法制备的薄膜的形貌、结构和室温下的荧光光谱作了比较,并对ZnO薄膜紫外光(UV)的发射机理作了初步的分析,发现与PLD方法相比,热氧化法制备的ZnO薄膜所产生的紫外光发射强度(17816任意相对强度单位a.u.)、半高宽(10nm)和紫外光与可见光的强度比(大约200∶1),都非常优异,认为热氧化法制备的ZnO薄膜的树枝状微晶界面及其方向的随机性有利于紫外光的发射和观测,而PLD方法制备的ZnO薄膜是垂直于衬底的六角柱状结构,紫外光最强的方向在垂直于侧面的方向上。     

Output characterization of a frequency shifted feedback laser:theory and experiment

Frequency shifted feedback cavity (FSFC) lasers can generate continuous broadband radiation or periodic picosecond pulses. To better understand these two different regimes a passive FSFC is modeled and its output in the time and frequency domains is calculated. Calculations of the FSFC output indicate that peaks observed using a Fabry-Perot spectrum analyzer can only occur when the FSFC cavity length and intracavity frequency shift satisfy a resonance condition. Calculations also indicate that a radio frequency spectrum analysis of the intensity should show sharp peaks which are independent of the FSFC frequency shift. Additionally, pulses propagating in a FSFC are insensitive to frequency shift detunings, suggesting applications as a stable source of ultrashort laser pulses. Experimental evidence confirming these findings is reported. Results indicate that a FSFC does not support conventional laser modes