随机介质介电常量的涨落与光波的空间分布

采用随机介质统计模型和时域有限差分方法模拟了二维随机介质中光场的空间分布及其同介电常量涨落的关系。模拟中,散射微粒随机地分布在均匀介质中以增加光子在介质中滞留时间。不同随机强度α,形成不同空间结构的随机介质。通过小心地调节入射光波长,得到了不同随机介质中的场强分布。结果表明,随机介质介电常量的涨落导致在介质的某些区域形成一些特殊结构。这些结构延长了光子在其中的滞留时间,产生了光波的局域态,起到了光腔的作用,使得随机介质在抽运激励下能够产生激光辐射。这样的光腔在介质中是随机分布的,其数量与分布特征与介电常量涨落的强弱有关。     

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。     

ZnO纳米颗粒受激发射的时间分辨特性

在飞秒脉冲激光激发下,观察到了均匀沉积法获得的ZnO纳米颗粒的受激辐射现象,并从频域和时域两方面研究了ZnO纳米多晶的室温激射特性。氧化锌纳米颗粒中出现激子-激子散射导致的激射阈值为7.2 GW·cm-2,激射模式类似于F-P谐振腔模式,时域谱则表现为寿命曲线中出现快速衰减成分。与荧光的时间衰减曲线不同,P带时间衰减具有对称结构,高斯拟合结果只有几个ps,接近条纹相机的时间分辨率极限。研究ZnO纳米颗粒的受激发射与激光特性对揭示ZnO晶体的内部结构和激子激发态的性质、激光产生的机理等有重要意义。     

散射颗粒分布对二维随机激光器腔内各模式的影响

采用时域有限差分法数值求解Maxwell方程组,分析了二维随机激光器中光波模式的频谱特性.计算表明,处于不同分布区域的散射颗粒对腔内不同模式的影响有很大差异,处于该模式空间分布区域的散射颗粒比区域外的颗粒对模式有更大的影响.这个特性是随机激光器所独有,并可以用来进行随机激光器的模式选择.     

LDA侧面泵浦Nd:YAG激光器的热效应分析

 在高斯光强近似下对泵浦LD光强分布模型进行修正，建立了LDA侧面泵浦固体激光介质内热源分布的数值模型。用有限元法计算模拟了三角均匀分布侧面泵浦结构激光棒瞬态温升过程及稳态温度分布情况。讨论比较了泵浦源的高斯强度近似和均匀强度近似下激光棒内温度分布情况，并对激光棒的类热透镜的焦距进行实验测量。实验和数值计算说明了LDA泵浦结构和冷却场的非均匀分布使实际温度场偏离均匀泵浦时的二次曲线分布模型，激光晶体热效应产生的类透镜会聚作用的不对称导致了激光器输出光束质量在x，y方向上的不同。     

分布反馈激光特性的实验研究

研究了以脉冲YAG倍频激光泵浦Rh6G染料,产生分布反馈激光振荡的辐射光谱、转换效率、时间过程以及可调谐特性;发现在大于泵浦阈值下的分布反馈激光输出的多谱线结构与双脉冲辐射现象,以及分布反馈激光脉冲变窄的自开关效应.作为详细的实验观察和分析.     

无序介质中不同模式的激光辐射与散射体密度的关系

以ZnO粉末无序介质激光为例,在介质增益和光场损耗的条件下研究了光子在无序介质中的无规行走行为.在泵浦光强和体积一定的情况下,运用数值模拟研究了不同频率的光受激辐射对散射体密度的要求;得到了不同散射体密度下激光的辐射谱：当散射体密度大于临界密度时才能有激光辐射产生;当散射体密度进一步增大时,辐射谱中出现更高更窄的分离尖锋.     

增益腔模大失配型垂直外腔面发射激光器侧向激射抑制

垂直外腔面发射激光器(Vertical external cavity surface emitting laser,VECSEL)的侧向激射是制约其高性能工作的关键。我们设计了室温下量子阱增益峰与表面腔模大失配(30 nm)的增益芯片结构,并证实该结构可以有效抑制泵浦功率增加时VECSEL的侧向激射增强问题。增益芯片基底温度为20℃时,VECSEL正向激射波长位于980 nm,侧向激射波长位于950 nm,当泵浦功率逐步增加时,侧向激射强度随着正向激射的出现而迅速降低。这是因为激光正向激射时量子阱的受激辐射能级与正向激射激光模式匹配,正向激射的激光模式可以获取更高的模式增益,在与侧向模式的竞争中处于优势地位。当基底温度控制在0℃与10℃时,量子阱本征增益峰值与表面腔模失配度增大,此时VECSEL仍然表现出稳定的侧向激射抑制效果。     

单片双波长输出垂直外腔面发射激光器EI北大核心CSCD

报道了利用垂直外腔面发射激光器(Vertical external cavity surface emitting laser,VECSEL)的增益谱与腔模的大失配设计实现VECSEL双波长同时激射的方法,设计了稳定的振荡腔结构,理论预测了这种VECSEL的三种工作状态并进行了实验验证。随着VECSEL泵浦功率增加,增益芯片内部工作温度逐步升高,VECSEL依次出现带边波长激射、双波长激射及腔模波长激射三种工作状态。最初VECSEL的激射波长位于带边模式决定的激光波长(952.7 nm),随着泵浦功率增加,增益芯片热效应增强,腔模波长与带边波长出现模式竞争,此后出现双波长激射现象。双波长峰值强度接近时VECSEL激光输出功率达到359 mW,激光波长分别位于954.2 nm和1001.2 nm,在该位置附近VECSEL的输出功率曲线呈现明显的二次阈值现象。当泵浦功率持续增加,激光输出波长变为腔模波长激射,激光波长位于1002.4 nm。在单波长及双波长工作状态下VECSEL的光斑形貌均为高斯形貌的圆形对称激光光束,激光光束发散角半角由5.7°增加到7.9°。这种单芯片双波长输出VECSEL方案未来在抗干扰激光雷达以及频率转换太赫兹激光等方面有着很好的应用潜力。     

二维随机介质中的能量分布和频谱特性

研究了二维随机介质中能量的空间分布和一些特定区域的频谱特性.结果显示介质中能量呈局域化分布,其分布与介质中颗粒的随机分布有关,还与激发光的波长有密切的关系,介质中能量聚集的区域和介质的准封闭区域有一定关系但并非一一对应.所考察的各准封闭区域的频谱均随时间变化,在同一激发光的激励下各准封闭区域的频谱均不相同,而同一区域在不同的激发光的激励下频谱也不相同.当激发波长与准封闭区域的结构参数偏差较大时,频谱中谱峰会出现此消彼涨的模式竞争现象,而且能量衰减得较快;而当激发波长与该结构参数较接近时,相应的频谱中存在一 关键词： 激光物理 有限时域差分法 局域化 频谱特性     

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.     

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.     

ZnO random laser spectra under nanosecond pumping

Spectra of ZnO random lasers were obtained using a Peltier-cooled CCD camera Videoscan-285 at a single shot of nanosecond pumping. It was demonstrated that these spectra differ essentially from lasing spectra under picosecond pumping: as a rule, the line widths are significantly larger and the spectra often change essentially from shot to shot in a random manner on the same pumping spot. We suggest that large line widths can be the result of many lasing acts appearing during a single pumping pulse and of the lasing frequency changing in every lasing act. The random variations of spectra from shot to shot can be called forth by spontaneous emission fluctuations.     

ZnO粉末中无序激射现象时间分辨的研究

采用抽运-探测时间分辨方法实验研究了半导体材料ZnO纳米颗粒粉末中的无序激射现象.在2 67nm激光的抽运下，通过精确控制抽运光的能量和样品表面的抽运面积，获得了宽度小于1n m的单模无序激射光谱和多峰的多模无序激射光谱.时间分辨的抽运-探测结果显示，此时样 品的上能级寿命仅为几个皮秒，证明了ZnO粉末的单模无序光谱是受激辐射的结果. 关键词： 高散射介质 多重散射 抽运-探测 无序激射     

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 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.     

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.     

基于钠硼铝硅酸盐玻璃的PbSe量子点红外单模光纤激光

根据实验制备的钠硼铝硅酸盐PbSe量子点玻璃及其透射电子显微镜(TEM)图、吸收谱和发射谱,计算机数值模拟了以PbSe量子点作为激活增益介质的红外单模光纤激光。应用遗传算法,通过数值求解粒子数速率方程和激光谐振腔振荡方程,优化计算了量子点光纤激光器(QDFL)的最佳抽运波长、光纤长度、掺杂浓度及出射镜反射率。结果表明:饱和抽运功率为2 W,在1676nm激光波长处,QDFL最大输出功率可达1.36 W,抽运效率达68%。与通常的掺稀土离子(Yb3+、Er3+)的光纤激光器相比,QDFL具有抽运效率高、激励阈值低、掺杂密度可调、光纤饱和长度短等特点。由于量子点辐射波长的尺寸依赖特性,容易形成多波长激射或波长可调的新型激光器。     

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     

Mode competition of bichromatic laser emission in two-dimensional strong scattering random medium

The mode competition of the bichromatic laser emission in two-dimensional (2D) random medium was studied. Based on the time-dependent theory, a model for dual-wavelength random laser was established. The Maxwell equations and rate equations were combined and solved by using the finite different time domain (FDTD) method. Results show that the emission intensity of both wavelength ranges increases simultaneously with the increasing of the surface-filling fraction. The mode dominance of the bichromatic emission can be switched by changing the scatterer radius. The emission intensity of the longer wavelength increases when enlarging the excitation area. The controllability of these lasing modes may provide a potential application of optical switches.