掺杂硅石纳米粉末的染料随机激光发射

针对染料随机激光器特性与应用研究,讨论了无序介质中荧光粒子和额外散射粒子的发光性质、浓度变化以及颗粒尺寸与随机激光发射阈值之间的关系。采用时域有限差分法直接求解Maxwell方程组及速率方程组的方法,模拟仿真出了相应的发射谱线及浓度变化与激光发射阈值的关系曲线。所设计的样品中Rh6G-SiO2的质量分数为4%,其阈值大小为8.5 J/pulse时,额外微小散射体TiO2的加入对该介质随机激光发射产生的影响很小,可忽略不计。在此基础上,通过进一步模拟分析了处于不同条件和背景下随机激光的阈值特性。     

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     

Maximum coefficient of light extinction in a nonabsorbing dispersive medium

The maximum value of the light extinction coefficient μ, which can be observed in a dispersive medium with a relative refractive index n of the scattering particles, is studied within the framework of a quasi-crystalline approximation for nonabsorbing dispersive media consisting of monodisperse spherical scatterers. A change in the diffraction parameter x of the scattering particles and their volume concentration c _v is accompanied by nonmonotonic variations of the extinction coefficient, and the function μ(x, c _v ) exhibits several maxima. The dimensions and concentrations of particles are determined, for which the extinction coefficient reaches the absolute maximum μ^max. The μ^max value exhibits a monotonic growth with increasing relative refractive index n of the scattering particles. The conditions of validity of the Ioffe-Regel criterion of radiation localization have been studied. It is established that the localization in nonabsorbing dispersive media can be observed only for n ? 2.7. The intervals of x and c _v in which the criterion of radiation localization is satisfied in dispersive media consisting of particles with n = 3.0 and 3.5 are determined.     

Effect of particle size and shape on nonresonant random laser action of dye-doped polymer random media

We present an experimental study of the light emission from dye-doped polymer random media dispersed with TiO₂ particles of various sizes, shapes, and structures. Random lasing with nonresonant feedback, similar to that for spherically shaped particles that are used for conventional random lasers, is observed for almost all types of particles and aggregates. The efficiency of random lasing for each medium is analyzed using the relationship between the emission spectrum and the transport mean free path (TMFP), which is measured by enhanced backscattering experiments. Results show that the peak emission intensity depends strongly on the particle shape and structure, whereas the spectral linewidth is governed by the TMFP.     

Lasing with resonant feedback in weakly scattering random systems

Laser action in active random media in the weak scattering regime far from Anderson localization is investigated by coupling Maxwell's equations with the rate equations of a four-level atomic system. We report systematic lasing action with resonant feedback and show that the lasing modes mostly consist of traveling waves spatially extended over the whole system. Next we address the question of the origin of the feedback mechanism in such a system where no disorder-induced long-lived resonances are available, and present strong evidence that they correspond to the quasimodes of the passive system. This in turn provides an original way to access the spatial distribution of the quasimodes of a non-Hermitian system.     

Scattering Processes of Excited Muonic Atoms by Molecular Hydrogen

The coupled-channel approach of an excited exotic atom scattering from a hydrogen molecule in the rigid-rotator approximation is developed. The K-matrix method is used to solve the set of coupled collisional equations in the space-fixed (SF) framework for the systems pμ+H₂, dμ+D₂, pμ+D₂, and dμ+H₂. The elastic and Stark mixing cross-sections for scattering on molecular ortho- and para-states are calculated for the principle quantum number n = 2 and collisional energies E≥0.1 eV.     

Surface polaritons on semi-infinite anisotropic media

We have investigated the nature of polaritons which propagate on the surface of semi-infinite anisotropic dielectric and magnetic media and find that the surface polaritons are regular surface waves [i.e., the penetration constant α (ω) is real] only for special orientations of the wave vector k and of the normal to the surface n relative to the principal axes. In the case of biaxial crystals, regular surface polaritons occur when k and/or n is along a principal axis of the dielectric tensor ε(ω) for dielectric media [or the magnetic permeability tensor μ(ω) for magnetic media]. When k and n are both directed along principal axes, the surface polaritons are TM with E in the saggital plane for dielectric media. One finds that surface polaritons which are coupled photon-virtual surface dipole excitation modes can exist on “surface active” anisotropic media in addition to the familiar coupled photon-real surface dipole excitation modes. When neither k nor n are along principal axes, the surface polaritons that occur are, with the exception of special orientations, “generalized” surface waves [i.e., α(ω) is complex]. The Poynting vectors of the propagating surface polaritons are always in the plane of the surface but not, in general, along k. These results, which hold for uniaxial crystals, are also applicable to gyromagnetic media and to gyrodielectric media. This theory is also applicable to surface polaritons which propagate along the interface between two media, one or both of which is anisotropic, and with one or the other serving as the “surface active” medium.     

二维随机介质中准态模的频谱时间演化特性

采用时域有限差分法数值求解Maxwell方程组，分析了平面随机介质中光波模式的频谱时间演化特性.随机介质的特征可以用散射颗粒的随机构形、介质参数（如颗粒的尺寸和填充率等）以及介质形态等因素来描述.这些因素决定了随机介质在准稳态下模式的频率特性与数量，但具有不同因素的随机介质中模式的产生、选择与演化，具有大致相同的特征与速度.平面随机介质的这些冷腔特性，与传统光腔中模式的产生、选择和演化的特征非常相似. 关键词： 激光物理 随机激光器 无序介质中的光学特性     

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.     

Dependence of viscoelastic parameters of nematic liquid crystals on pretilt angle and temperature

The viscoelastic parameters of nematic liquid crystal (LC) E7 in both splay and twist relaxation modes are investigated as functions of pretilt angle and temperature by the technique of dynamic laser light scattering. The results show that the elastic constants of the liquid crystal in the two modes not only depend on temperature, but also depend on pretilt angle. There is a critical pretilt angle (β_C) at which nematic LC begins exhibiting elastic property, and beyond which the elastic constants increase with pretilt angle, then keep constant after exceeding another angle (β_S, with β_S > β_C). This phenomenon is observed to be universal for different nematic LC’s, and significant not only in the understanding of the molecular mechanism of exhibiting viscoelastic properties in nematic LC, but also for their practical purpose.     

The stochastic properties of a molecular-dynamical Lennard-Jones system in equilibrium and nonequilibrium states

The K-entropy and the t _m time of dynamical memory (the time of forgetting initial conditions during numerical integration) of a classical system of particles whose interactions are governed by the Lennard-Jones potential were calculated by the method of molecular dynamics. The K value was a characteristic of a system of many particles, and the t _m value proved to increase logarithmically as fluctuations of the total energy of the system decreased; that is, as the accuracy of numerical integration increased. Two different K-entropy values corresponding to the same total energy of the system were found to exist, namely, K _e for the equilibrium and K _n for the nonequilibrium state. The rate of kinetic energy relaxation (t _r ^?1 ) was shown to equal K _n, and the K _n value was found to be a more fundamental characteristic than (t _r ^? ). The density dependences of K _e (monotonic) and K _n (nonmonotonic) were calculated. The transition from dynamical (Newtonian) correlations to stochastic for the velocity autocorrelation function was considered. The reasons for the finiteness of dynamical memory in physical processes are discussed. The duration of dynamical correlations in real systems is limited by quantum uncertainty and is of the order of picoseconds.     

Light scattering by polariton modes of KNbO3 in orthorhombic phase

The polariton modes of A₁, B₁ and B₂ species in biaxial KNbO₃ are observed by near forward Raman scattering. The oscillator strengths and dielectric constants along the principal axes are determined from the polariton dispersion. The mode assignments of each species are also discussed.     

本征态的局域化与增益介质中局域态的放大

用时域有限差分方法(FDTD)模拟了二维增益、非增益随机介质中的光学现象。在非增益介质中，观察到了局域态空间分布及其频率特征。在增益介质中，观察到局域态的放大过程。模拟结果显示非增益介质中的局域态相当于传统激光腔的腔模，随机激光是由局域态谐振产生的。     

Conformal field theory and the symmetries of string field theory

The two-dimensional conformal field theory representation of Witten's bosonic string field theory is discussed. The basic overlap equations, K_n symmetry and BRST invariance are proved directly, without the usual expansion in oscillators. The conformal field theory approach naturally provides local overlap identities which (when integrated over half the string) can be used to verify properties of the cubic action. In particular, a recently proposed diffeomorphism invariance is shown to be free of anomalies. Finally, a new class of symmetries, including generalizations of the K_n symmetries which are local in spacetime, are presented.     

活性无序介质中泵浦功率对局域模发射的影响

基于具有局域-非周期-类结构的活性无序介质中的复杂局域模与合适泵浦光的相互作用模型,研究活性无序介质中泵浦功率对局域模发射的影响,定性解释了泵浦功率对局域模发射的影响.结果表明,局域-非周期-类结构的无序介质的透射谱只与介质的结构有关,而与介质的损耗和增益无关,不同的激发模具有不同的泵浦功率阈值.     

Random lasing in an organic light‐emitting crystal and its interplay with vertical cavity feedback

The simultaneous vertical‐cavity and random lasing emission properties of a blue‐emitting molecular crystal are investigated. The 1,1,4,4‐tetraphenyl‐1,3‐butadiene samples, grown by physical vapour transport, feature room‐temperature stimulated emission peaked at about 430 nm. Fabry‐Pérot and random resonances are primed by the interfaces of the crystal with external media and by defect scatterers, respectively. The analysis of the resulting lasing spectra evidences the existence of narrow peaks due to both the built‐in vertical Fabry‐Pérot cavity and random lasing in a novel, surface‐emitting configuration and threshold around 500 μJ cm⁻². The anti‐correlation between different modes is also highlighted, due to competition for gain. Molecular crystals with optical gain candidate as promising photonic media inherently supporting multiple lasing mechanisms.     

An analysis of the radial flow in the heterogeneous porous media based on fractal and constructal tree networks

In this paper, an analysis of the radial flow in the heterogeneous porous media based on fractal and constructal tree networks is presented. A dual-domain model is applied to simulate the heterogeneous porous media embedded with a constructal tree network based on the fractal distribution of pore space and tortuosity nature of flow paths. The analytical expressions for seepage velocity, pressure drop, local and global permeability of the network and binary system are derived, and the transport properties for the optimal branching structure are discussed. Notable is that the global permeability (K_n) of the network and the volume fraction (f_n) occupied by the network exhibit linear scaling law with the fractal dimension (D_p) of channel diameter bylogK_n∼0.46D_p and logf_n∼1.03D_p, respectively. Our analytical results are in good agreement with the available numerical results for steady-state soil vapor extraction and indicate that the fractal dimension for pore space has significant effect on the permeable properties of the media. The proposed dual-domain model may capture the characteristics of heterogeneous porous media and help understanding the transport mechanisms of the radial flow in the media.     

Rate equation approach for diode lasers

The light output and the electron density within the active layer of a semiconductor laser have been calculated from the steady state solution of a rate equation approach. The assumed rate equation model takes into account the spontaneous emission into the lasing modes. Simple analytical approximation formulae have been found under the assumption of a power-law dependence between optical gaing and electron densityn (g ∞n′). The approximations are compared with numerical results from the rate equations. With the exception of a small region near the lasing threshold good agreement has been found. The theoretical results are compared with experimentally measured light outputs from single and double heterostructure lasers. From the comparison below threshold the power law exponent/may be evaluated. The comparison at threshold suggests that the spontaneous emission term decreases at threshold. This decrease is in agreement with experimentally measured changes of the emission bandwidth at threshold.     

Light scattering in a layer of correlatively arranged optically soft particles

The light scattering by an ensemble of monodisperse spatially correlated optically soft spherical particles is studied in the interference approximation. A model of the interaction of particles is proposed in which the spatial correlation between particles is determined by a radius R _c exceeding the particle radius R _p. The radial distribution function is calculated in the Percus-Yevick approximation for hard spheres of the radius R _c. To simulate the radiation scattering from an individual particle of the radius R _p, the Mie equations are used. It is shown that, in a medium of correlated small nonabsorbing particles of the radius R _c > R _p, an abnormal wavelength dependence of the refractive index is possible at a low volume concentration of particles. The results obtained explain some experimentally observed features of the scattering in sodium borosilicate glasses with a small concentration of scattering centers.