Experimental evidence of absolute and convective instabilities in optics

Experimental evidence of convective and absolute instabilities in a nonlinear optical system is given. In optics, the presence of spatial nonuniformities brings in additional complexity. Hence, signatures characterizing these two regimes are derived based on analytical and numerical investigations. The corresponding noise-sustained and dynamical patterns are observed experimentally in a liquid crystal layer subjected to a laser beam with tilted feedback.     

Dual-Frequency Generation in Quantum Cascade Lasers of the 8-μm Spectral Range

Results of an investigation of quantum cascade lasers of the 8-μm spectral range generating dual-frequency radiation at room temperature are presented. The dependences of radiation intensity for two lines of laser generation obtained as a result of investigation of the generation spectra revealed the presence of saturation and decay of the long-wavelength line of generation and linear growth of the intensity of the short-wavelength one. Based on analysis of the obtained data, a mechanism of the dual-frequency generation in the studied QCL samples is proposed Generation.     

Spicules of glass sponges as a new type of self-organizing natural photonic crystal

The results of studying the optical characteristics of basal spicules of sea glass sponges Hyalonema sieboldi are presented. These objects are shown to be a natural nanocomposite biomaterial and to consist of a large number of axial layers of hydrated quartz that possesses a number of unique optical properties. It is found that, upon the excitation of spicules by second-harmonic pulses from an Nd:YAG laser, a considerable increase in the fluorescence intensity in the long-wavelength range with a maximum at 770 nm, saturation and anomalously large fluorescence lifetimes are observed. It has been shown experimentally that spicules exposed to femtosecond radiation pulses exhibit nonlinear optical properties. Based on the data obtained, it is concluded that spicules of glass sponges are a new type of self-organizing natural photonic crystals and are a promising object for photonics.     

Optical nonuniformities in nuclear-pumped lasers with plane uranium layers

Optical nonuniformities in the nuclear-pumped gas lasers are calculated. The nonuniformities are formed due to inhomogeneous energy deposition by the fission fragments from thin plane uranium layers that irradiate the laser active gas. The results of calculations agree with experimental data.     

Effect of nonlinear gain on the phase noise of Y-branch lasers

Effect of nonlinear gain on the phase noise of modulated grating Y-branch (MGY) lasers is investigated by experiments and simulations. The phase noise is first characterized by measuring the frequency modulation noise spectrum of the MGY laser, and some interesting phenomena are observed. In order to understand the underlying physic mechanism of those phenomena, simulations are performed with taking Langevin noise sources and nonlinear gain terms into account. Simulated results show that, in the presence of the nonlinear gain, fluctuations of side-modes can bring excess phase noise to the lasing mode, especially when the side-modes are on the long-wavelength side of the main mode. Furthermore, it has been found that the phase noise hopping phenomenon can be induced by a bi-stable state of the laser, which is also closely related to the nonlinear gain. The investigation is helpful for explaining the complicated phase noise characteristics of the MGY laser.     

Coherence and saturation properties of second-harmonic generation with a nonlinear crystal inside the laser cavity

The basic equations for second-harmonic generation including noise are derived for the case that the nonlinear crystal is put inside the laser cavity. A realistic model of a (detuned) laser with two-level atoms in single-mode operation is taken using the nonlinear theory of laser noise which describes the laser saturation effects, the phase diffusion and the intensity fluctuations. The reaction of the second-harmonic field on the fundamental field is taken into account as well as the reaction of the fundamental field on the laser. The nonlinear crystal is described by microscopic anharmonic oscillator equations (without introducing nonlinear susceptibilities by perturbation theory). The saturation of the polarization of the nonlinear medium is taken into account exactly with the only assumption that the influence of third and higher harmonics should be small. The electromagnetic field is described semiclassically by stochastic equations. In all equations, the damping is introduced simultaneously with Markoffian fluctuating forces by coupling to heatbaths. The equations are solved exactly in the stationary state without noise (the time dependent solution including noise will be presented in a subsequent paper). The most important saturation effect is a frequency shift which depends on the laser intensity.     

Ultrafast nonlinear optical response of Ag:BaTiO3 composite films at the near-ultraviolet wavelengths

The ultrafast nonlinear optical response of Ag:BaTiO3 composite films synthesized by pulse laser deposition (PLD) is studied at the near-ultraviolet (400 nm) wavelength. The pulse duration of the laser used in the measurement is 200 fs. The real and imaginary parts of the third-order nonlinear susceptibility x(3) of the composite materials are measured. The composite films indicate self-focusing effect and nonlinear saturation, the values of Rex(3) and Imx(3) are measured to be 3.42 × 10-10 and -1.37 × 10-10 esu respectively, which are much lower than those reported before. It is mainly because that the duration of the laser used in the measurements is too short to induce the hot electron excitation and the thermal effect. The ultrafast nonlinear saturated absorption response of the Ag:BaTiO3 composite films may have potential applications for femtosecond laser mode locking.     

Effect of spatial refractive-index nonuniformities existed in a large-scale rapid growth KDP crystal on third-harmonic conversion

Effect of spatial refractive-index nonuniformities existed in a large-scale rapid growth KDP crystal (a type I frequency doubler at the wavelength of 1053 nm) on third-harmonic conversion is studied. Spatial refractive-index nonuniformities due to imperfections in the crystal structure which prohibit the attainment of phase matching at all locations in the crystal are measured with the orthogonal polarization interferometry technique. And a third-harmonic conversion experiment is conducted on a large-scale facility to investigate the influence of refractive-index nonuniformities on conversion efficiency and third-harmonic beam nearfield. According to the experimental results, refractive-index nonuniformities in this large-scale rapid growth KDP doubler do not reduce the third-harmonic conversion efficiency, but affect the third-harmonic beam nearfield.     

Effect of picosecond radiation intensity on nonlinear absorption of glasses with silver nanoparticles

The dependence of the process of nonlinear absorption on the intensity of laser radiation of a picosecond duration was studied with the use of the second harmonic of a Nd:YAG laser (λ = 532 nm, τ = 55 ps). The values of the nonlinear absorption coefficient and of the imaginary part of the third-order nonlinear susceptibility of materials were measured. The nonlinearity of absorption in the materials under study was found to be caused by the saturation of absorption.     

Increasing hydrodynamic efficiency by reducing cross-beam energy transfer in direct-drive-implosion experiments

A series of experiments to determine the optimum laser-beam radius by balancing the reduction of cross-beam energy transfer (CBET) with increased illumination nonuniformities shows that the hydrodynamic efficiency is increased by ～35%, which leads to a factor of 2.6 increase in the neutron yield when the laser-spot size is reduced by 20%. Over this range, the absorption is measured to increase by 15%, resulting in a 17% increase in the implosion velocity and a 10% earlier bang time. When reducing the ratio of laser-spot size to a target radius below 0.8, the rms amplitudes of the nonuniformities imposed by the smaller laser spots are measured at a convergence ratio of 2.5 to exceed 8 μm and the neutron yield saturates despite increasing absorbed energy, implosion velocity, and decreasing bang time. The results agree well with hydrodynamic simulations that include both nonlocal and CBET models.     

不同激光脉冲宽度下甲苯的光学非线性特性的研究

在不同激光脉冲激发下,用Z 扫描法测量了甲苯的三阶光学非线性,发现其非线性折射率随着脉冲宽度增加而增大（从150 fs时的3.7×10－16 cm2/W到4 ns时的2.6×10－14 cm2/W）.在波长532 nm、光强4.73×1014 W/m2 和21 ps的脉冲作用下,甲苯溶液呈现反饱和吸收现象.结果表明,该吸收效应主要来自于导致该分子中苯环结构π－π电子跃迁的双光子吸收的贡献.     

Nonlinear theory of the ablative Rayleigh-Taylor instability

A fully nonlinear sharp-boundary model of the ablative Rayleigh-Taylor instability is derived and closed in a similar way to the self-consistent closure of the linear theory. It contains the stabilizing effect of ablation and accurately reproduces the results of 2D DRACO simulations. The single-mode saturation amplitude, bubble and spike evolutions in the nonlinear regimes, and the seeding of long-wavelength modes via mode coupling are determined and compared with the classical theory without ablation. Nonlinear stability above the linear cutoff is also predicted.     

电荷束团横向非均匀密度分布引起的发射度增长

直线加速器中电荷束团的非线性效应是导致束流发射度增长的一个重要原因。本文给出了直线加速器中几种常见的横向非均匀电荷密度分布的有限长空间电荷束团所具有的非线性自场能,得到了由束团非均匀电荷密度分布引起的发射度增长。     

双模瑞利-泰勒不稳定性的预热烧蚀效应研究

针对双模扰动下的烧蚀瑞利-泰勒不稳定性增长问题,采用高精度的数值计算方法,研究了不同预热程度下模耦合产生的多个高次谐波幅值的发展和演化问题。研究表明,三种预热烧蚀条件下,当扰动基模满足长波与短波耦合方式时,谐波中的长波模态占主导,而短波模发展明显受到抑制;当满足短波与短波耦合时,耦合结果带来了许多新的增长较快的长波模态,此时短波模增长呈现小幅震荡形式。比较两种耦合方式可以发现,长波结构在烧蚀瑞利-泰勒不稳定性弱非线性阶段都占主导地位,尤其是短波与短波耦合中气泡与尖钉表现出不同于两个基模的长波模结构。进一步分析预热效应对模耦合增长的影响,发现预热程度越强就越能削弱耦合谐波的增长,这说明预热对烧蚀瑞利-泰勒不稳定性具有致稳作用,这对惯性约束聚变工程中控制烧蚀瑞利-泰勒不稳定性发展具有重要意义。     

The features of nonlinear absorption during resonance one- and two-photon excitation of the basic exciton transition in colloidal CdSe/ZnS quantum dots

The features of the nonlinear absorption of CdSe/ZnS quantum dots (colloidal solution) in the case of resonant one- and two-photon excitation of the basic exciton transition by powerful ultra-short laser pulses were determined. In one-photon excitation, with an increasing intensity of impulses, a decrease in absorption (bleaching) is relayed by an increase in absorption, which is associated with the process of the filling of the states (saturation) of a two-level system with the lifetime of the excited state depending on the light intensity. The arising Fresnel or Fraunhofer diffraction of the laser ray that pass through a colloidal solution with a high concentration of quantum dots is associated with the formation of the transparency channel and self-diffraction of laser ray on an induced diaphragm. In two-photon excitation, the features of the nonlinear absorption and luminescence tracks (the dependence of luminescence intensity on distance) were explained by the influence, in addition to the two-photon absorption, of the processes that are responsible for the slower growth of nonlinear absorption and luminescence quenching at high intensities of laser pulses.     

Three-dimensional simulation of long-wavelength free-electron lasers with helical wiggler and ion-channel guiding

A three-dimensional simulation of a steady-state amplifier model of a long-wavelength free-electron laser (FEL) with realizable helical wiggler and ion-channel guiding is presented. The set of coupled nonlinear differential equations for electron orbits and fields of TE 11 mode in a cylindrical waveguide are solved numerically by the Runge-Kutta algorithm with averages calculated by the Gaussian quadrature technique. Self-fields and space-charge effects are neglected, and the electron beam is assumed to be cold and slippage is ignored. The parameters correspond to the Compton regime. Evolution of the radiation power and growth rate along the wiggler is studied. Ion-channel density is chosen to obtain optimum efficiency. Simulations are preformed for the FEL operating in the neighborhood of 35 GHz and 16.5 GHz for the electron beam energies of 250 keV and 400 keV, respectively. The result of the saturated efficiency was found to be in good agreement with the simple estimation based on the phase-trapping model.     

掺半导体玻璃(SDG)波导非线性饱和的导波测量方法

掺半导体玻璃波导的折射率分布是通过多模波导有效折射率测量得到的.当耦合进入波导的激光功率密度变化时,可在不同模深度处,测量到波导的非线性折射率系数,并获得波导非线性折射率系数与功率密度的关系.从而得到波导的非线性饱和值为1.54×10-4.     

掺杂对CH样品Rayleigh-Taylor不稳定性增长的影响

在神光II激光装置上进行了辐射驱动不同掺杂样品的单模Rayleigh-Taylor(RT)不稳定性实验.结果显示:与纯碳氢(CH)样品相比,掺Br的CH样品的扰动更早、更快地进入非线性区,产生二次谐波,并且掺Br比例越高,CH样品扰动进入非线性区的时间越早,相同时刻扰动的二次谐波的幅度越高.这是因为密度梯度效应抑制了二次谐波的产生,掺Br比例越高,密度梯度标长越小;同时密度梯度效应还抑制三次谐波对基模增长的负反馈,造成基模具有更大的线性增长,导致线性饱和幅值大于经典值0.1λ.     

Experimental characterization of nonlinear harmonic radiation from a visible self-amplified spontaneous emission free-electron laser at saturation

Nonlinear harmonic radiation was observed using the VISA self-amplified, spontaneous emission (SASE) free-electron laser (FEL) at saturation. The gain lengths, spectra, and energies of the three lowest SASE FEL modes were experimentally characterized. The measured nonlinear harmonic gain lengths and center spectral wavelengths decrease with harmonic number, n, which is consistent with nonlinear harmonic theory. Both the second and third nonlinear harmonics energies are about 1% of the fundamental energy. These experimental results demonstrate for the first time the feasibility of using nonlinear harmonic SASE FEL radiation to produce coherent, femtosecond x rays.     

Laboratory evidence for stochastic plasma-wave growth

The first laboratory confirmation of stochastic growth theory is reported. Floating potential fluctuations are measured in a vacuum arc centrifuge using a Langmuir probe. Statistical analysis of the energy density reveals a lognormal distribution over roughly 2 orders of magnitude, with a high-field nonlinear cutoff whose spatial dependence is consistent with the predicted eigenmode profile. These results are consistent with stochastic growth and nonlinear saturation of a spatially extended eigenmode, the first evidence for stochastic growth of an extended structure.