5fs驱动激光脉冲的高次谐波选择性优化

研究了在紧聚焦实验条件下, 采用5 fs激光脉冲与氩气相互作用产生的高次谐波特性. 通过优化系统色散、气体靶气压与位置等参数, 观察到在60–73 eV波段范围内高次谐波光谱接近一个量级的增强. 进一步通过对单原子模型和传播方程的数值求解及模拟相位匹配实验参数下高次谐波的产生过程, 发现相位匹配在所观察到的实验现象中起着关键作用, 得到了理论上与实验规律一致的结果. 关键词： 极紫外激光 高次谐波 超快光谱     

Theoretical analysis of porous radiant burners under 2-D radiation field using discrete ordinates method

This paper describes a theoretical study to investigate the heat transfer characteristics of porous radiant burners (PRBs). In the present work, a 2-D rectangular model is used to solve the governing equations for porous medium and gas flow before the premixed flame to the exhaust gas. The gas and the solid phases are considered in non-local thermal equilibrium and combustion in the porous medium is modeled by considering a non-uniform heat generation zone. The homogeneous porous media, in addition to its convective heat exchange with the gas, may absorb, emit and scatter thermal radiation. The radiation effect in the gas flow is neglected but the conductive heat transfer is taken into account. In order to analyze the thermal characteristics of porous burners, the coupled energy equations for the gas and porous medium in steady condition are solved numerically and the discrete ordinates method (DOM) is used to obtain the distribution of radiative heat flux in the porous media. Finally, the effects of various parameters on the performance of porous radiant burners are examined. The present results are compared with some reported theoretical and experimental results by other investigators and good agreement is found.     

Generation of terahertz radiation by the optical breakdown induced by a bichromatic laser pulse

Experimental results are presented on generation of terahertz radiation by the air breakdown induced by a high-intensity laser pulse having not only a fundamental component, but also a second-harmonic one. A theoretical explanation of the experimental data is proposed, based on a model of field ionization of a gas by a bichromatic laser.     

压气机特性通用数学表达式

压气机特性通用数学表达式张娜，林汝谋，蔡睿贤（中国科学院工程热物理研究所／北京１０００８０）关键词压气机，特性线１引言求解燃气轮机全工况问题首先要涉及部件特性和负荷特性的描述。各部件中，压气机实验花费昂贵，特性线数据一般很少公开发表。即便已掌握了特性...     

掩膜板凸出环隔离压缩式纳米压印施压气体的研究

在半导体微纳加工技术中, 纳米压印由于具备低成本、高产出、超高分辨率等诸多优势而备受研究者和半导体厂商的青睐, 有望成为下一代光刻技术的重要备选支撑技术之一. 然而在其施压流程中, 由于气体诱捕或陷入所造成的气泡缺陷问题直接关系到图案复制的成功率和完整性, 因此避免气泡缺陷, 阻止气泡进入模穴是亟待解决的关键问题. 提出一种适用于在气体环境中进行气压压缩式纳米压印工艺并避免气体进入掩膜板基板间隙的方法. 采用带有刻蚀一定宽度凸出环的掩膜板, 凸出环与基板形成环板毛细缝隙, 图形转移介质流体在其中形成毛细液桥, 使掩膜板-介质-基板形成独立的封闭腔, 转移介质黏附力所产生的静摩擦力及介质流体表面张力所诱导的毛细力抵抗施压气体, 有效地阻止气体进入空穴形成气泡缺陷.通过理论解析推导求出针对具有不同表面特性转移介质流体的凸出环有效宽度, 为掩膜板制备提供理论依据. 关键词： 纳米压印 凸出环 毛细液桥 静摩擦力     

Generating molecular rovibrational coherence by two-photon femtosecond photoassociation of thermally hot atoms

The formation of diatomic molecules with rotational and vibrational coherence is demonstrated experimentally in free-to-bound two-photon femtosecond photoassociation of hot atoms. In a thermal gas at a temperature of 1000 K, pairs of magnesium atoms, colliding in their electronic ground state, are excited into coherent superpositions of bound rovibrational levels in an electronically excited state. The rovibrational coherence is probed by a time-delayed third photon, resulting in quantum beats in the UV fluorescence. A comprehensive theoretical model based on ab initio calculations rationalizes the generation of coherence by Franck-Condon filtering of collision energies and partial waves, quantifying it in terms of an increase in quantum purity of the thermal ensemble. Our results open the way to coherent control of a binary reaction.     

Consequence analysis of blast wave from accidental gas explosions

Recently, consequence analyses of accidental gas explosions are often carried out to assess the risk of chemical plants, hazardous-materials sites and new energy systems. In these consequence analyses, it is indispensable to adequately predict the blast-wave (pressure-wave) intensity from gas deflagrations. Some prediction models already exist; however, most of them are based on the theory for explosives and adjusting parameters are needed for evaluating gas deflagrations. In this study, new prediction methods for gas deflagrations were developed. From theoretical analysis of blast-wave generation by a gas deflagration, an evaluation equation of the blast-wave intensity was derived. As the scale of gas deflagration becomes larger, flame front instability (especially hydrodynamic instability) would be more effective and the flame propagating velocity starts to be accelerated. Therefore, the equation was modified considering the effect of flame instability. The evaluations by this modified equation agreed well with the results of large scale experiments. By this analysis, it was found that not only total energy release but also combustion reaction rate has to be introduced into the prediction of gas deflagrations. Using this concept, a modified scale model to predict the blast-wave intensity was developed by improving the previous scale model introducing the term of combustion reaction rate as burning velocity. Furthermore, scale analysis was performed to develop the new scaling law. The universal relationship between scaled distance and overpressure has been realized by this new scaling law for gas deflagrations. In summary, these results provide new methods for accurate prediction of the blast-wave intensity from gas deflagrations.     

Generation of strong Langmuir fields during optical breakdown of a dense gas

Results are presented from theoretical studies and computer simulations of the resonant excitation of Langmuir waves during the ionization of a homogeneous gas by high-intensity laser radiation. Two mechanisms for the formation of nonuniform resonant structures in the discharge are examined: plasma-resonance ionization instability, resulting in the density modulation along the electric field vector, and gas breakdown in the field of a transversely inhomogeneous laser beam (a Bessel beam produced by an axicon lens). In both cases, the transition of the plasma density through the critical value is accompanied by the generation of intense Langmuir waves, the formation of fast ionization fronts, and the appearance of long-lived quasi-turbulent states.     

Singlet delta oxygen generation for Chemical Oxygen-Iodine Lasers

The development of Chemical Oxygen-Iodine Lasers is based on the generation of singlet delta oxygen. To improve the overall efficiency of these lasers, it is necessary to increase the generator production and yield of singlet delta oxygen at low and high pressure, respectively, for subsonic and supersonic lasers. Furthermore, the water vapor content must be as low as possible. A generator model, based on gas-liquid reaction and liquid-vapor equilibrium theories associated with thermophysical evaluations is presented. From model predictions, operating conditions have been drawn to attain the following experimental results in a bubble-column: by increasing the superficial gas velocity, the production of singlet delta oxygen is largely improved at low pressure; by mixing chlorine with an inert gas before injection in the reactor, this yield is maintained constant up to higher pressure. A theoretical analysis of these experimental results and their consequences for both subsonic and supersonic lasers are presented.This work was performed with the support of the Direction des Recherches, Etudes et Techniques.     

Special features of electron beam-excited XeCl-laser generation

The results of experimental investigations on generation in Ar(Ne): Xe: CCl₄(HCl) mixtures at excitation by an electron beam are given. It is shown that the use of CCl₄ and HCl with an Ar buffer gas gives equal radiative energies and efficiencies and ā change of Ne for Ar causes a change in generation spectrum. The special features of XeCl-laser generation are discussed.     

Generation of terahertz radiation by the axicon focusing of ionizing laser pulses

JETP Letters - Experimental results on the generation of terahertz radiation in the gas breakdown by high-power quasi-monochromatic laser pulses focused by an axicon lens are presented. A...     

Extreme nonlinear optics in a femtosecond enhancement cavity

Intrinsic to the process of high-order harmonic generation is the creation of plasma and the resulting spatiotemporal distortions of the driving laser pulse. Inside a high-finesse cavity where the driver pulse and gas medium are reused, this can lead to optical bistability of the cavity-plasma system, accumulated self-phase modulation of the intracavity pulse, and coupling to higher-order cavity modes. We present an experimental and theoretical study of these effects and discuss their implications for power scaling of intracavity high-order harmonic generation and extreme ultraviolet frequency combs.     

Harmonic generation by an intense laser pulse in neutral andionized gases

Reported are the results of a harmonic generation experiment in a simple gas (hydrogen) using 1-ps, 1-μm laser pulses with a range of intensities extending from below to far above the laser ionization saturation threshold. The scaling with intensity above saturation of the third harmonic generated by a single laser-pulse in a filled gas cell is observed not to fit with a simple model that takes into consideration volume ionization effects alone. In another experiment, a pump-probe type, an upper limit on the conversion efficiency of third-harmonic generation in a preformed plasma is determined. It is found to be in agreement with the efficiency predicted by a relativistic harmonic generation theory     

Analysis of the force action of a gas jet on the surface of a liquid

The results of theoretical and experimental investigations of the interaction of a gas jet with the surface of a viscous liquid are reported. Relations are derived for calculating the force exerted by the gas jet on the liquid surface. A dependence of the gas jet compression ratio in the nozzle outlet on the pressure in front of the nozzle is revealed. The values of the shape factor for the indentation formed by the gas jet on the surface of the liquid are determined for various diameters of the indentation and the distances between the outlet hole of the nozzle and the liquid surface. The theoretical conclusions formulated here are confirmed by the results of experiments.     

Plasma dynamically induced frequency shifts in high-order harmonic generation in nitrogen

Experiments and theoretical calculations on high-harmonic generation in nitrogen are presented in the regime of laser pulses of a 300-ps duration, where the plasma dynamics following the ionization of the medium plays a decisive role. The experiments are performed with ～4-GW Ti:sapphire laser pulses, giving rise to fully saturated ionization. The shifts between the exact harmonic frequency in the extreme ultraviolet and the integer multiple of the fundamental frequency are caused by the self-phase modulation of the laser pulse due to the time-dependent free-electron density in the plasma generated in the focal zone. Well-calibrated atomic resonances in the extreme ultraviolet measured through absorption in a secondary gas jet are used as frequency markers in the extreme ultraviolet for the accurate determination of the sign and magnitude of the frequency shifts. A theoretical model including both plasma dynamics and harmonic generation from atoms and ions has previously been developed, and successfully applied to explain the frequency red shift observed in xenon [Phys. Rev. Lett. 96, 123904 (2006)]. The plasma-dynamical model is extended and applied to the results of the harmonic generation in nitrogen, fully explaining the observed harmonic frequency shifts in the 9th and 13th harmonic.     

Released momentum distribution of a Fermi gas in the BCS-BEC crossover

We develop a time-dependent mean-field theory to investigate the released momentum distribution and the released energy of an ultracold Fermi gas in the BCS-BEC crossover after the scattering length has been set to zero by a fast magnetic-field ramp. For a homogeneous gas we analyze the nonequilibrium dynamics of the system as a function of the interaction strength and of the ramp speed. For a trapped gas the theoretical predictions are compared with experimental results.     

三代微光像增强器分辨力计算理论模型

分辨力和传递函数MTF是微光像增强器的2个重要参数。长期以来，人们对于三代微光像增强器阴极发出的电子初能量分布没有统一的认识，从而没有一个公认的分辨力和MTF计算模型。通过理论分析和假设，给出一定条件下一个分辨力计算模型。把实际测得的第一近贴距、第二近贴距、阴极电压和荧光屏电压等数值代入分辨力计算模型中，可以得到分辨力理论值。经与实际测量值进行比对，发现二者偏差值在12.3%以内，此理论模型基本符合实际需求。该分析方法和所得结果有一定实用价值，可作为设计三代微光像增强器的技术参考。     

Low‐wavelength emission of Nd‐doped lasers

This paper gives an overview of the results obtained with diode‐pumped Neodymium‐doped crystals operating below 900 nm. Operation at such low wavelengths requires considering the strong thermal population of the lower level of the laser transition. Based on a theoretical study and simulations, the paper presents the challenges related to the design of these three‐level lasers. Experimental results are given with Nd:YAG and Nd:vanadate crystals. It is explained how to deal with the line competition with emission at 946 nm or 912 nm. Finally, intracavity second‐harmonic generation is presented. The output powers reach a few hundred mW at wavelengths below 450 nm. Hence, the paper demonstrates the potential of Nd‐doped diode‐pumped solid‐state lasers for applications in the blue range, in replacement of gas lasers such as helium‐cadmium lasers.