Generation of an extreme ultraviolet supercontinuum with a multicycle chirped laser and a static electric field

We theoretically present a method for generating an ultrabroad extreme ultraviolet(XUV) supercontinuum by using the combination of a multicycle chirped laser and a static electric field.At a low laser intensity,the spectral cutoff is extended to the 495th order harmonic,and the bandwidth of the supercontinuum spectrum is broadened to 535 eV.At a high laser intensity,the harmonic cutoff is enlarged to the 667th order,and a supercontinuum covering a bandwidth of 1035 eV is generated.In these two cases,the long quantum path is removed,and the short quantum path is selected.Especially for the relatively high laser intensity,an isolated 23-attosecond pulse with a bandwidth of about 170.6 eV is directly obtained.Finally,we also analyze the influences of the chirp parameter and the duration of the chirped pulse as well as the static field strength on the supercontinuum.     

基于可调谐光纤激光器的C2H2气体光声光谱检测

研制了基于可调谐掺Er光纤激光器的共振式光声光谱乙炔气体检测系统,结合波长调制和锁相放大器的二次谐波信号检测技术,有效地消除了光声池窗片和池壁吸收入射光而引起的背景噪声,通过对该系统的光学、声学和电子检测系统的优化,实现了低浓度乙炔气体的流动式检测.实验结果证明,当气体浓度较低时,二次谐波振幅与气体浓度成正比,其线性响应相关度达到0.999 53.在常温常压和3.5 mW平均光功率以及100 ms锁相积分时间条件下,乙炔气体的极限检测灵敏度达到了0.3 ppm(1 ppm=1μg·mL-1)(SNR=1时),系统用可调谐掺Er光纤激光器代替半导体激光器作光源,降低了成本,为发展低成本、实用、便携式微量气体光谱榆测仪器奠定了基础.若采用多光程光声池,或者采用EDFA提高激光功率,可大幅度提高信噪比,将极限检测灵敏度提高至ppb(1 ppb=1 ng·mL-1)量级.     

Role of entropy generation minimization in thermal optimization

Thermal optimization is very important for improving the performances of thermal systems. In engineering, the entropy generation minimization(EGM) has been widely used to optimize and evaluate the performances of thermal systems.However, the consistency between the EGM and the optimization objective should be specified when the EGM is used.In this paper, we discuss the view angle of irreversibility of entropy generation, and show that entropy generation directly reflects the exergy destruction or the ability loss of doing work. As the design objective in a thermal system is not often consistent with the view angle of irreversibility of entropy generation, the EGM may not lead to the optimal value of the design objective. In heat transfer and heat-work conversion, the inconsistence between the design objectives and the EGM is shown with some examples, and the applicability of the EGM is found to be conditional. The "entropy generation paradox" in heat exchanger analyses is also discussed, and it is shown that there is no direct monotonic relation between the minimum entropy generation rate and the best heat transfer performance of heat exchangers.     

从机械能守恒的角度推导单摆简谐振动的周期

本文给出了利用基本的物理原理结合高等数学公式的方法推导单摆做简谐运动的周期公式,可以简明扼要地理解单摆简谐运动的过程.     

An improved injector bunching geometry for ATLAS

The bunching system of the ATLAS positive ion injector (PII) has been improved by relocating the harmonic buncher to a point significantly closer to the second stage sine-wave buncher and the injector LINAC. The longitudinal optics design has also been modified and now employs a virtual waist from the harmonic buncher feeding the second stage sine-wave buncher. This geometry improves the handling of space charge for high-current beams, significantly increases the capture fraction into the primary rf bucket and reduces the capture fraction of the unwanted parasitic rf bucket. Total capture and transport through the PII has been demonstrated as high as 80% of the injected dc beam while the population of the parasitic, unwanted rf bucket is typically less than 3% of the total transported beam. To remove this small residual parasitic component a new traveling-wave transmission-line chopper has been developed reducing both transverse and longitudinal emittance growth from the chopping process. This work was supported by the U.S. Department of Energy under contract W-31-109-ENG-38.     

Far infrared sub-nanosecond pulse generation in GaP with a time-synchronized mode-locked double-frequency Nd: Glass laser system

Light pulses of 149 m wavelength and 700 ps duration are generated by non-collinear phase-matched difference frequency mixing of laser pulses at 1053.5 and 1061 nm in a (110) cut GaP crystal. The pump laser pulses are generated in a time-synchronized mode-locked double-frequency Nd:glass laser system consisting of a silicate glass branch and a phosphate glass branch. A photon conversion efficiency of 4 × 10^–6 is achieved. The non-linear susceptibility constant of GaP is determined to be d ₁₄ = (10 ± 1) pm V^–1.     

IR-poled second-harmonic generation in glass

Infrared (IR)-induced second-harmonic generation in the chalcogenide glasses is observed. A phenomenological approach of IR picosecond non-linear optical (NLO) response in glass is developed for the middle IR spectral range (5–15 μm). The observed effect is explained within the framework of fifth-order NLO susceptibilities. A model that reproduces the basic characteristics of the experimental data, in which the optical non-linearities caused by photoinduced electron–phonon anharmonic interactions, is proposed. The role of the IR-induced phase matching conditions in the observed phenomenon is discussed.     

Design of polarization retarder suitable for nonlinear optical systems

We present a design technique of retarders suitable for nonlinear optical systems. A thin sample of LiNbO₃ crystal is utilized as a retarder for wide-ranging applications. Analysis shows that the same crystal can be used for single-wave or two-wave retardations, changing the orientation of the crystal only. As an application, the retarder is used to polarize two waves orthogonal, as necessary for difference frequency mixing in an AgGaSe₂ crystal.     

Laser pulse heating of steel surface and flexural wave analysis

Laser gas-assisted material processing finds wide application in industry. The modelling of heating, elastic response of the substrate material, and the wave analysis gives insight into the laser workpiece interaction. In the present study, laser gas-assisted heating of steel is considered. The normal component of the thermal stress is taken as the source of load for the flexural wave generation in the material. The flexural wave generated is simulated and the wave characteristics are analyzed at four locations at the workpiece surface. The numerical scheme employing a control volume approach is introduced when solving the governing equations of flow and heat transfer while finite element and spectran element methods are used when solving the stress and wave equations. It is found that the normal component of the stress is tensile. The dispersion effect of the workpiece material, interference of the reflected beam, and partial overlapping of second mode of the travelling wave enable to identify a unique pattern in the travelling wave in the substrate.     

Bound states of the Klein－Gordon and Dirac equations for scalar and vector harmonic oscillator potentials

We give the exact bound states of the Klein－Gordon and Dirac equations with equal scalar and vector harmonic oscillator potentials.