Laser drilling of stainless steel with nanosecond double-pulse

Nanosecond double-pulse laser drilling is reported in this paper. The double-pulse herein represents two closely conjoint pulses with 21 ns pulse duration and about 52 ns interpulse separation, which are acquired by temporal pulse shaping. Percussion drilling with such double-pulse is performed in stainless steel samples with different laser fluences, sample's thickness, repetition rates and ambient pressures. The experimental results show that the drilling rates of double-pulse drilling are more than one order of magnitude higher than that of conventional single-pulse drilling in air. Differences in the processing results between single-pulse and double-pulse with various processing parameters are investigated. In addition the ablation mechanisms of the double-pulse drilling are discussed.     

导线非共线的分子器件输运性质的第一性原理研究

分子器件是电子器件向小体积化发展的极限,分子器件中的电子在输运过程中体现出明显的量子效应,分子导线与分子接触的位置和导线间的角度等器件结构因素都会对分子器件的输运性质产生较大的影响.迄今为止,尚未见利用第一性原理量子输运计算方法研究导线非共线的分子器件输运性质的报道.本文以金-苯(硫醇)-金结构的分子器件为例,利用基于非平衡格林函数理论和密度泛函理论的第一性原理量子输运计算方法对其输运性质进行了系统研究,特别注重于研究随着非共线导线间导线夹角角度的变化及导线和苯(硫醇)分子接触位置的不同对器件输运性质的影响.计算表明,金导线与苯(硫醇)的接触位置及导线的夹角等器件结构细节不仅能够定量地影响金-苯(硫醇)-金分子器件的电流大小,还能够定性地改变器件的输运性质,使得部分器件结构出现负微分电阻效应.研究结果对全面理解分子器件的输运性质具有一定的指导意义.     

Effect of experimental conditions on surface hardness measurements of calcified tissues via LIBS

This paper reports on the effects of LIBS experimental conditions on the measurement of the surface hardness of calcified tissues. The technique mainly depends on a previously demonstrated correlation between the intensity ratio of ionic to atomic spectral lines and the hardness of the target material. Three types of calcified tissues have been examined, namely enamel of human teeth, shells, and eggshells. Laser-induced breakdown spectra were obtained under two different experimental conditions. In the first nano and picoseconds, laser pulses were used in a single-pulse arrangement, while in the second, single- and double-pulse regimes with nanosecond laser excitation were utilized. The results show that the ionic to atomic spectral line intensity ratios are higher in the case of picosecond laser pulse for both Ca and Mg spectral lines. This effect has been justified in view of the repulsive force of the laser-induced shock waves which depends clearly on the target surface hardness and on the laser irradiance. The electron densities ratio (pico/nano) is shown to be strongly depending on the laser irradiance too. In the case of calcium, single-pulse ratios are higher than the double-pulse ratios, while there is no appreciable difference between both in the case of magnesium. The results obtained herein suggest that double-pulse nanosecond arrangement and the choice of a minor element such as Mg furnishes the best experimental conditions for estimating the surface hardness via LIBS spectra. To validate this method, it has been applied on two previously measured groups of teeth enamel, the first is of ancient Egyptians, and the second from Nubians and Ugandans. The results support the usefulness of this method for similar real-life applications.     

On the detection of closed orbits in time-dependent systems by using double-pulse lasers

In the photodetachment of atoms or negative ions by a double-pulse laser, the first pulse of the double-pulse laser generates waves and the delayed second pulse may detect them. The phenomenon of the excitation and detection of waves by a double-pulse laser can be used to identify the closed orbits in the system. We demonstrate this phenomenon with a negative hydrogen ion (H⁻) by analyzing the total population excited by a double-pulse laser in a time-dependent field for different physical parameters. By analyzing the total excited population using a double-pulse laser, we can uncover all the closed orbits existing in the system. We demonstrate that this can be realized by scanning the first pulse position and the time delay between the two pulses.     

Bandwidth enhancement for parametric amplifiers operated in chirped multi-beam mode

In this paper we discuss the bandwidth enhancement that can be achieved in multi-Joule optical parametric chirped pulse amplification (OPCPA) systems exploiting the tunability of parametric amplification. In particular, we consider a pair of single pass amplifiers based on potassium-dideuterium-phosphate (DKDP), pumped by the second harmonic of Nd:glass and tuned to amplify adjacent regions of the signal spectrum. We demonstrate that a bandwidth enhancement upto 50% is possible in two configurations; in the first case, one of the two amplifiers is operated near its non-collinear broadband limit; to allow for effective recombination and recompression of the outgoing signals this configuration requires filtering and phase manipulation of the spectral tail of the amplified pulses. In the second case, effective recombination can be achieved simply by spectral filtering: in this configuration, the optimization of the parameters of the amplifiers (pulse, crystal orientation, and crystal length) does not follow the recipes of non-collinear OPCPA.     

双脉冲电子直线感应加速器实验研究

 Mini-LIA为MHz重复频率双脉冲电子直线感应加速器,由双脉冲功率系统、热阴极电子枪注入器及金属玻璃磁芯感应加速腔等组成。在此平台的实验获得了数百ns间隔（即MHz重复频率）的双脉冲高压,每个脉冲幅值达到80 kV,脉冲半高全宽为80 ns;在感应腔加速间隙处测得双脉冲加速电场;在加速器出口处测量得到流强约1.1 A的双脉冲电子束流。实验结果表明：利用硅堆隔离汇流装置可实现MHz重复频率的双脉冲高压,金属玻璃磁芯感应加速腔和六硼化镧热阴极电子枪均适合MHz重复频率双脉冲工作方式。     

Broadband non-collinear double QPM optical parametric amplification in mid-infrared wave

The properties of optical parametric amplification (OPA) based on non-collinear double quasi-phase matching (NDQPM) with single periodically poled KTP (PPKTP) have been investigated theoretically. The NDQPM includes two different non-linear processes: one is optical parametric generation (OPG) and the other is difference frequency generation (DFG). The investigation of our numerical simulation focuses on the gain bandwidth of dependence upon non-collinear angle, grating period and crystal temperature. At a certain non-collinear angle and grating period with fixed temperature, there exists a broadest gain bandwidths of output mid-infrared pulse at 526 nm pump wavelength and certain signal wavelength in PPKTP. These are an optimal values of non-collinear angles and grating period. By accurately tuning the non-collinear angle or temperature near the optimal non-collinear angle, broadband mid-infrared tuning is obtained and an optimal operation of NDQPM can be realized. In this paper, the solutions of the coupled equations of the cascaded processes were discussed, and the spatial-temporal frequency (STF) band of the output idler pulse is analyzed by taking angular dispersion of amplified pulse beam into account. The idler pulse with a certain angular dispersion can improve the OPA bandwidth significantly. So, optical parametric chirped-pulse amplification can be realized in this configuration. For a broadband NDQPM both the acceptance angles and the acceptance temperature are smaller and the gain bandwidth is sensitive to non-collinear angles and temperature, it is important to control the precision of the non-collinear angles and the temperature in experiment.     

Comparison between single- and double-pulse LIBS at different air pressures on silicon target

A comparative study between single- and double-pulse laser-induced breakdown spectroscopy (LIBS) was performed on an n-type silicon(111) target. A new mobile double-pulse instrument for LIBS analysis was used for the measurements. The experiment was carried out at different air pressures of 0.7, 470 and 1000 hPa. It has been found that, in the case of double-pulse LIBS, the emission intensities of atomic and ionic lines are strongly enhanced at higher pressures. Using Stark broadening of the atomic lines of silicon, it was found that the electron number densities for single and double pulses are approximately the same (N_e∼10¹⁷ cm^-3). Plasma excitation and ionization temperatures were determined from a Boltzmann plot. The double-pulse laser-induced plasma was studied at different interpulse delay times of 1, 2, 5, 10, 15, 25 and 50 μs. The results indicated that the interaction between the laser, plasma and target gives higher atomic and ionic intensities at shorter interpulse delay times. PACS 52.38.Mf; 79.20.Ds; 52.50.Jm     

Tunnelling,non-collinearity and current-induced switching in metal/heterojunctions

Current-induced switching in heterojunctions such as Fe/Vac/Fe and Fe/Ge/Fe, including in the latter case homogeneous and inhomogeneous chemical disorder caused by holes (vacuum), is described theoretically in terms of a multi-scale approach based on ab initio calculations using the fully relativistic screened Korringa–Kohn–Rostoker method and the Landau–Lifshitz–Gilbert equation. It is found that (1) the presence of tunnelling can be a function of the relative angle between the orientations of the magnetization in the magnetic slabs; and (2) disorder is responsible for the occurrence of non-collinear magnetic ground states. Furthermore, it is found that the first terms in the expansion of the twisting energy in a power series in the cosine of this relative angle, namely the interlayer exchange energy term and the anisotropy term, can be used for a qualitative scheme not only to characterize the occurrence of non-collinear ground states, but also for the critical current needed to induce switching.     

Plasma property effects on spectral line broadening in double-pulse laser-induced breakdown spectroscopy

Double-pulse Laser-Induced Breakdown Spectroscopy (LIBS) in an orthogonal configuration was used to investigate plasma temperature and electron density effects on Mg II emission spectral line broadening. The experiments were carried out with two Nd:YAG lasers, one operating at 355 nm for ablation and the other one at 1064 nm for plasma reheating in air at atmospheric pressure. Temporally resolved plasma temperature and electron density were measured at various delay times. Data in this study show prolonged emission of Mg II (280.27 nm) as well as enhancement of the signal intensity when using double-pulse excitation compared to the single-pulse case. An enhancement of ～8× was attained with a delay between the laser pulses equal to 1 μs. The enhancement was accompanied by higher plasma temperature and increased electron density. The double-pulse LIBS configuration provides energy to sustain the plasma emission at a period in time when the linewidth is minimum, thereby improving the analytical capabilities of low spectral resolution instrumentation typically used in LIBS system.     

Non-collinear nanosecond optical parametric oscillator based on periodically poled LN with tilted domain walls

We report on a non-collinear, 1064 nm pumped, 10 kHz repetition rate ns-OPO which consists of a 13 mm long periodically poled lithium niobate (PPLN) crystal in a hemispherical optical cavity. The non-collinear phase-matching is achieved by tilting the domains by 60° with respect to the pump beam. This phase-matching avoids back conversion of signal and idler radiation into pump radiation and thus improves the spatial quality of the generated OPO radiation considerably. At a pump power of 5.5 W the OPO provided a conversion efficiency of up to 34%. The generated OPO pulses with a total power of up to 1.85 W were emitted in an almost diffraction limited beam with a M²-value of 1.1. The beam quality did not change when the pump power was varied in the range of 2–5 W. PACS 42.65.-k; 42.65.Yj     

Analysis of broadband third harmonic generation with non-collinear angular dispersion in KDP crystals

A new scheme with non-collinear sum frequency generation (SFG) process of broadband third harmonic generation (THG) with two KDP crystals (one doubler and one mixer) using angular dispersion was proposed. The principle of broadband harmonic generation with non-collinear angular dispersion was presented. The comparison between the schemes of non-collinear and collinear SFG process was performed. The effects of the angular dispersion on the conversion efficiency, the pulse shape and the spectrum of the third harmonic field were discussed. The results show that, if proper angular dispersion is added to the fundamental and the second harmonic field outside the mixing KDP crystal, respectively, the broadband third harmonic conversion efficiency can be improved significantly. However, the difficulty of this scheme arises due to the requirement of two gratings with different angular dispersion outside the mixer. With the new scheme, it can be simpler that only one grating is needed to realize the broadband phase matching in the non-collinear SFG process. Although the fundamental and the second harmonic field inside the mixing crystal are non-collinear except the center wavelength, a blazing grating with proper angular dispersion for the fundamental field (twice as that for second harmonic field) can yield the well compensation for phase mismatching in the SFG process. Consequently, the conversion efficiency and the characteristics of the third harmonic field for the broadband third harmonic generation can be improved significantly.     

Domain wall nucleation in epitaxial exchange-biased Fe/IrMn bilayers with highly misaligned anisotropies

The angular dependence of the magnetization reversal in epitaxial Fe/IrMn bilayers with collinear and non-collinear cubic and unidirectional anisotropies is investigated. Multistep loops with different magnetization reversal processes are observed for either positive or negative angles with respect to the Fe easy axis. The angular dependence of the switching fields displays the broken symmetry of the induced non-collinearity. The experimental results are reproduced with a generalized domain wall nucleation model that includes the induced anisotropy configuration and the peculiar asymmetric magnetic switching behavior. These results highlight the importance of the relative angle between anisotropies in epitaxial exchange bias systems with incoherent rotation reversal mechanism, opening a new pathway for tailoring the magnetic properties of such systems.     

Controlled two-pulse generation in Q-switch mode from a single laser using Q-modulator with frustrated total internal reflection

In this work a cavity design for double-pulse generation in Q-switched mode from a single laser is proposed, based on the construction of a second laser channel using a FTIR (frustrated total internal reflection) Q-modulator. A time interval between the two pulses from 500 ns to 120 μs is obtained in a Nd:YAG laser. A comparison with other methods and cavity designs for double-pulse generation is presented. The case when this technique is applied on a tunable laser with metastable upper laser level (Cr:LiCAF, Cr:LiSAF, Alexandrite, Co:MgF₂ or other) is also considered. Even though the method presented in the paper does not rely only on the cavity configuration proposed, some advantages can be obtained – both polarization and wavelength-independent tuning without polarization and wavelength restrictions in combination with the possibility of different wavelengths and polarizations in each pulse. Moreover, by using an active medium generating wavelengths around and up to 3 μm, the Pockels-cell-Q-switch optical transmission problems can be avoided. Received: 9 May 2001 / Revised version: 2 August 2001 / Published online: 15 October 2001     

Interferences in photo-detached electron spectra from a non-collinear tri-atomic anion

The electron flux oscillations in photo-detachment of a non-collinear tri-atomic anion have been studied by taking each atom of the system as a coherent source of detached-electron wave. These electron waves traversing along three different trajectories result in a quantum interference. An analytical expression of detached-electron flux is evaluated for various detached-electron energies and for different geometrical shapes of the system. The results show that the electron flux distributions exhibit molecular shape-induced oscillatory structures. These oscillations are explained using the semi- classical closed-orbit theory; the outgoing electron waves produced from one center are propagated in the vicinity of the sources at other centers. It is also observed that in a particular case our non-collinear tri-atomic system reduces to the collinear tri-atomic system recently published.     

Magnetism of Co overlayers and nanostructures on W(0 0 1): A first-principles study

The magnetic properties of Co nanostructures and a Co monolayer on W(0 0 1) have been studied in the framework of density functional theory. Different geometries such as planar and three-dimensional clusters have been considered, with cluster sizes varying between 2 and 13 atoms. The calculations were performed using the real-space linear muffin-tin orbital method (RS-LMTO-ASA). With respect to the stability of the magnetic state, we predict an antiferromagnetic (AFM) structure for the ground state of the planar Co clusters and a ferromagnetic (FM) state for the three-dimensional clusters. For the three-dimensional clusters, one of the AFM arrangements leads to frustration due to the competing FM and AFM exchange interactions between different atoms in the cluster, and gives rise to a non-collinear state with energy close to that of the FM ground state. The relative role of the Co–Co and Co–W exchange interactions is also investigated.     

闭合裂纹非共线混频超声检测试验研究

针对结构中微裂纹检测难题,发展了一种闭合裂纹非共线混频超声检测方法。在对非共线混频超声检测机理分析基础上,进行了结构中疲劳裂纹混频非线性超声检测实验。对有无裂纹试件中检测信号进行了滤波和时频分析,结果表明,可根据信号滤波后时域波形中是否存在明显的混频波包或时频分析中是否存在明显的和频分量,实现有无闭合裂纹的判识;通过移动激励探头的位置,控制两列入射声波在试件中的交汇位置,实现试件中不同深度位置的混频非线性检测。并根据测得的混频非线性系数沿试件深度方向上分布,实现了闭合裂纹沿深度方向上长度的测量。研究工作为结构中微裂纹定量评价做了有益探索。      

Self-focusing in two-color collinear optical Kerr measurements

We performed the femtosecond two-color collinear optical Kerr shutter (OKS) measurements in a thick Kerr medium. The time-resolved OKS signals showed a multi-peak structure and a large broadening with respect to that for non-collinear. By measuring the time-resolved OKS signals in the sample at different powers, we demonstrated that the abnormal profile of the OKS signals was mainly attributed to self-focusing and re-focusing of the pump beam in the Kerr medium and the group velocity difference between the pump and the probe beams.     

Simulation of spin-polarized scanning tunneling microscopy images of nanoscale non-collinear magnetic structures

We apply an efficient method to calculate spin-polarized scanning tunneling microscopy (SP-STM) images of nanostructures with complex non-collinear magnetic order. The model is based on the spin-polarized version of the Tersoff–Hamann model of STM and the independent orbital approximation for the surface electronic structure. For its application, only the knowledge of the arrangement of the magnetic moments of the surface atoms is required. In spite of its simplifications, calculated SP-STM images of periodic collinear and non-collinear magnetic spin structures are in many cases in excellent agreement with those obtained from computationally much more demanding ab initio calculations. Especially for surfaces of chemically equivalent atoms, the atomic scale SP-STM images are dominated by the magnetic structure and depend much less on the accurate electronic structure. This suggests the application of the method to more complex non-collinear magnetic structures such as domain walls in antiferromagnets, spin-spiral states, spin glasses, or disordered states. Based on the model, we predict SP-STM images of helical spin-spiral states in ultra-thin films. PACS 68.37; 75.70; 75.30     

Collinear and non-collinear sum-frequency mixing inβ-BBO for a tunable 195–198 nm all-solid-state laser system

We attained tunable UV laser radiation between 195 and 198 nm by sum-frequency mixing two synchronized flashlamp-pumped solid-state Q-switch lasers, a Nd:YAG laser frequency quadrupled to 266 nm and a tunable (730–770 nm) alexandrite laser. UV pulse energies of 0.12 mJ with repetition rates of 10 Hz were attained in collinear, as well as non-collinear sum-frequency interaction in a-Barium Borate (BBO) crystal with a conversion efficiency of 2.5%. Theoretical models for the non-collinear phase-matching interaction were investigated at UV wavelengths below 200 nm.