A novel monochromator for experiments with ultrashort X‐ray pulses

Aiming at advancing storage‐ring‐based ultrafast X‐ray science, over the past few years many upgrades have been undertaken to continue improving beamline performance and photon flux at the Femtoslicing facility at BESSY II. In this article the particular design upgrade of one of the key optical components, the zone‐plate monochromator (ZPM) beamline, is reported. The beamline is devoted to optical pump/soft X‐ray probe applications with 100 fs (FWHM) X‐ray pulses in the soft X‐ray range at variable polarization. A novel approach consisting of an array of nine off‐axis reflection zone plates is used for a gapless coverage of the spectral range between 410 and 1333 eV at a designed resolution of E/ΔE = 500 and a pulse elongation of only 30 fs. With the upgrade of the ZPM the following was achieved: a smaller focus, an improved spectral resolution and bandwidth as well as excellent long‐term stability. The beamline will enable a new class of ultrafast applications with variable optical excitation wavelength and variable polarization.     

Shaping of few-cycle laser pulses via a subwavelength structure

We theoretically investigate the propagation of few-cycle laser pulses in resonant two-level dense media with a sub- wavelength structure, which is described by the full Maxwell-Bloch equations without the frame of slowly varying envelope and rotating wave approximations. The input pulses can be shaped into shorter ones with a single or less than one optical cycle. The effect of the parameters of the subwavelength structure and laser pulses is studied. Our study shows that the media with a subwavelength structure can significantly shape the few-cycle pulses into a subcycle pulse, even for the case of chirp pulses as input fields. This suggests that such subwavelength structures have potential application in the shaping of few-cycle laser pulses.     

Propagation of extremely short pulses in a graphene–boron nitride bilayer

In this study we investigate the propagation of extremely short optical pulses in a thin film formed by a graphene grown on a boron nitride substrate. Conduction electrons of the system are described on the basis of the long-wavelength effective Hamiltonian in the case of low temperatures; the electromagnetic field being taken into account within the framework of the classical Maxwell equations. The time evolution of the pulse?s shape for different speeds and maximum amplitudes of an extremely short pulse is analyzed.     

超快激光的新前沿—阿秒科学

超短脉冲激光正在进行着从飞秒（1fs=10^-15s)向阿秒（1attosecond=10^-18s)的跨越,这一跨越对于激光原理和激光应用来说都有很重要的意义,文章通过阿秒脉冲与飞秒脉冲比较来介绍超快激光的新前沿－阿秒科学,包括阿秒科学的诞生,现状以及由于阿秒脉冲的产生而出现的阿秒科学的概况 。     

Sensitivity enhancement in natural-abundance solid-state 33S MAS NMR spectroscopy employing adiabatic inversion pulses to the satellite transitions

The WURST (wideband uniform rate smooth truncation) and hyperbolic secant (HS) pulse elements have each been employed as pairs of inversion pulses to induce population transfer (PT) between the four energy levels in natural abundance solid-state (33)S (spin I=3/2) MAS NMR, thereby leading to a significant gain in intensity for the central transition (CT). The pair of inversion pulses are applied to the satellite transitions for a series of inorganic sulfates, the sulfate ions in the two cementitious materials ettringite and thaumasite, and the two tetrathiometallates (NH(4))(2)WS(4) and (NH(4))(2)MoS(4). These materials all exhibit (33)S quadrupole coupling constants (C(Q)) in the range 0.1-1.0 MHz, with precise C(Q) values being determined from analysis of the PT enhanced (33)S MAS NMR spectra. The enhancement factors for the WURST and HS elements are quite similar and are all in the range 1.74-2.25 for the studied samples, in excellent agreement with earlier reports on HS enhancement factors (1.6-2.4) observed for other spin I=3/2 nuclei with similar C(Q) values (0.3-1.2 MHz). Thus, a time saving in instrument time by a factor up to five has been achieved in natural abundance (33)S MAS NMR, a time saving which is extremely welcome for this important low-gamma nucleus.     

Determination of pore space shape and size in porous systems using NMR diffusometry. Beyond the short gradient pulse approximation

The influence of finite length gradient pulses on NMR diffusion experiments on liquids confined to diffuse between two parallel planes is investigated. It is experimentally verified that the pore size decreases when determined using finite gradient pulses if the results are analyzed within the short gradient pulse approximation. The results are analyzed using the matrix formulation. The observed minima in the echo decay profiles are considerably less sharp than theoretical analysis would indicate and we suggest that this is due to the presence of a distribution of pore sizes in the sample. In addition, effects due to the presence of background gradients are discussed. It is argued that effects due to the finite length gradient pulses are relatively minor and in realistic applications the effects due to inhomogeneities in pore sizes and effects due to background gradients will constitute more serious problems in pore size determinations by means of NMR diffusometry.     

空间非均匀场下核间距对不对称分子谐波发射的影响

数值研究了空间非均匀场下核间距离对于不对称分子HeH2+发射高次谐波以及阿秒脉冲的影响。计算结果表明,通过适当调节HeH2+分子离子的核间距离以及空间非均匀参数,不仅谐波发射的截止能量得到了延伸,而且单一的短量子路径也被选择出来对谐波发射起作用。随后适当引入第二束控制激光场,谐波截止能量得到了进一步扩展,形成了两个带宽分别在431eV和372eV的连续平台区。 最后,通过适当的叠加谐波,可获得脉宽为31as-65as的一系列阿秒X射线光源。     

Sensitivity-enhanced IPAP-SOFAST-HMQC for fast-pulsing 2D NMR with reduced radiofrequency load

The SOFAST-HMQC experiment [P. Schanda, B. Brutscher, Very fast two-dimensional NMR spectroscopy for real-time investigation of dynamic events in proteins on the time scale of seconds, J. Am. Chem. Soc. 127 (2005) 8014-8015] allows recording two-dimensional correlation spectra of macromolecules such as proteins in only a few seconds acquisition time. To achieve the highest possible sensitivity, SOFAST-HMQC experiments are preferably performed on high-field NMR spectrometers equipped with cryogenically cooled probes. The duty cycle of over 80% in fast-pulsing SOFAST-HMQC experiments, however, may cause problems when using a cryogenic probe. Here we introduce SE-IPAP-SOFAST-HMQC, a new pulse sequence that provides comparable sensitivity to standard SOFAST-HMQC, while avoiding heteronuclear decoupling during (1)H detection, and thus significantly reducing the radiofrequency load of the probe during the experiment. The experiment is also attractive for fast and sensitive measurement of heteronuclear one-bond spin coupling constants.     

boldmath Broadband water window supercontinuum generation with a ω+3ω/2 multicycle two-colour pulse

We propose a method to generate a high-efficiency broadband water window supercontinuum with a ω+3ω/2 multicycle two-colour pulse. Our results reveal that the 3ω/2 laser pulse can simultaneously modulate the acceleration step and the ionization step, which not only broadens the bandwidth but also enhances the yield of the generated supercontinuum. An ultra-broadband supercontinuum from 290 eV to 555 eV covering the whole water window is generated. Using this method, we expect that an isolated 62-as pulse with a minor pre-pulse can be directly obtained.     

原子在红外激光场中产生高次谐波及阿秒脉冲随波长的变化规律

利用强场近似理论,研究了在长波红外激光场(波长800—2000 nm)驱动下氢原子产生的高次谐波,分析了在截止位置附近高次谐波的转换效率随激光波长的变化规律.发现在截止位置附近原子发射高次谐波的转换效率比平台区域的转换效率低,但获得的阿秒脉冲的宽度会随波长的增大而缩短. 关键词： 强场近似 高次谐波 阿秒脉冲