Measuring protein self-diffusion in protein-protein mixtures using a pulsed gradient spin-echo technique with WATERGATE and isotope filtering

Here we report a modified pulsed gradient spin-echo (PGSTE) pulse sequence to measure diffusion coefficients. This approach incorporates WATERGATE combined with isotopic filtering into a standard PGSTE experiment. Doing this eliminates much of the disadvantages from the combination of diffusion encoding and heteronuclear selection intervals and allows for facile modification of the diffusion pulse sequence with flexibility of the time period between RF pulses. The new diffusion pulse sequence is demonstrated using an 15N-labeled peptide and an 15N-labeled protein in a mixture with a protein of similar size.     

Magic Angle Spinning NMR Spectroscopy with Composite RF Pulses

Using numerical optimization procedures it is shown that it is possible to design composite 180 degrees RF pulses for MAS NMR spectroscopy by explicitly taking into account the variation of the resonance offset of each crystallite during the application of the RF pulses. When using composite RF pulses in experiments such as TOSS, where the delays between the RF pulses have to be critically adjusted, an optimization of these delays can lead to the desired performance characteristics.     

Broadband 13C-13C adiabatic mixing in solution optimized for high fields

Experiments which require mixing among spins with large frequency differences are generally performed with sequences based on composite pulses or computer-optimized cycles. Adiabatic pulses generally offer several advantages over other approaches, including greater single spin inversion bandwidths and tolerance to RF inhomogeneity. Here, a novel theoretical framework is presented in order to understand how spin-spin interactions are influenced by adiabatic inversion pulses, and insights from this approach are used to design more efficient adiabatic coherence exchange experiments. For very large frequency differences, this new approach generally offers improved results over previously applied mixing sequences, as applied to 13C-13C experiments which are the basis of modern sidechain assignment techniques in proteins. It is also anticipated that the approach presented here will be applicable to the analysis of various alternative approaches to adiabatic mixing.     

Optimization of Adiabatic Selective Pulses

Adiabatic RF pulses play an important role in spin inversion due to their robust behavior in presence of inhomogeneous RF fields. These pulses are characterized by the trajectory swept by the tip of theB_effvector and the rate of motion upon it. In this paper, a method is described for optimizing adiabatic inversion pulses to achieve a frequency-selective magnetization inversion over a given bandwidth in a shorter time and to improve slice profile. An efficient adiabatic pulse is used as an initial condition. This pulse allows for flexibility in choosing its parameters; in particular, the transition sharpness may be traded off against the inverted bandwidth. The considerations for selecting the parameters of the pulse according to the requirements of the design are discussed. The optimization process then improves the slice profile by optimizing the rate of motion along the trajectory of the pulse while preserving the trajectory itself. The adiabatic behavior of the optimized pulses is fully preserved over a twofold range of variation in the RF amplitude which is sufficient for imaging applications in commercial high-field MRI machines. Design examples demonstrate the superiority of the optimized pulses over the conventional sech/tanh pulse.     

改进型零附加相位光谱相位相干电场重构系统对啁啾脉冲的测量

现有的基于光谱相位相干电场重构法(SPIDER)的脉冲测量系统,在测量啁啾脉冲时容易出现误差.本文提出一个改进型零附加相位光谱相位相干电场重构系统(MZAP-SPIDER),来解决上述问题.在实验上,利用改进后的SPIDER系统测量了钛宝石飞秒激光器输出的脉冲及其经80mm长的BK7玻璃块展宽得到的啁啾脉冲.结果表明,该系统能胜任啁啾脉冲的相位测量.     

Quantum logical operations for spin 3/2 quadrupolar nuclei monitored by quantum state tomography

This article presents the realization of many self-reversible quantum logic gates using two-qubit quadrupolar spin 3/2 systems. Such operations are theoretically described using propagation matrices for the RF pulses that include the effect of the quadrupolar evolution during the pulses. Experimental demonstrations are performed using a generalized form of the recently developed method for quantum state tomography in spin 3/2 systems. By doing so, the possibility of controlling relative phases of superimposed pseudo-pure states is demonstrated. In addition, many aspects of the effect of the quadrupolar evolution, occurring during the RF pulses, on the quantum operations performance are discussed. Most of the procedures presented can be easily adapted to describe selective pulses of higher spin systems (>3/2) and for spin 1/2 under J couplings.     

Generation of double pulses in-line by using reflective Dammann gratings

Doubled femtosecond laser pulses in-line are needed in the collinear pump-probe technique, collinear second harmonic generation frequency-resolved optical gating (SHG FROG) and the spectral phase interferometry for direct electric-field reconstruction (SPIDER), etc. Normally, it is generated by using a Michelson's structure. In this paper, we proposed a novel structure with two-layered reflective Dammann gratings and the reflective mirrors to generate doubled femtosecond laser pulses in line without transmission optical elements. Angular dispersion and spectral spatial walk-off are both compensated. In addition, this structure can also compress the positive chirped pulse, which cannot be realized with a Michelson's structure. By adopting triangular grating and blazed gratings, the efficiency of the system would in principle be increased as the Michelson's scheme. Experiments demonstrated that this method should be an alternative approach for generation of the double compressed pulses of femtosecond laser for practical applications.     

Generation of few-cycle laser pulses:Comparison between atomic and molecular gases in a hollow-core fiber

We numerically study the pulse compression approaches based on atomic or molecular gases in a hollow-core fiber.From the perspective of self-phase modulation(SPM), we give the extensive study of the SPM influence on a probe pulse with molecular phase modulation(MPM) effect. By comparing the two compression methods, we summarize their advantages and drawbacks to obtain the few-cycle pulses with micro- or millijoule energies. It is also shown that the double pump-probe approach can be used as a tunable dual-color source by adjusting the time delay between pump and probe pulses to proper values.     

光纤零色散波长的精确测量

提出了一种测量单模光纤零色散波长的新方法,即让可调谐的连续波（波长λｓ）和波长λｐ位于光纤正常色散区的泵浦脉冲串在光纤中共同传输,交叉相位调制与群速度色散的相互作用可使得连续波的强度得到调制。对连续波λｓ进行调谐,若连续波被调制成亮脉冲,则表明λｓ位于光纤负色散区;若连续波被调制成暗脉冲,则表明λｓ位于光纤正色散区;若连续波的强度不能被调制,则表明λｓ位于光纤的零色散波长λ０附近。利用这一方法对长度为０．９８ｋｍ的色散位移光纤的零色散波长进行了测量,测量精度可达０．５ｎｍ。     

Two-dimensional high-resolution NMR spectra in matched B(0) and B(1) field gradients

In a recent publication we presented a method to obtain highly resolved NMR spectra in the presence of an inhomogeneous B(0) field with the help of a matched RF gradient. If RF gradient pulses are combined with "ideal" 90 degrees pulses to form inhomogeneous z rotation pulses, the line broadening caused by the B(0) gradient can be refocused, while the full chemical shift information is maintained. This approach is of potential use for NMR spectroscopy in an inhomogeneous magnetic field produced by an "ex-situ" surface spectrometer. In this contribution, we extend this method toward two-dimensional spectroscopy with high resolution in one or both dimensions. Line narrowing in the indirect dimension can be achieved by two types of nutation echoes, thus leading to depth-sensitive NMR spectra with full chemical shift information. If the nutation echo in the indirect dimension is combined with a stroboscopic acquisition using inhomogeneous z-rotation pulses, highly resolved two-dimensional correlation spectra can be obtained in matched field gradients. Finally, we demonstrate that an INEPT coherence transfer from proton to carbon spins is possible in inhomogeneous B(0) fields. Thus, it is possible to obtain one-dimensional (13)C NMR spectra with increased sensitivity and two-dimensional HETCOR spectra in the presence of B(0) gradients of 0.4 mT/cm. These schemes may be of some value for ex-situ NMR analysis of materials and biological systems.     

Combined simulated annealing and Shinnar-Le Roux pulse design of slice-multiplexed RF pulses for multi-slice imaging

Slice-multiplexed RF pulses have recently been introduced for simultaneous multi-slice imaging. Their novel aspect is that each slice is given a different linear phase profile, and hence a different slice-rephasing requirement, by the pulse. During readout, extra slice gradients are applied such that when one slice is rephased, the others are dephased to prevent aliasing. In this paper, an improved method of designing slice-multiplexed RF pulses is presented: component pulses which are optimized with simulated annealing for a specific rephasing are combined using Shinnar-Le Roux methods. In this way, non-linearities at higher flip angles are taken into account and more slices can be excited. Bloch simulations show the phase and amplitude profile of component pulses are faithfully preserved in the multiplexed pulse. Three- and four-slice multiplex pulses are demonstrated in gradient- and spin-echo in-vivo imaging.     

周期量级激光脉冲的Thomson散射

用经典辐射理论对线偏振周期量级激光脉冲的线性Thomson散射进行分析，从理论上得到它可产生亚阿秒脉冲的结论. 计算显示，在电子相对论因子为50、激光脉冲中心波长为1μm、归一化光场强度为0.01的情况下，用包含1.5个光周期的激光脉冲，可获得0.2as(半高全宽)的散射脉冲输出. 还对光场载波包络初相φ_ce和电子进入光场的初相φ_in对散射脉冲的影响作了分析讨论，结果表明，在适当的φ_ce和φ_in条件下，能实现单个阿秒脉冲输出，并可对脉冲宽度和频率进行调谐. 关键词： 线性Thomson散射 周期量级激光脉冲 载波包络初相 阿秒脉冲     

Study of dynamics in pentachlorophenol in the solid state by 1D Nuclear Quadrupole Resonance exchange spectroscopy

The paper reports studies of the dynamic process in pentachlorophenol (PCP) in the solid state by one-dimensional Nuclear Quadrupole Resonance (NQR) exchange spectroscopy employing shaped rf pulses. Pentachlorophenol exhibits intramolecular hydrogen bonding between the hydroxyl proton and one of the o-chlorines. Reorientation of the hydrogen bond with migration of the hydroxyl proton from one o-chlorine to the other is possible in a wide temperature range. In this process the two o-chlorines are exchanged; at the same time this implies the exchange of the two m-chlorines as well, which we have chosen to monitor, modeling it as a two-site exchange process. A detailed analysis of the appropriate Bloch-McConnell equation is performed to formulate the relevant kinetic matrix. The exchange pulse sequence implemented by us involves suitably modulated shaped RF pulses to achieve simultaneous on-resonance excitation of the two exchanging sites. This approach results in a clear definition of the exchange time window and requires only a short experiment time. Kinetic parameters, namely, the spin-lattice relaxation rate and the exchange rate are deduced from the experimental signal intensities. Variable temperature measurements in the range 213 K–318 K have been carried out to extract the activation parameters as well.     

Reconfigurable generation of high-repetition-rate optical pulse sequences based on time-domain phase-only filtering

We propose and demonstrate a fiber-based phase-only filtering technique for programmable optical pulse shaping, in which the filtering operation is implemented in the time domain by means of an electro-optical (EO) phase modulator. The technique has been applied for generating customized ultrahigh-repetition-rate optical pulse sequences (>40 GHz) from single input pulses by driving the EO phase modulator with a periodic electronic waveform (RF tone). The generated output pulses are replicas of the input pulse and both the repetition rate and the envelope profile of the generated sequences can be controlled and tuned electronically using this approach.     

Comparative experimental analysis of composite pulses in 14N NQR

Experimental results of comparing composite pulses in nitrogen-14 NQR, that are analogous to common 90 degrees RF pulses in powder, are presented. All tested pulses have been taken from publications in journals. Comparative diagrams of the measurement results for induction signals and echo signals are presented. The results of the measurements demonstrate that the best outcomes are achieved when the composite pulse (45)0(95)180(164)0 is used.     

Evolution Strategy Optimization for Selective Pulses in NMR

We present a first set of improved selective pulses, obtained with a numerical technique similar to the one proposed by Geen and Freeman. The novelty is essentially a robust and efficient “evolution strategy” which consistently leads, in a matter of minutes, to “solutions” better than those published so far. The other two ingredients are a “cost function,” which includes contributions from peak and average radiofrequency power, and some understanding of the peculiar requirements of each type of pulse. For example, good solutions for self-refocusing pulses and “negative phase excitation pulses” (which yield a maximum signal well after the end of the pulse) are found, as may have been predicted, among amplitude modulated pulses with 270° tip angles. Emphasis is given to the search for solutions with low RF power for selective excitation, saturation, and inversion pulses. Experimental verification of accuracy and power requirements of the pulses has been performed with a 4.7 T Sisco imager.     

Introduction to: A k-space analysis of small-tip-angle excitation

The article “A k-space analysis of small-tip-angle excitation” introduced a spatial frequency interpretation of the effect of RF excitation pulses. This introduction describes where the initial ideas for this paper came from, and traces out some of the applications that have been developed using this perspective.     

超短脉冲倍频中三阶非线性效应的影响及补偿

 为了提高超短脉冲二倍频的转换效率和改善出射倍频光的脉冲形状,对超短脉冲倍频中三阶非线性效应的影响进行了理论分析和数值模拟,并采用初始相位失配的方法来补偿三阶非线性效应的影响。结果表明：用KDP晶体二倍频中心波长为800 nm的超短脉冲,当入射功率密度大于100 GW/cm²时,三阶非线性效应是倍频转换效率的主要影响因素。对脉宽为50 fs,入射功率密度为250 GW/cm²的超短脉冲在KDP晶体（2 mm）中的二次谐波变换,当初始相位失谐0.9 mrad时,转换效率提高了10%,同时由三阶非线性效应引起的强度调制得到明显抑制,出射基频光和倍频光的脉冲形状得到明显改善。     

Application of optimal control theory to the design of broadband excitation pulses for high-resolution NMR

Optimal control theory is considered as a methodology for pulse sequence design in NMR. It provides the flexibility for systematically imposing desirable constraints on spin system evolution and therefore has a wealth of applications. We have chosen an elementary example to illustrate the capabilities of the optimal control formalism: broadband, constant phase excitation which tolerates miscalibration of RF power and variations in RF homogeneity relevant for standard high-resolution probes. The chosen design criteria were transformation of I(z)-->I(x) over resonance offsets of +/- 20 kHz and RF variability of +/-5%, with a pulse length of 2 ms. Simulations of the resulting pulse transform I(z)-->0.995I(x) over the target ranges in resonance offset and RF variability. Acceptably uniform excitation is obtained over a much larger range of RF variability (approximately 45%) than the strict design limits. The pulse performs well in simulations that include homonuclear and heteronuclear J-couplings. Experimental spectra obtained from 100% 13C-labeled lysine show only minimal coupling effects, in excellent agreement with the simulations. By increasing pulse power and reducing pulse length, we demonstrate experimental excitation of 1H over +/-32 kHz, with phase variations in the spectra <8 degrees and peak amplitudes >93% of maximum. Further improvements in broadband excitation by optimized pulses (BEBOP) may be possible by applying more sophisticated implementations of the optimal control formalism.     

Bloch方程的精细时程积分及其在射频脉冲设计中的应用

射频脉冲的频率选择性直接影响磁共振成像的质量，而射频脉冲的优化设计又归结为对Bloch方程的求解.尽管在某些情况下Bloch方程存在解析解，但由于其缺乏通用性而且形式上过于复杂而难于得到实用.本文提出一种Bloch方程的精细时程积分算法，并结合全局优化算法给出一个完整的射频脉冲设计方案.精细积分算法具有高效、高精度的特点，对于射频脉冲的设计很有裨益.数值算例表明，设计所得的射频脉冲具有较好的频率选择性. 关键词： 磁共振成像 射频脉冲 Bloch方程 精细时程积分