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.     

Recoupling of heteronuclear dipolar interactions with rotational-echo double-resonance at high magic-angle spinning frequencies

Heteronuclear dipolar recoupling with rotational-echo double-resonance (REDOR) is investigated in the rapid magic-angle spinning regime, where radiofrequency irradiation occupies a significant fraction of the rotor period (10-60%). We demonstrate, in two model (13)C-(15)N spin systems, [1-(13)C, (15)N] and [2-(13)C, (15)N]glycine, that REDOR DeltaS/S(0) curves acquired at high MAS rates and relatively low recoupling fields are nearly identical to the DeltaS/S(0) curve expected for REDOR with ideal delta-function pulses. The only noticeable effect of the finite pi pulse length on the recoupling is a minor scaling of the dipolar oscillation frequency. Experimental results are explained using both numerical calculations and average Hamiltonian theory, which is used to derive analytical expressions for evolution under REDOR recoupling sequences with different pi pulse phasing schemes. For xy-4 and extensions thereof, finite pulses scale only the dipolar oscillation frequency by a well-defined factor. For other phasing schemes (e.g., xx-4 and xx-4) both the frequency and amplitude of the oscillation are expected to change.     

Influences of finite gain bandwidth on pulse propagation in parabolic fiber amplifiers with distributed gain profiles

The evolutions of the pulses propagating in decreasing and increasing gain distributed fiber amplifiers with finite gain bandwidths are investigated by simulations with the nonlinear Schr¨odinger equation. The results show that the parabolic pulse propagations in both the decreasing and the increasing gain amplifiers are restricted by the finite gain bandwidth. For a given input pulse, by choosing a small initial gain coefficient and gain variation rate, the whole gain for the pulse amplification limited by the gain bandwidth may be higher, which is helpful for the enhancement of the output linearly chirped pulse energy. Compared to the decreasing gain distributed fiber amplifier, the increasing gain distributed amplifier may be more conducive to suppress the pulse spectral broadening and increase the critical amplifier length for achieving a larger output linearly chirped pulse energy.     

Influences of finite gain bandwidth on pulse propagating in parabolic fiber amplifiers with distributed gain profiles

The evolutions of the pulses propagating in decreasing and increasing gain distributed fiber amplifiers with finite gain bandwidths are investigated by simulating with the nonlinear Schrödinger equation. The results show that the parabolic pulse propagations in both the decreasing and the increasing gain amplifiers are restricted by the finite gain bandwidth. For a given input pulse, by choosing small initial gain coefficient and gain variation rate, the whole gain for the pulse amplification limited by the gain bandwidth may be higher, which is helpful to the enhancement of the output linearly chirped pulse energy. Compared to the decreasing gain distributed fiber amplifier, the increasing gain distributed amplifier may be more conducive to suppress the pulse spectral broadening and increase the critical amplifier length for achieving a larger output linearly chirped pulse energy.     

Ultrafast pulsed mode effects on the performance of grating-assisted output couplers of finite extent

The performance of grating-assisted output couplers of finite extent excited by finite ultrafast pulsed modes is characterized numerically by the finite-difference time-domain method. The results for pulsed excitations with extremely long pulse envelopes are compared with those that contain only a few optical cycles for gratings that are both long and short compared with the equivalent pulse length. We demonstrate that the shortest length of either the grating or the pulse dictates the far-field performance of the grating-assisted output couplers.     

固态NMR中自旋Ⅰ=1体系的魔术回波序列

虽然组合脉冲固体回波大大地减轻了有限脉冲宽度的影响,但是它们并没有完全消除有限脉冲宽度带来的失真,而且产生另一种失真,即所谓的位相失真。本文分析了组合脉冲序列,将它用来构成魔术回波序列,这样就能消除位相失真。理论和实验的结果都证明,自旋I=1的组合脉冲魔术回波在线型的测量上优于组合脉冲固体回波序列,基本上解决了²D核核磁共振中有限脉冲宽度带来的难题。     

Optimization of grating size in chirped-pulse-amplification. laser system

We performed calculations for a chirped-pulse-amplification laser system with a sub-100-fs pulse duration including the finite size of gratings in an aberration-free stretcher/compressor design. Both the higher-order phase dispersion and the spectral clipping, which is due to the finite size of the optics in the stretcher and the compressor, are taken into account in the calculation. We discuss the influence of both phase dispersion and spectral clipping on the important parameters of the recompressed pulse, such as pulse duration, pulse contrast, and peak intensity, employing different phase-dispersion compensation methods. The calculations show that the use of a small compressor grating reduces the higher-order phase errors and smoothes the sharp spectral cut-off caused by spectral clipping in the stretcher, which consequently improves the pulse contrast ratio of the main pulse to the pedestal. Received: 1 April 2000 / Published online: 8 November 2000     

Finite pulse width artifact suppression in spin-1 quadrupolar echo spectra by phase cycling

A phase cycling scheme for suppressing spectral artifacts introduced in quadrupolar echo spectroscopy of spin-1 nuclei due to finite pulse width effects is presented. The phase cycling scheme is developed using the formalism of average Hamiltonian theory and fictitious spin-1 operators. A simulation and experiment on deuterated polyethelene is performed highlighting the spectral artifact introduced by finite pulse widths and successful removal with the proposed phase cycling scheme.     

Storage and release of femtosecond laser pulses in a resonant photonic crystal

Femtosecond laser pulses with suitable pulse areas are shown to decelerate and stop inside a resonant photonic crystal. The release of the stored fields can be achieved through pulse collision by use of an external optical pulse as a control field. The duty cycle efficiency of the storage and release of light is shown to be as much as 96%. The proposed technique is applied to a real photonic system with finite thickness of atomic layers.     

基于ANSYS的脉冲激光辐照石英玻璃的温度场数值模拟

采用有限元仿真软件ANSYS 12.0对脉冲功率激光辐照石英玻璃建立了热力学模型,对其表面温度场进行了数值模拟,得到了在不同激光功率密度下的瞬态温度场分布,并对模拟结果进行了分析和研究,为激光辐照石英玻璃实验提供依据.     

Study of femtosecond soliton dynamics in photonic crystal fiber using the moment method

We investigate the dynamics of femtosecond solitons in photonic crystal fibers (PCFs) by including high-order dispersion terms until to sixth-order in the generalized nonlinear Schrödinger equation, in addition to the nonlinear effects of the self phase modulation, self steepening and Raman scattering. We calculate theoretically the pulse parameters using the moment method. In the case of the fundamental soliton, our computed equations are coupled and difficult to solve analytically. However, we use the finite difference method to calculate numerically pulse parameters using an initially hyperbolic secant pulse at 1550-nm with different peak powers along 10m-PCF. Our numerical results show that the nonlinear regimes allow obtaining pulse compressions and initial pulse amplitudes. Furthermore, we remark a pulse broadening, and weak shifts of the peak power positions and frequencies in the critical and dispersive regimes. The use of an initial chirp provides a better pulse compressions and especially for low input powers. Also, the initial positive chirp reduces the optimal compression position lengths, while the negative one increases them. Therefore, we conclude that our theoretical calculations and numerical simulation results show that the moment method associated with the finite differences method is effective for the study of femtosecond pulse dynamics in PCFs.     

Plasma ion evolution in the wake of a high-intensity ultrashort laser pulse

Experimental investigations of the late-time ion structures formed in the wake of an ultrashort, intense laser pulse propagating in a tenuous plasma have been performed using the proton imaging technique. The pattern found in the wake of the laser pulse shows unexpectedly regular modulations inside a long, finite width channel. On the basis of extensive particle in cell simulations of the plasma evolution in the wake of the pulse, we interpret this pattern as due to ion modulations developed during a two-stream instability excited by the return electric current generated by the wakefield.     

缝隙回热器式脉管制冷机的数值分析

提出一种缝隙回热器式脉管制冷机,并进行了计算机模拟.运用有限差分数值计算方法,对其进行了较全面的热力分析,着重讨论了一些运行参数和结构参数的影响.     

Periodic pulse train conformation based on the temporal Radon-Wigner transform

By using the Radon-Wigner transform (RWT), we analyze the temporal selfimaging or Talbot effect for producing well-conformed pulse trains with variable repetition rates and duty-cycles. The relationships linking the selfimaging conditions with the fractional orders of the RWT are first obtained for unchirped pulse trains. Then, we extend the analysis to chirped pulse sequences by deriving the conditions to be fulfilled by an equivalent unchirped pulse train producing the same selfimage irradiances. This result becomes relevant for observing well-defined high order fractional selfimaging, which are of interest due to their repetition rate multiplication. Besides, the effect of the finite extension of the pulse train on the selfimage quality is analyzed and a condition is found for relating the required minimum pulse number with the chirp parameter of the pulses.     

Dynamics of competitive evolution on a smooth landscape

We study competitive DNA sequence evolution directed by in vitro protein binding. The steady-state dynamics of this process is well described by a shape-preserving pulse which decelerates and eventually reaches equilibrium. We explain this dynamical behavior within a continuum mean-field framework. Analytical results obtained on the motion of the pulse agree with simulations. Furthermore, finite population correction to the mean-field results are found to be insignificant.     

Diffuse X-ray scattering near a two-dimensional solid–liquid phase transition at the <Emphasis Type="Italic">n</Emphasis>-hexane–water interface

Spatial profiles of single-shot microcraters produced by tightly focused femtosecond laser pulses with variable pulse energies are measured by means of a laser confocal microscope. Dependences of crater depth on laser intensity at different pulse energies appear as overlapping saturating curves with the same threshold, indicating the presence of nonlinear absorption and absence of nonlocal ablation effects. A monotonic twofold increase in absorption nonlinearity is related to the transition from minor defect-band absorption to fundamental band-to-band absorption.     

Tunable-chirp pulse compression in quasi-phase-matched second-harmonic generation

We demonstrate continuously tunable compensation of linear chirp on a first-harmonic pump pulse to produce a near-transform-limited second-harmonic output pulse through the use of a chirped, fanned, periodically poled lithium niobate quasi-phase-matching grating. Compensation of positive and negative chirps is possible through reversal of device orientation. The device is simple and monolithic and can be applied to compensation of a higher-order phase with minor modification.     

Simulation model for the effects of nonlinear polarization on the propagation of intense pulse lasers

A simulation model including the effects of nonlinear polarization of a background neutral gas on the propagation of intense short pulse lasers is presented. The model includes the finite response time of the gas by using a soft-core Coulombic potential for the bound electrons. A test problem for self-focusing of a pulse is presented. Agreement with theoretical predictions for the self-focusing of an intense laser pulse in a gas with a finite-response-time nonlinear polarization included is found.     

Supercontinuous high harmonic generation from asymmetric molecules in the presence of a terahertz field

We have investigated high-order harmonic generation from asymmetric molecules. It is found that supercontinuous high harmonics,which are produced from asymmetric molecules by significantly steering the ionization,are broken down when the electric field of the 5-fs driving laser pulse is increased to 0.16 a.u. The high harmonic generation from asymmetric molecules with the presence of a terahertz field is also investigated. This reveals that the terahertz controlled laser pulse significantly increases the energy difference between photons,emitted from the ejected electrons,in the first and second halves of the optical cycle at the centre of the driving laser pulse. In this way,a 200-eV broadband supercontinuum can be produced in the plateau,from which a 60-as pulse with a bandwidth of 60 eV can be directly obtained with a minor post-pulse.     

Polarized Fluorescence in Tryptophan Excited by Two-Photon Femtosecond Laser Pulses

The shape of an optical self-similar pulse that propagates in a medium of finite length with noninstantaneous nonlinear absorption is found analytically. The pulse has an asymmetric shape and, in addition, frequency modulation, which confirms that it belongs to a new type of self-similar propagation modes of laser radiation—chirped pulses with a self-similar shape. The existence of pulses of this kind is also confirmed by results of computer simulation.