Clocking ultrafast wave packet dynamics in molecules through UV-induced symmetry breaking

We investigate the use of UV-pump-UV-probe schemes to trace the evolution of nuclear wave packets in excited molecular states by analyzing the asymmetry of the electron angular distributions resulting from dissociative ionization. The asymmetry results from the coherent superposition of gerade and ungerade states of the remaining molecular ion in the region where the nuclear wave packet launched by the pump pulse in the neutral molecule is located. Hence, the variation of this asymmetry with the time delay between the pump and the probe pulses parallels that of the moving wave packet and, consequently, can be used to clock its field-free evolution. The performance of this method is illustrated for the H(2) molecule.     

Phase control of rotational wave packets and quantum information

Lasers can create rotational wave packets in gas-phase molecules which periodically revive as field-free, aligned distributions. We control the wave packet evolution with relatively weak laser pulses at fractional revivals which modify the phase between wave packet components. We demonstrate two phase control effects in oxygen: coherently switching revivals off and on, and doubling the revival frequency. When viewed as a quantum logic system, these effects correspond to a Hadamard and a T operation.     

Field-free alignment of molecules observed with high-order harmonic generation

High-order harmonic generation is demonstrated to provide a sensitive way for an extensive study of dynamic processes in the field-free alignment of strong-field-induced molecular rotational wave packets. The time-dependent harmonic signal observed from field-free-aligned N2, O2, and CO2 has been found to include two sets of beat frequency for pairs of coherently populated rotational states. One of them is the well-known frequency component characterizing the field-free alignment of molecules, and the other is ascribed to the beat that arises from coherence embedded in the wave packet. We discuss the effect of each frequency component on the revival signal observed with the harmonic generation.     

Switched wave packets: a route to nonperturbative quantum control

The dynamic Stark effect due to a strong nonresonant but nonionizing laser field provides a route to quantum control via the creation of novel superposition states. We consider the creation of a field-free "switched" wave packet through adiabatic turn-on and sudden turn-off of a strong dynamic Stark interaction. There are two limiting cases for such wave packets. The first is a Raman-type coupling, illustrated by the creation of field-free molecular axis alignment. An experimental demonstration is given. The second case is that of dipole-type coupling, illustrated by the creation of charge localization in an array of quantum wells.     

Nonintrusive measurement of field-free molecular alignment

A technique was provided to nonintrusively quantify the field-free alignment of molecules by measuring the transient birefringence caused by the aligned molecules. Pure heterodyne alignment signals were obtained from the difference of the signals under an external electric field with opposite polarity and equal magnitude. The results demonstrated that the pure heterodyne alignment signal directly reproduced the revival structure of molecular alignment and its Fourier transform spectroscopy represented the actual populations of the different J states in the rotational wave packet.     

All-optical ultrafast polarization switching of terahertz radiation by impulsive molecular alignment

We experimentally demonstrate ultrafast polarization switching of terahertz (THz) radiation generated by dual-color driving pulses composed of orthogonally polarized fundamental and second-harmonic waves, which can be controlled by field-free molecular alignment in air by modulating the relative phase between the two field components as a transient dynamic wave plate. By fine-tuning the time delay to properly match the molecular alignment revivals, a significant polarization modulation of the THz radiation is observed and both linearly and elliptically polarized THz radiations can be obtained.     

Dynamical analysis of the effect of elliptically polarized laser pulses on molecular alignment and orientation

In this Letter, we study the molecular alignment and orientation driven by two elliptically polarized laser pulses.It is shown that the field-free molecular alignment can be achieved in a three-dimensional(3D) case, while the field-free molecular orientation is only along the x and y directions, and that the field-free alignment and orientation along different axes are related to the populations of the rotational states. It is demonstrated that changing the elliptic parameter is efficient for controlling both in-pulse and post-pulse molecular alignment and orientation. The delay time also has an influence on the field-free molecular alignment and orientation.     

飞秒时间分辨质谱和光电子影像对分子激发态动力学的研究

分子量子态的研究,特别是分子激发态演化过程的研究不仅可以了解分子量子态的基本特性和量子态之间的相互作用,而且可以了解化学反应过程和反应通道间的相互作用.飞秒时间分辨质谱和光电子影像是将飞秒抽运-探测分别与飞行时间质谱和光电子影像相结合的超快谱学方法,为实现分子内部量子态探测,研究分子量子态相互作用及超快动力学过程提供了强有力的工具,可以在飞秒时间尺度下研究单分子反应过程中的光物理或光化学机理.本文详细介绍了飞秒时间分辨质谱和光电子影像的技术原理,并结合本课题组的工作,展示了这两种方法在量子态探测及相互作用研究领域,特别是激发态电子退相、波包演化、能量转移、分子光解动力学以及分子激发态结构动力学研究中的广泛应用.最后,对该技术的发展前景以及进一步的研究工作和方向进行了展望.     

Tomographic Description of a Quantum Wave Packet in an Accelerated Frame

The tomography of a single quantum particle (i.e., a quantum wave packet) in an accelerated frame is studied. We write the Schrödinger equation in a moving reference frame in which acceleration is uniform in space and an arbitrary function of time. Then, we reduce such a problem to the study of spatiotemporal evolution of the wave packet in an inertial frame in the presence of a homogeneous force field but with an arbitrary time dependence. We demonstrate the existence of a Gaussian wave packet solution, for which the position and momentum uncertainties are unaffected by the uniform force field. This implies that, similar to in the case of a force-free motion, the uncertainty product is unaffected by acceleration. In addition, according to the Ehrenfest theorem, the wave packet centroid moves according to classic Newton’s law of a particle experiencing the effects of uniform acceleration. Furthermore, as in free motion, the wave packet exhibits a diffraction spread in the configuration space but not in momentum space. Then, using Radon transform, we determine the quantum tomogram of the Gaussian state evolution in the accelerated frame. Finally, we characterize the wave packet evolution in the accelerated frame in terms of optical and simplectic tomogram evolution in the related tomographic space.     

Dynamics of molecular alignment steered by a few-cycle terahertz laser pulse

The field-free alignment of molecule ClCN is investigated by using a terahertz few-cycle pulse (THz FCP) based on the time-dependent density matrix theory. It is shown that a high degree of molecular alignment can be obtained by changing the matching number of the THz FCPs in the adiabatic regime and the non-adiabatic regime. The matching number can affect both the maximum value of the alignment and the time at which it is achieved. It is also found that a higher degree of alignment can be achieved by using the THz FCP at lower intensity and there exists an optimal threshold of molecular alignment with the increase of the field amplitude. Also found is the frequency sensitive region in which the degree of maximum alignment can be enhanced greatly by modulating the center frequencies of different THz FCPs. The investigation demonstrates that comparing with a THz single-cycle pulse, a better result of the field-free alignment can be created by a THz FCP at a constant rotational temperature of molecule.     

Selective alignment of molecular spin isomers

We experimentally demonstrate field-free, spin-selective alignment of ortho- and para molecular spin isomers at room temperature. By means of two nonresonant, strong, properly delayed femtosecond pulses within a four wave mixing arrangement, we observed selective alignment for homonuclear diatomics composed of spin 1/2 (15N) or spin 1 (14N) atoms. The achieved selective control of the isomers' angular distribution and rotational excitation may find applications to analysis, enrichment, and actual physical separation of molecular spin modifications.     

Field-free molecular alignment and its application

Field-free molecular alignment can be achieved by nonadiabatic rotational excitation of molecules with strong femtosecond laser pulses. We experimentally and theoretically demonstrate that the degree of alignment can be improved with multi-pulse excitation. An approach is proposed to tune the frequency of few-cycle pulses using field-free alignment of molecules.     

Numerical approaches to time evolution of complex quantum systems

We examine several numerical techniques for the calculation of the dynamics of quantum systems. In particular, we single out an iterative method which is based on expanding the time evolution operator into a finite series of Chebyshev polynomials. The Chebyshev approach benefits from two advantages over the standard time-integration Crank-Nicholson scheme: speedup and efficiency. Potential competitors are semiclassical methods such as the Wigner-Moyal or quantum tomographic approaches. We outline the basic concepts of these techniques and benchmark their performance against the Chebyshev approach by monitoring the time evolution of a Gaussian wave packet in restricted one-dimensional (1D) geometries. Thereby the focus is on tunnelling processes and the motion in anharmonic potentials. Finally we apply the prominent Chebyshev technique to two highly non-trivial problems of current interest: (i) the injection of a particle in a disordered 2D graphene nanoribbon and (ii) the spatiotemporal evolution of polaron states in finite quantum systems. Here, depending on the disorder/electron-phonon coupling strength and the device dimensions, we observe transmission or localisation of the matter wave.     

H-在金属面附近光剥离的波包动力学研究

利用波包演化和自关联函数方法对H^-在金属面附近光剥离的波包动力学进行了研究.结果表明,金属面附近光剥离电子的波包演化和回归结构与H^-到金属面的距离、激光脉冲的脉冲宽度和初始动量都有一定的关系.因此,可以通过改变离子表面距离和激光脉冲的参数对光剥离电子的动力学性质进行调控研究.除此之外,光剥离电子的镜像态寿命对波包的演化和自关联函数也会产生一定的影响:考虑镜像态寿命的影响时,随着时间的演化,波包概率密度的振幅逐渐减小,波包整体上有明显的衰减,寿命对波包演化过程中的干涉有削弱的作用;通过对电子波包的自关联函数研究,发现无限长寿命的电子波包有很好的量子回归现象,而当考虑寿命因素后光剥离电子波包随着时间的演化会发生周期性的坍塌和扩散,经过一段时间后,该回归现象消失.本文的理论研究可以为表面附近电子波包动力学的实验研究提供一定的参考价值. 关键词： 波包 演化和回归 自关联函数 金属面     

Alignment-dependent strong field ionization of molecules

We demonstrate a method to measure strong field laser ionization of aligned molecules. The method employs a macroscopic field-free dynamic alignment, which occurs during revivals of rotational wave packets produced by a femtosecond laser pulse. We investigate the dependence of strong field ionization of N2 on molecular orientation. We determine that N2 molecules are four times more likely to ionize when aligned parallel to the field than when aligned perpendicular to it.     

Selective excitation of the molecular rotational wave packet by two time-delayed laser pulses

In this paper, we investigate the control of the molecular wave packet of a linear molecule by two femtosecond laser pulses. It is shown that the odd and the even rotational wave packets created by a single laser pulse can be selectively excited by accurately controlling the time delay of another laser pulse. By inserting the peak of the second laser pulse at the position of maximum or minimum value around quarter or three quarter rotational period of the slope curve with odd (or even) rotational wave packet contribution that is created by the first laser pulse, the odd rotational wave packet can be enhanced (or suppressed) while the even rotational wave packet is suppressed (or enhanced). As a result, the molecular alignments around quarter and three quarter rotational periods can be obtained. Moreover, it is also shown that by inserting the second laser pulse around the quarter or three quarter rotational periods, the changes in the maximum degree of the molecular alignment for the odd and the even rotational wave packet contributions are consistent with their corresponding slope curves at these positions.     

Coherent Hall effect in a semiconductor superlattice

The coherent Hall effect denotes the transient Hall response of impulsively excited coherent charge-carrier wave packets in a solid. We report the first experimental study of this phenomenon (i) using a semiconductor superlattice in crossed electric and magnetic fields as a model for three-dimensional materials and (ii) employing a contactless optoelectronic technique to probe the transient currents. Two field regimes with distinctly different oscillatory wave packet dynamics are found, separated from each other by a transition region where all oscillations are suppressed.     

De Sitter spacetime as a momentum measuring apparatus

We discuss the evolution of a quantum wave packet in the expanding de Sitter spacetime using the plane wave solutions of the Dirac equation. We concentrate on the case of large negative times when the packet approaches the event horizon and confirm that the evolution accords with that expected from the classical trajectories. We point out that in certain conditions the packet can split into two components that become localized at different parts of the horizon and that this effect can be seen, in an idealized sense, as a measuring process for the momentum of the particle, in direct analogy with the measurement of spin in a Stern-Gerlach experiment.     

灰色系统理论在波包演化近似计算中的应用

基于灰色系统理论, 对波包分布进行GM(1, 1)模型时间响应函数替换, 通过变分法等数学技术得到波包波函数的模拟演化行为, 并进行误差分析和修正。在实际应用中, 只需要对其发展系数参量进行确定, 就能较好地得到波函数的近似演化行为。以高斯型波包为例, 将数学建模中的灰色系统理论引入物理学的计算当中, 提供了一种近似计算方法, 在涉及波包演化的计算中可得到应用。     

Inelastic scattering of broadband electron wave packets driven by an intense midinfrared laser field

Intense, 100 fs laser pulses at 3.2 and 3.6 μm are used to generate, by multiphoton ionization, broadband wave packets with up to 400 eV of kinetic energy and charge states up to Xe(+6). The multiple ionization pathways are well described by a white electron wave packet and field-free inelastic cross sections, averaged over the intensity-dependent energy distribution for (e, ne) electron impact ionization. The analysis also suggests a contribution from a 4d core excitation, or giant resonance, in xenon.