Spectrally resolved coherent transient signal for ultracold rubidium molecules

We present spectrally resolved pump-probe experiments on the photoassociation of ultracold rubidium atoms with shaped ultrashort laser pulses. The pump pulse causes a free-bound transition leading to a coherent transient signal of rubidium molecules in the first excited state. In order to achieve a high frequency resolution the bandwidth of the pump pulse is reduced to a few wavenumbers. The frequency dependence of the transient signal close to the D1 atomic resonance is investigated for characteristic pump-probe delay times. The observed spectra, which show a pronounced dip for pump-probe coincidence, are interpreted using quantum dynamical calculations.     

Optical pump-probe scanning tunneling microscopy for probing ultrafast dynamics on the nanoscale

The development of a method for exploring the ultrafast transient dynamics in small organized structures with high spatial resolution is expected to be a basis for further advances in current science and technology. Recently, we have developed a new microscopy technique by combining scanning tunneling microscopy (STM) with ultrashort-pulse laser technology, which enables the visualization of ultrafast carrier dynamics even on the single-atomic level. A nonequilibrium carrier distribution is generated using ultrashort laser pulses and its relaxation processes are probed by STM using the optical pump-probe method realized in STM by the pulse-picking technique. In this paper, the fundamentals of the new microscopy technique are overviewed.     

Clusters on surfaces: high-resolution spectroscopy at low temperatures

With controlled growth in nanometer-sized pits we produced silver and gold clusters on a graphite surface. We give a summary of the preparation method and discuss the scanning tunneling imaging and the crystalline orientation of the clusters. The electronic structure of the clusters was studied by an in-situ combination of ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling spectroscopy (STS). For both techniques we obtained an energy resolution in the range of 10 meV employing low sample temperatures. Dynamic final-state effects together with averaging over a cluster-size distribution result in characteristic spectral shapes in UPS, which can be understood referring to STS data taken on individual clusters. Finally, directions for future experiments are pointed out. Received: 13 April 2000 / Accepted: 6 November 2000 / Published online: 9 February 2001     

Theoretical study of the time-resolved photoelectron spectrum of Na 2F: effects of thermal initial conditions

The excited state dynamics of the Na₂F cluster initiated by a femtosecond laser pulse is studied considering a thermally excited initial sample. Within a pump-probe set-up, the time-dependent photoelectron spectrum is calculated, which is shown to be a sensitive tool to study intramolecular motion of the cluster. Temperature effects are taken into account through thermal averaging over the time-dependent spectra obtained from different initial vibrational states of the cluster. The nuclear motion upon laser excitation is described by full-dimensional quantum wavepacket propagation using explicit, realistic pump and probe pulses. The characteristic features of the time-resolved photoelectron spectra of the Na₂F cluster, identified as due to periodic bending motion of the cluster as well as to the excitation of the stretching mode, are found to be robust against increasing vibrational temperature of the cluster beam. This finding is important for possible future experiments.     

Determination of the temporal response function in femtosecond pump-probe systems

A method of determination of the true temporal response function for pump-probe type of experiments with femtosecond time resolution is presented. An analytical formula allowing calculation of group velocity dispersion (GVD) modified and sample thickness-sensitive pump-probe cross correlation function is provided. The reliability of the formula is further experimentally verified with measurements of ultrafast stimulated Raman scattering and transient absorption signals. Received: 31 October 2000 / Revised version: 6 February 2001 / Published online: 27 April 2001     

Probing angular correlations in sequential double ionization

We study electron correlation in sequential double ionization of noble gas atoms and HCl in intense, femtosecond laser pulses. We measure the photoelectron angular distributions of Ne+ relative to the first electron in a pump-probe experiment with 8 fs, 800 nm, circularly polarized laser pulses at a peak intensity of a few 10(15) W/cm2. Using a linear-linear pump-probe setup, we further study He, Ar, and HCl. We find a clear angular correlation between the two ionization steps in the sequential double ionization intensity regime.     

NaLi分子飞秒含时光电子能谱的理论研究

运用三态模型和含时波包方法,研究了NaLi分子处于强飞秒抽运-探测激光场中的波包动力学过程和时间分辨光电子能谱,并且揭示了飞秒激光脉冲参数与NaLi分子光电子能谱之间的关系.研究发现:对于不同的激光波长,波包的振动周期是不同的,而且随着抽运-探测脉冲延迟时间的不同,NaLi分子光电子能谱的谱峰高度和位置发生变化;当λ1=352 nm并且?t=400 fs时,外阱中相应的光电离信号(0.5 eV处)明显强于内阱中相应的光电离信号(1.35 eV处).计算结果表明,NaLi分子激发态41Σ+上波包动力学的一些信息能够通过其光电子能谱反映出来.这些结果可以为实验上实现分子的光控制以及量子操控过程提供一些有价值的参考信息,并为进一步的理论研究提供重要依据.     

Visualization of electron orbitals in scanning tunneling microscopy

Scanning tunneling microscopy (STM) is one of the main techniques for direct visualization of the surface electronic structure and chemical analysis of multi-component surfaces at the atomic scale. This review is focused on the role of the tip orbital structure and tip-surface interaction in STM imaging with picometer spatial resolution. Fabrication of STM probes with well-defined structure and selective visualization of individual electron orbitals in the STM experiments with controlled tunneling gap and probe structure are demonstrated.     

Picosecond-resolution study of nonlinear scattering in carbon black suspensions in water and ethanol

The physical origin of the nonlinear scattering of light induced by focusing of intense laser pulses onto carbon black suspensions (CBS's) is discussed through the interpretation of limitation, time-resolved pump-probe transmission, and angularly and time-resolved scattering experiments performed with picosecond resolution. Comparison of the experimental results performed on two different solutions of CBS's in water and in ethanol leads us to the conclusion that the scattering phenomenon begins most probably with growing microplasmas and that the influence of the solvent is significant only at longer times.     

Time-resolved current response of a nanosecond laser pulse illuminated STM tip

Time-resolved dependence of the transient current through a ns laser pulse illuminated scanning tunneling microscope (STM) tip/sample gap in tunneling mode and out of tunneling range is presented. A self-designed fast STM-preamplifier (bandwidth 35 MHz) allows one to resolve the fine structure of the transient signal as well as the observation of some effects that are undetectable by using conventional low-band preamplifiers. The dependence of the threshold laser pulse intensity, which corresponds to the beginning of electron emission from tip (in non-tunneling mode), as a function of the tip/sample distance was investigated. At tip/sample distances from tunnel contact up to approximately 1 μm a linear dependence is found. This behavior is in good agreement with the theory for field enhancement in a STM tip/sample system. In tunneling mode a ns (fast component) as well as a μs (slow component) current response was found as a result of the laser pulse illumination. These data suggest the tip bending to be an important factor in clarifying the thermal/mechanical mechanism of laser-assisted surface nanomodification. Received: 4 May 1998 / Accepted: 29 January 1999 / Published online: 28 April 1999     

Ultrafast dynamics in ZnO thin films irradiated by femtosecond lasers

The damage morphologies, threshold fluences in ZnO films were studied with femtosecond laser pulses. Time-resolved reflectivity and transmissivity have been measured by the pump-probe technique at different pump fluences and wavelengths. The results indicate that two-phase transition is the dominant damage mechanism, which is similar to that in narrow band gap semiconductors. The estimated energy loss rate of conduction electrons is 1.5 eV/ps.     

Ultrafast emission of ions during laser ablation of metal for 3D atom probe

The 3D atom probe(3DAP) is an imaging instrument based on the controlled field evaporation of single atoms from a sample having a tip shape with an end radius of 50 nm. In the fs laser assisted 3DAP the evaporation is induced by the laser pulses so that the physical process involved in this 3DAP analysis might correspond to the very early stages of the ablation process. In this paper we present the principle of the 3DAP and we discuss the existing models of the fs assisted evaporation. At last, we test the relevance of these models with pump-probe experiments on tungsten tips in the tomographic atom probe.     

Influence of phonon-associated tunneling rate on transport through a single-molecule transistor

We study the electronic transport through a single-molecule transistor (SMT) by considering the phonon-associated tunneling rate. We find that the electron-phonon interaction (EPI) changes the constant conductivities of the leads into a multi-channel structure of single vibration frequency. This interference of the multi-channel tunneling process results in a bias-dependent tunneling rate and obscures the conductance peaks at large bias voltage. The bias-dependent tunneling rate further causes a remarkable conductivity gap between the chemical potential of the leads (n=0) and the first phonon sideband (n=1). These anomalies are consistent with the experimental observations in transport experiments.     

Li2分子含时波包动力学的理论研究

利用含时量子波包方法计算得到了Li2分子的光电子能谱,并运用波包动力学理论对含有不同参量的光电子能谱现象给出了合理的解释。通过分析文中的直观图像,研究了波包的动力学过程。结果表明,泵浦-探测脉冲的延迟时间对光电子能谱的形状有重要的影响;在较短延迟时间下,能谱独特的四峰现象是由光诱导势的产生引起的。     

Li2分子含时波包动力学的理论研究

利用含时量子波包方法计算得到了Li2分子的光电子能谱,并运用波包动力学理论对含有不同参量的光电子能谱现象给出了合理的解释.通过分析文中的直观图像,研究了波包的动力学过程.结果表明,泵浦-探测脉冲的延迟时间对光电子能谱的形状有重要的影响;在较短延迟时间下,能谱独特的四峰现象是由光诱导势的产生引起的.     

Dielectric function dynamics during femtosecond laser excitation of bulk ZnO

Using a broad band dual-angle pump-probe reflectometry technique, we obtained the ultrafast dielectric function dynamics of bulk ZnO under femtosecond laser excitation. We determined that multiphoton absorption of the 800-nm femtosecond laser excitation creates a large population of excited carriers with excess energy. Screening of the Coulomb interaction by the excited free carriers causes damping of the exciton resonance and renormalization of the band gap causing broadband (2.3–3.5 eV) changes in the dielectric function of ZnO. From the dielectric function, many transient material properties, such as the index of refraction of ZnO under excitation, can be determined to optimize ZnO-based devices.     

飞秒激光参数对三态阶跃型分子Autler-Townes分裂的影响

利用三态模型和含时波包法, 研究了K₂分子在强飞秒泵浦-探测激光场中泵浦/探测场强、波长对光电子能谱Autler-Townes(AT)分裂的影响．通过分别改变两激光场的场强或者波长预测AT峰移和间距,并且首次量化了AT分裂的峰移和间距．光电子能谱在共振时显示为对称双峰,失谐时为非对称双峰。AT分裂间距随泵浦场强增大而增大,但不因探测场强改变而改变．     

On the role of athermal electrons in non-linear photoemission from Ag(100)

The non-linear photoelectron spectra obtained by short laser pulses from a Ag(100) surface show a high-energy electron emission due to an athermal electron distribution created by the laser pulse. By comparing the photoemission at normal and non-normal emission geometry it is possible to evidence the independence of the hot electron photoemission on the parallel momentum and on different final-state configurations. A photoemission correlation measurement evidences that non-photoelectric effects, as tunneling or thermally assisted photoemission, do not contribute to the electron yield. Various theoretical models are discussed on the basis of the present data.     

纳秒激光作用下二氧化钒薄膜相变特性

采用脉冲激光沉积法制备了VO2薄膜,应用X射线衍射和X射线光电子能谱分析表明样品为多晶薄膜。采用泵浦-探针方法研究了二氧化钒薄膜的相变特性,实验结果表明当激光重复频率为160 Hz时,样品最小相变响应为12ns,相变恢复时间与激光能量按照自然指数关系变化。仿真结果表明当激光能量一定时,二氧化钒薄膜相变恢复时间与衬底材料的热导率、热扩散系数有关。     

Manipulating Photoelectron Distributions for Rare Gas Atoms Ionized with Polarized Femtosecond Laser Pulses

Above-threshold ionizations of rare gas atoms excited by polarized femtosecond laser pulses are investigated. The photoelectron momentum spectra are obtained applying the strong-field approximation (SFA) theory. It is found that, distribution of the emitted photoelectrons varies with different polarizations of laser pulses. We have interpreted the relationship between the observed distribution and the laser polarization taking advantage of tunneling ionization theory and simple-man model. The polarization sensitivity indicates that one can easily manipulate the photoelectron distribution by controlling the polarization of the exciting pulse.