Parallel excitation pathways in ultrafast interferometric pump-probe correlation measurements of hot-electron lifetimes in metals

Hot electron (E-E_Fermi=0.75 to 1.55 eV) lifetimes for cesiated Cu(100) and Cu(111) surfaces are measured via interferometric time-resolved two-photon photoemission with a 19-fs intensity FWHM mode locked Ti:sapphire laser at 1.55 eV. The data are analyzed using the optical Bloch equations and a laser pulse characterized in situ via surface second-harmonic generation interferometric autocorrelation. It is found that the retrieved hot-electron lifetimes are unphysically fast, and have a strong dependence on the temperature of the sample and the polarization of the laser. A simple explanation for the data is that the measured signal consists of contributions from transitions through both virtual and real intermediate states. Received: 26 July 2000 / Accepted: 8 September 2000 / Published online: 12 October 2000     

Statistical inference in coupled quantum systems

We discuss the problem of statistical inference in a coupled system. The method is based on the concept of inference introduced in [1].     

超快激光抽运-探测中探针光时间延迟量的 实时测量原理与光学设计

为了在飞秒激光抽运-探测(pump-probe)的超快测量中对探针光时间延迟量进行实时检测与有效控制,提出了一种四路同步移相干涉的光学测量系统.应用琼斯理论对该光学测量系统进行了优化设计与计算,推导出同步移相干涉系统各帧干涉图相应点的光强表达式,确定了相邻干涉图之间的相移步长,并给出了相位延迟量测量的解析表达式.最后,对光学系统在检测中可能存在的误差进行了计算分析,结果表明,系统的设计不仅能满足超快pump-probe精度的基本要求,而且优于目前同步移相干涉测量的光学设计,对于800 nm中心波长的探针光在理论上可达阿秒级的时间分辨率. 关键词： 超快抽运-探测 时间延迟量 同步移相干涉 琼斯矩阵     

Femtosecond pump-probe investigation of ultrafast silver nanoparticle deformation in a glass matrix

Single-color femtosecond pump-probe experiments were performed to investigate the time dependence of laser-induced ultrafast desorption and deformation processes of silver nanoparticles in glass. After laser excitation at wavelengths close to the surface plasmon resonance, transient extinction changes were found to exhibit dynamics on quite different time scales ranging from sub-picoseconds to some hundred ps. The slowest observed decay component is identified as characteristic for the deformation/desorption processes. Possible mechanisms for these processes are discussed. Received: 3 April 2000 / Revised version: 3 July 2000 / Published online: 20 September 2000     

Nature of quantum recurrences in coupled higher dimensional systems

We investigate recurrence phenomena in coupled two degrees of freedom systems. It is shown that an initial well localized wave packet displays recurrences even in the presence of coupling in these systems. We discuss the interdependence of time scales namely classical period and quantum revival time and explain the significance of initial conditions.     

Periodic oscillation of quantum diffusion in coupled one-dimensional systems

We analytically show that quantum diffusion in the coupled system composed of two identical chains exhibits a well-defined periodic oscillation in both transverse and longitudinal directions with a frequency determined by the interchain hopping strength, no matter whether the chains are periodic or non-periodic. We illustrate the result through numerical work on the coupled periodic chains and the quasiperiodic Aubry-Andre-Harper(AAH) chains with various modulations of onsite potentials supporting extended, critical, and localized states. We further numerically show that quantum diffusion in the coupled chains of different degrees of disorder W exhibits an exponential decay oscillation similar to the behavior of an underdamped harmonic oscillator, with a decay time inversely proportional to the square of W and a slight frequency change proportional to the square of W. Moreover, quantum diffusions in the coupled systems composed of two different chains are numerically studied, including periodic/disordered chains, periodic/AAH chains, and two different AAH chains, which exhibit the same behavior of underdamped periodic oscillation if the onsite potential difference between two chains is smaller than the interchain hoping strength.Existence of this universal periodic oscillation is a result of spectral splitting of the iso-spectra of two chains determined by interchain hopping, independent of system size, boundary condition, and intrachain onsite potentials. Because the oscillation frequency and spreading distance of wavepacket can be tuned separately by interchain hopping and intrachain potentials, the periodic oscillation of quantum diffusion in coupled chains is expected to find applications in control of quantum states and in designing nanoscale quantum devices.     

Voltage-controlled negative refractive index in vertically coupled quantum dot systems

The authors demonstrate that negative index of refraction can be achieved by tuning the tunneling rate between InGaAs quantum dots layers via simply applying a bias voltage across the layers. As the bias voltage is changed, the index of refraction is tunable from negative through zero to positive. Moreover, the large negative refractive index and little loss can be achieved at the same time.     

Retardation effects in quantum dot systems coupled via one-dimensional waveguides

We investigate the retardation effect on the radiative decay and entanglement of two quantum dots. The retardation effect is found to be very weak if the dots are coupled to free-space vacuum reservoir. To enhance the effect, we propose to embed the dots inside a one-dimensional waveguide. It is found that populations and entanglement can saturate to non-vanishing values with appropriate conditions. Furthermore, entanglement sudden-rise and sudden-fall are also observed due to this non-Markovian retardation.     

Masterequation,H-theorem and stationary solution for coupled quantum systems

We extend the Zwanzig projector formalism to coupled systems taking into account the mutual interactions of the reduced density matrices of both systems. In the Born- and Markoff-approximation we end up with a bilinear masterequation for occupation probabilities, in contrast to the usually studied linear equations. We derive theH-theorem for this equation and show that the stationary solution is the canonical or more generally a grand canonical density matrix.     

Nuclear quadrupole hyperfine quantum beats in HCl and pyrimidine measured by the pump-probe method using REMPI/LIF detection

Coherent time resolved IR-UV double resonance spectroscopy with REMPI and/or LIF detection has been used to measure the nuclear hyperfine structure of a diatomic and a polyatomic molecule. The pump-probe technique was applied and the experimental set up was optimized to achieve highest spectral resolution. Following excitation of the HCl fundamental vibrational transition by a nanosecond IR laser pulse, the nuclear quadrupole coupling constants were determined to be eQq = −69.51(22) MHz for H³⁵Cl and eQq = −54.40(16) MHz for H³⁷Cl in the J = 1 and J = 2 states of the υ = 1 level. Nuclear (Cl) spin-rotation interaction was shown to be active with the corresponding coupling constant being C _I = 0.068(10) MHz for H³⁵Cl and C _I = 0.049(8) MHz for H³⁷Cl. For pyrimidine a C-H stretch vibration ν ₁₃ was excited and the quadrupole tensor elements for the rovibronic states J _{K a} ,K _c = 1₁₀ and 1₀₁ of the υ ₁₃ = 1 level were found to be χ _aa = −3.095(10) MHz, χ _bb = 0.227(10) MHz and χ _cc = 3.322(10) MHz. In this case the residual frequency error was reduced to 8 kHz. The results of these jet experiments independently confirm those from millimeter wave and microwave measurements on static gas samples.     

Self-sustained ultrafast pulsation in coupled vertical-cavity surface-emitting lasers

We demonstrate that high-frequency, narrow-band, self-pulsating intensity oscillation can be generated with two coupled vertical-cavity surface-emitting lasers (VCSELs) and that such oscillation is greatly enhanced by collecting one of the two field lobes. The coupled VCSELs provide an ideal source for high-repetition-rate (over 40 GHz), sinusoidallike modulated laser light with Gaussian-like near- and far-field profiles. We also show that the modulation frequency can be tuned by inter-VCSEL separation or by a dc bias level.     

Factorizing coupled quantum systems and damped nonlinear Schrödinger equations

We show that the wave function of a coupled quantum system may factorize for certain coupling operators, resulting in wave functions and effective nonlinear Hamiltonians for the subsystems. Systems of coupled harmonic oscillators with discrete or continuous spectra are considered, where all degrees of freedom move in time-dependent coherent Glauber states.We present the general formalism and study two examples in detail. The problem of radiation damping results under drastic assumptions in exponentially damped harmonic motion, obeying a nonlinear Schrödinger equation. In the second example, a different type of coupling is studied which yields inverse power law damping.     

单个量子态的操纵和测量:2012年诺贝尔物理学奖述评

塞尔日.阿罗什和大卫.维因兰德获得了2012年诺贝尔物理学奖.他们独立地发展了突破性的实验方法,成功地实现了对单个量子态的测量和控制,由此展示了量子世界的基本特性.同时促进了基于量子技术的量子信息和原子钟的发展.     

Dispersion-independent high-visibility quantum interference in ultrafast parametric down-conversion

We demonstrate the effective removal of intrinsic distinguishability between entangled-photon pairs in femtosecond spontaneous parametric down-conversion. High-visibility quantum interference is recovered (an increase to 96% from 17%) while preserving the high photon-flux density associated with the use of long nonlinear crystals. This new technique is expected to serve as a basic component in the preparation of multiphoton entangled states.     

量子点操控的光子探测和圆偏振光子发射

半导体量子点是研究光子与电子态相互作用的优选固态体系,并在光子探测和发射两个方向上展现出独特的技术机遇.其中基于量子点的共振隧穿结构被认为在单光子探测方面综合性能最佳,但受到光子数识别、工作温度两个关键性能的制约.利用腔模激子态外场耦合效应,有望获得圆偏振态可控的高频单光子发射.本文介绍作者提出的量子点耦合共振隧穿（QD-cRTD）的光子探测机理,利用量子点量子阱复合电子态的隧穿放大,将QD-cRTD光子探测的工作温度由液氦提高至液氮条件,光电响应的增益达到10⁷以上,并具备双光子识别能力;同时,由量子点能级的直接吸收,原型器件获得了近红外的光子响应.在量子点光子发射机理的研究方面,作者实现了量子点激子跃迁和微腔腔模共振耦合的磁场调控,在Purcell效应的作用下增强激子自旋态的自发辐射速率,从而增强量子点中左旋或右旋圆偏振光的发射强度,圆偏度达到90%以上,形成一种光子自旋可控发射的新途径.     

Understanding fundamental processes in poly(9,9-dioctylfluorene) light-emitting diodes via ultrafast electric-field-assisted pump-probe spectroscopy

Femtosecond electric-field-assisted pump-probe measurements are a new approach to the study of fundamental processes within organic optoelectronic devices. Here we report a detailed study of organic light-emitting diodes based on poly(9,9-dioctylfluorene), using both temporal and spectral information to investigate polaron generation, due to field-induced singlet dissociation, and their subsequent recombination. The fundamental event in electroluminescence is time resolved: we find that initially free polarons coalesce into intermediate pairs of both singlets and triplets multiplicity which subsequently decay into the neutral state. Our results indicate that the efficiency of singlet formation, beta approximately 0.7, is much higher than expected from simple state degeneracy arguments (beta approximately 0.25).     

Heterodyne spectral interferometry for multidimensional nonlinear spectroscopy of individual quantum systems

A novel implementation of transient nonlinear spectroscopy is presented that allows the study of the transient nonlinear polarization emitted from individual electronic transitions. Both the amplitude and the phase of the polarization are retrieved, permitting the implementation of multidimensional spectroscopy in the excitation pulse delays. The technique is shown to detect mutual coherent coupling in a group of individual, localized excitonic transitions.     

Optical pump-probe measurements of local nuclear spin coherence in semiconductor quantum wells

We demonstrate local manipulation and detection of nuclear spin coherence in semiconductor quantum wells by an optical pump-probe technique combined with pulse rf NMR. The Larmor precession of photoexcited electron spins is monitored by time-resolved Kerr rotation (TRKR) as a measure of nuclear magnetic field. Under the irradiation of resonant pulsed rf magnetic fields, Rabi oscillations of nuclear spins are traced by TRKR signals. The intrinsic coherence time evaluated by a spin-echo technique reveals the dependence on the orientation of the magnetic field with respect to the crystalline axis as expected by the nearest neighbor dipole-dipole interaction.     

Laser steered ultrafast quantum dynamics of electrons in LiH

The response of the electronic system of LiH to a few-cycle strong field is computed by a time-dependent multiconfiguration method using a large, adaptive, basis set. The intensity, pulse duration, polarization, and phase of carrier frequency can all be tuned to steer the motion of the electrons. It is shown possible to, e.g., direct the electrons to move along the Li-H bond or normal to it. By shifting the phase, the electrons can be driven toward the Li nucleus or away from it. When the pulse is polarized not along the bond the result is a rotation of the charge density.