High-order Raman scattering of X rays by optical phonons and generation of ultrafast X-ray transients

We study the modulation of x-ray diffraction in ideal crystals by a copropagating wave of optical vibrations generated by a fs-laser pulse. Our results suggest that in the synchronous interaction regime the output diffracted x-ray field has the form of ultrafast transients with a time structure even shorter than the period of the excited vibrational mode. The behavior is explained in terms of high-order Raman scattering of x rays by optical phonons.     

Synthesis of periodic femtosecond pulse trains in the ultraviolet by phase-locked Raman sideband generation

We demonstrate a new technique for femtosecond-pulse generation that employs ultrafast modulation of a laser field phase by impulsively excited molecular rotational or vibrational motion with subsequent temporal compression. An ultrashort pump pulse at 800 nm performs impulsive excitation of a molecular gas in a hollow waveguide, and a weak delayed probe pulse at 400 nm is scattered on the temporal oscillations of its dielectric index. The resultant sinusoidal phase modulation of the probe pulse permits probe pulse temporal compression by use of both positively and negatively dispersive elements. The potential of this new method is demonstrated by the generation of a periodic train of 5.8-fs pulses at 400 nm with positive group-delay dispersion compensation.     

Probing nonequilibrium dynamics with white-light femtosecond pulses

Femtosecond pulsed lasers have become an invaluable tool for examining ultrafast nonequilibrium dynamics. With pulsewidths of a few hundred femtoseconds (fs) to less than 10 fs, these lasers can clearly provide unprecedented temporal resolution. By amplifying ultrashort laser pulses to sufficient levels of energy per pulse, it is possible to exploit the nonlinear optical properties of certain materials to generate extremely broadband pulses. These pulses retain the time structure of the incident pulse, but contain a spectral bandwidth extending from the infrared to as far as the ultraviolet. By generating white-light pulses, it becomes possible to probe ultrafast nonlinear processes over a large range of energies. In this paper, the process of generating white-light ultrashort pulses will be presented, along with a discussion of different probing techniques and procedures necessary for modeling the transient optical data. Finally, results from pump-probe measurements using a white-light probe on indium phosphide (InP) films will be presented as a demonstration of this technique.     

飞秒时间分辨实验中相关函数和时间零点的确定

结合飞秒激光在研究分子激发态弛豫动力学中的应用,介绍了几种飞秒时间分辨实验中确定泵浦激光脉冲与探测激光脉冲的相关函数和时间零点的方法.对于波长在可见波段的泵浦和探测激光脉冲,我们可以利用非线性光学的技术手段,探测泵浦光与探测光的和频光的强度随二者间的时间延迟的变化来确定相关函数和时间零点;对于波长在紫外甚至更短的波段的泵浦和探测激光脉冲,由于单脉冲能量比较低,目前还很难利用技术手段来测定泵浦激光与探测激光的相关函数及时间零点,可以利用某些原子气体(如Xe)或某些具有短寿命态的分子作平行实验进行间接测量.     

CS_2飞秒超快非线性特性的实验研究

采用中心波长800 nm、脉宽30 fs的超短激光脉冲,通过飞秒光开关技术对CS_2的飞秒超快非线性特性进行了实验研究.在探测光强与抽运光强比为1∶10时,得到了较理想的光克尔时间分辨曲线.通过实验测定的光克尔信号强度与激发光和探测光偏振方向夹角的依赖关系表明:30 fs的超短激光脉冲激发CS_2的克尔信号主要是源于光诱导双折射效应,而非用200 fs的超短激光脉冲时来自瞬态栅的自衍射效应.     

Ultrafast control of coherent population transfer with a train of femtosecond pulse pairs

We investigate the ultrafast control of coherent population transfer in a Λ-type three-level system with a train of pump-Stokes femtosecond pulse pairs, where the pulse sequences can be produced either by optical delay line or by pulse shaping with sinusoidal phase modulation. It is shown that when the pump and Stokes pulses in each pair are applied in the counterintuitive order, similar to that in the stimulated Raman adiabatic passage technique, due to temporal quantum interference (besides optical interference in the case of overlapped subpulses), ultrafast control of coherent population transfer can be achieved by scanning the inter-pair time delay or by changing the sinusoidal phase modulation parameters. This method has potential applications in ultrafast control of chemical reactions and quantum information processing.     

基于瞬态光克尔效应的超快光学快门技术

系统介绍了几种基于瞬态光克尔效应的超快光学快门技术,包括传统的光克尔双折射快门技术,瞬态光栅快门技术,以及利用瞬态克尔透镜效应的光学快门技术。这些技术都是在泵浦-探测的实验配置基础上,利用门控光对克尔介质折射率的瞬态调制导致的信号光的相位（偏振态）、传播方向或光束发散角等光学特性的改变实现对信号光的超快时间分辨测量。对比讨论了这些超快快门技术的工作原理和实验配置,结果表明瞬态光束偏折快门技术相比其他快门技术具有无偏振配置要求、无相位匹配条件、超宽带光谱响应范围的特点,在光与物质相互作用的超快动力学研究中具有更为广阔的应用前景。     

基于高聚物光诱导激子漂白的无腔光学双稳态

基于已建立的高聚物光诱导激子漂白动力学理论，模拟计算了一类基态非简并高聚物在光激发和弛豫过程中的瞬态无腔光学双稳态．结果表明，基于高聚物的超快激发和弛豫，可获得瞬态饱和吸收型和增强吸收型无腔光学双稳态，其开关时间均在ps和fs量级．随着入射探测光脉冲相对入射泵浦光脉冲延迟时间的增加，无腔光学双稳态可由饱和吸收型向增强吸收型转变，转变的快慢与高聚物光诱导漂白的弛豫的快慢成正比 关键词：     

Generation and stabilization of ultrafast optical pulse trains with monolithic mode-locked laser diodes

In this paper, we report on the generation and the stabilization of ultrafast optical pulse trains exceeding 100 GHz from monolithic mode-locked laser diodes (MLLDs) combined with some new techniques such as subharmonic synchronous mode-locking (SSML) and repetition-frequency multiplication (RFM) method. Key subjects to increase the pulse repetition frequencies of the MLLDs such as fast absorption recovery and harmonic mode-locking operation are discussed. 500 GHz optical pulse generation from a short-cavity, graded-index separated confinement heterostructure MLLD and THz-rate pulse generation by harmonic mode-locking are reported. We also demonstrate the stabilization of a 160 GHz MLLD by the SSML with subharmonic-frequency optical pulse injection and reveal that the SSML is very promising as a stabilization technique of the ultrafast MLLD beyond the limitations by the electronic device speed. A method to accurately measure the timing jitter of such ultrafast optical pulse train, all-optical down converting using a nonlinear optical device, is also presented. We also mention another choice for ultrafast optical pulse generation using the MLLD combined with a dispersive medium such as an optical fiber. We demonstrate here the generations of stable 84–256 GHz optical pulse trains by the RFM method of the MLLD stabilized by the SSML.     

Proposal and analysis of a reconfigurable pulse shaping technique based on multi-arm optical differentiators

We propose and numerically investigate an optical pulse re-shaping method based on multi-arm ultrafast optical differentiators. In this approach, the desired (arbitrary) optical pulse shape is synthesized by coherently overlapping different successive time derivatives of an input optical pulse (not necessarily a Gaussian-shape pulse), including the input pulse itself, with suitable relative weights. Time derivatives of (sub-)picosecond pulses can be obtained using first and higher-order ultrafast optical differentiators practically implemented with integrated waveguide or fiber-based linear filtering technologies. Different output pulse shapes can be generated from the same platform by properly programming the relative weights among the different pulse derivatives. The effective bandwidth of the output waveform is not necessarily limited by the input pulse bandwidth but rather it depends on the highest derivative order used for the pulse synthesis. Our results reveal that interesting transform-limited pulse shapes (including flat-top and parabolic waveforms) can be synthesized from Gaussian-like (e.g. Gaussian, sech) pulses using a simple and practical three-arm ultrafast differentiation system with amplitude-only relative weights.     

富勒烯有机-无机杂化材料的非共珍三阶非线性光学特性

采用中心波长800 nm、脉宽30 fs的超短激光脉冲,通过飞秒光开关技术(Optical Kerr Shutter,OKS)对富勒烯有机-无机杂化材料的飞秒超快非线性特性进行了实验研究.获得270 fs的开关时间,所得的富勒烯有机-无机杂化材料的三阶非线性系数X~(3)约为4.5×10~(-4) esu,比C_(60)分子的三阶非线性系数X~(3)高一个数量级.通过实验测定的光克尔信号强度与激发光和探测光偏振方向夹角的依赖关系表明:30 fs的超短激光脉冲激发富勒烯有机-无机杂化材料的克尔信号主要是源于光诱导双折射效应,而非用200 fs的超短激光脉冲时来自瞬态栅的自衍射效应.     

富勒烯有机-无机杂化材料的非共珍三阶非线性光学特性

采用中心波长800 nm、脉宽30 fs的超短激光脉冲,通过飞秒光开关技术（Optical Kerr Shutter,OKS）对富勒烯有机-无机杂化材料的飞秒超快非线性特性进行了实验研究.获得270 fs的开关时间,所得的富勒烯有机-无机杂化材料的三阶非线性系数χ(3)约为4.5×10－14 esu,比C60分子的三阶非线性系数χ(3)高一个数量级.通过实验测定的光克尔信号强度与激发光和探测光偏振方向夹角的依赖关系表明：30 fs的超短激光脉冲激发富勒烯有机-无机杂化材料的克尔信号主要是源于光诱导双折射效应,而非用200 fs的超短激光脉冲时来自瞬态栅的自衍射效应.     

Femtosecond pump-probe photoionization-photofragmentation spectroscopy of azobenzene cation

We report the studies of ultrafast dynamics of azobenzene cations using femtosecond photoionization-photofragmentation spectroscopy. In our experiment,a femtosecond pump pulse first prepares an ensemble of azobenzene cations via photoionization of neutrals. A delayed probe pulse then brings the evolving ionic system to higher states that ultimately undergo ion fragmentation. The dynamics is followed by monitoring either the parent-ion depletion or fragment-ion formation as a function of the pump-probe delay time. The observed transients for azobenzene cations are characterized by a constant ion depletion modulated by a rapidly damped oscillatory signal with a period of about 1 ps. Theoretical calculations suggest that the oscillation arises from a vibration motion along the twisting inversion coordinate involving displacements in CNNC and phenyl-ring torsions. The oscillation is damped rapidly with a time constant of about 1.2 ps,suggesting that energy dissipation from the active mode to bath modes takes place on this time scale.     

Transmission Properties of the Femtosecond Pulse Through a Near-field Optical Fiber probe

The experimental results are given on the broadening characteristics of the time and frequency profile of the femtosecond pulse passed through a SNOM fiber probe. The power threshold for the spectral and temporal broadening was 158 μw for pulses with 250 fs width and the repetition rate of 250 kHz at 830 nm wavelength. The time broadening reached lps for 250 fs incident pulse of 500 μw. A finite-difference time-domain (FDTD) simulation was carried out to illuminate the transmission properties of ultrafast pulse through the SNOM probe. The broadening influence on temporal resolution spectrum and near-field fluorescence excited by two-photon technique are discussed.     

Optical second harmonic generation and its photoinduced dynamics in ferroelectric semiconductor Sn<Subscript>2</Subscript>P<Subscript>2</Subscript>S<Subscript>6</Subscript>

By means of optical pump–probe technique, the ultrafast dynamics of nonlinear optical response of the ferroelectric semiconductor Sn₂P₂S₆ crystal excited with a femtosecond laser pulse has been investigated. It has been shown that, under the action of femtosecond pulses, change in optical second harmonic generation occurs in the sample, which can be due to screening of existing electric polarization.     

Multiframe observation of an intense femtosecond optical pulse propagating in air

We have demonstrated femtosecond time-resolved and picosecond time-interval successive observations of a single femtosecond optical pulse propagating in air with ultrafast self-modulation such as filamentation. A quadruple femtosecond probe pulse crossing an intense propagating pulse at picosecond intervals was able to capture directly four successive images of the propagating pulse as in a movie but with femtosecond time resolution. From this observation, we can directly analyze and recognize the propagation process, which is significantly affected by pulse-energy fluctuation, and (or) atmospheric turbulence from shot to shot.     

Single-shot time-frequency imaging spectroscopy using an echelon mirror

We demonstrate single-shot time-frequency imaging spectroscopy with an echelon mirror for measuring ultrashort laser pulses as well as ultrafast responses of materials using the same optical setup. The echelon mirror produces a spatially encoded time delay for the probe pulse whereby both the probe and pump pulses are focused on samples with small spot size. Using the optical Kerr gate apparatus, we successfully mapped the time-frequency images of ultrashort laser pulses and subsequently evaluated the chirp characteristics with the phase-retrieval procedure on a single-shot basis. By simply replacing the Kerr medium with samples, we could also visualize the phonon-polariton oscillations in ferroelectric LiNbO3.     

Spatiotemporal stability of a femtosecond hard-x-ray undulator source studied by control of coherent optical phonons

We report on the temporal and spatial stability of the first tunable femtosecond undulator hard-x-ray source for ultrafast diffraction and absorption experiments. The 2.5-1 Angstrom output radiation is driven by an initial 50 fs laser pulse employing the laser-electron slicing technique. By using x-ray diffraction to probe laser-induced coherent optical phonons in bulk bismuth, we estimate an x-ray pulse duration of 140+/-30 fs FWHM with timing drifts below 30 fs rms measured over 5 days. Optical control of coherent lattice motion is demonstrated.     

Ultrafast nonlinear optical response of carbon nanotubes functionalized with water soluble porphyrin

A new nanocomposite is obtained by functionalizing carbon nanotubes (CNTs) with a water soluble metalloprophyrin using a simple chemical technique and characterized by optical absorption, IR, and Raman spectroscopy. Results from spectroscopic studies indicate the noncovalent nature of interaction between CNTs and porphyrin. The ultrafast nonlinear response is characterized by measuring the nonlinear absorption coefficient and refractive index by z-scan technique in the femtosecond pulse regime. The nanocomposite is found to exhibit two-photon absorption (TPA) with a reasonably large nonlinear optical coefficient, whereas pure CNTs is known to exhibit saturable absorption. Design of such water soluble nanocomposites offers scope for obtaining materials with enhanced ultrafast optical nonlinearity.