与794nm飞秒激光精确同步的无直流背底的1064nm脉冲光的产生

基于非共线光参量放大(NOPA)，以宽带794nm飞秒激光的倍频光为抽运光，以连续的He-Ne激光为信号光，产生了与宽带794nm飞秒激光精确同步的无直流背底的1064nm的脉冲光.实验结果显示该1064nm的光脉冲可作为光参量啁啾脉冲放大系统的抽运激光链的种子光，从而实现用全光学方法实现OPCPA系统抽运光和信号光的精确同步.还将非共线光参量放大器置于经特殊设计的He-Ne激光腔内，也同样成功得到了无直流背底的1064nm的光脉冲.经一次光参量放大后所得到的1064nm光的光谱和空间啁啾特性与非共线光参量放大器置于He-Ne激光腔外时得到的1064nm的光脉冲相同，而其单脉冲能量约为腔外NOPA的10倍. 关键词： 非线性光参量放大 光参量啁啾脉冲放大 时间同步     

Adaptive shaping of two-cycle visible pulses using a flexible mirror

We present a double-pass non-collinear optical parametric amplifier (NOPA) that delivers 4-fs visible–near-infrared pulses. The paper discusses geometrical and temporal properties of the NOPA configuration, which allow us to broaden the gain band of simultaneous parametric amplification in excess of 250 THz. The key elements of the bandwidth enhancement include (i) the use of a broadband second-harmonic pump, (ii) optimization of the incidence angle of all monochromatic spectral components of the pump in order to produce a favorable offset of the corresponding phase-matching curves, and (iii) selection of the chirp rate and time delay between the stretched pump and seed pulses. We next devise a two-stage compressor that incorporates a flexible mirror for adaptive pulse shaping and develop a simple and trustworthy feedback loop based on a one-dimensional spectral measurement. Our rapid numerical algorithm for adaptive control of the flexible membrane is found superior to more complex search routines that are less resistant to the fluctuations of the laser intensity. The automated optimization procedure results in the generation of two-cycle pulses with a carrier wavelength at ∼600 nm. The absence of deep modulation on the parametrically amplified spectrum in combination with the adaptive phase correction lead to a high quality of the temporal profile and allow concentrating 90% of the pulse energy within only a 7.5-fs time window. Received: 8 April 2002 / Revised version: 26 June 2002 / Published online: 25 September 2002 RID="*" ID="*"Present address: Institut für Photonik, Technische Universit?t Wien, Gusshausstrasse 27, 1040 Vienna, Austria RID="**" ID="**"Corresponding author. Fax: +81-3/5841-4165, E-mail: kobayashi@phys.s.u-tokyo.ac.jp     

High-repetition-rate near-infrared noncollinear ultrabroadband optical parametric amplification in KTiOPO4

We report efficient noncollinear optical parametric amplification (NOPA) of ultrabroadband near-IR pulses tunable in the 1.1-1.5 μm range at a repetition rate of 250 kHz. Improved generation of smooth near-IR continua (extending over ~1.0-1.6 μm) at 250 kHz was achieved by weakly focusing ~20% of the 1 W driving laser beam into a sapphire plate with longer focal length lenses than previously reported. Using bulk potassium-titanyl phosphate (KTiOPO(4)) pumped at 800 nm, powers as high as 11 mW (14% pump conversion efficiency) and signal pulse durations as short as 23 fs were obtained after a single white-light seeded NOPA stage.     

Theory of energy- and time-resolved two-photon photoemission from metal surfaces – influence of pulse duration and excitation condition

The theory of energy- and time-resolved two-photon photoemission (2PPE) spectra of metal surfaces is presented using density matrix formulation for a three-level system consisting of an initial occupied, intermediate unoccupied and final photoelectron states. A perturbation expansion method is employed to calculate the energy-resolved 2PPE spectrum for continuous light beams. We have obtained analytical expressions of the 2PPE spectrum corresponding to a step-by-step one-photon process through the intermediate state and a direct two-photon-ionization process via virtual transition. It is demonstrated that the intermediate state can also be populated via the nonresonant virtual process. This indicates an absolute importance of “pure dephasing” associated with the transition between the initial and intermediate states. Evolution of the 2PPE spectrum as a function of the pump photon energy is calculated to demonstrate the conditions under which the intrinsic linewidth (total dephasing time) can be deduced from the lineshape analysis. It is also found that the intensity ratio of the two peaks due to the initial and the intermediate states in 2PPE spectrum can be used to estimate the pure dephasing time. Transient behavior of the excited-state population following pulse excitation is calculated with a focus on how the ultrafast relaxation times of the excited states such as image-potential states of metal surfaces are deduced from the transient 2PPE response observed with a pulse laser with much longer duration. The time-resolved 2PPE spectra are calculated for varying detuning from the resonant excitation from the initial state to the intermediate state. Transient responses of the 2PPE signal due to direct ionization and step-by-step processes are also calculated to demonstrate that the nonresonant former process has an influence on the analysis of the cross-correlation trace of the intermediate state, by which the population relaxation time is estimated. Attempts are also made to apply the present theory to a recent time-resolved 2PPE study of the relaxation dynamics of the image-potential states as well as hot electrons in Cu(100) and Ag(100) surfaces. Received: 23 May 2000 / Accepted: 2 September 2000 / Published online: 12 October 2000     

两腔级联纠缠增强的理论分析

高纠缠度的纠缠源是实现高保真度量子信息传输与处理的保障,因为受到光学元器件自身性能不完美的限制,通过有效的操控手段来提高光场的纠缠度是十分必要的.连续变量Einstein-Podolsky-Rosen纠缠态光场可以利用工作在阈值以下的非简并光学参量放大器来获得.将两个非简并光学参量放大器级联,可以利用第二个光学腔来操控第一个光学腔输出的纠缠态光场,在一定条件下实现光场的纠缠增强.本文通过理论分析设计出两种光学腔级联的实验系统,其中,纠缠产生装置采用具有三共振结构的半整块驻波腔,输出到目前为止世界上单腔获得两组份纠缠态光场纠缠度的最高值,操控光学腔采用驻波腔或四镜环形腔的结构.详细对比分析了不同结构的操控腔对纠缠增强效果的影响,得出利用不同腔形作为操控腔的最佳实验方案.同时分析了级联腔输出光场的纠缠度随不同物理参量的变化关系,得出进一步优化的最佳实验系统参量,为实验获得更高纠缠度的纠缠态光场提供了依据.     

Broadband difference frequency mixing between visible and near-infrared pulses for few-cycle pulse generation with stable carrier-envelope phase

Difference frequency generation between broadband visible noncollinear optical parametric amplifier (NOPA) pulses and the fundamental pump laser pulses allows the generation of ultrashort infrared pulses with passively stabilized carrier-envelope phase. A simple prism compressor for the visible NOPA pulses is sufficient to generate few-cycle pulses in the infrared and no additional compression is needed. We theoretically investigate the concept, explain the principles, and demonstrate it for high repetition rate, long pulse durations, and various wavelengths by applying it to a Ti:sapphire and an Yb:KYW-based laser systems. For the latter sub-15 fs phase stable pulses around 1.8 μm with an energy of 100 nJ are obtained at 100 kHz repetition rate.     

Entangled and disentangled decoherence of intermediate electron-hole pairs in two-photon photoemission from surface bands: beyond the adiabatic approximation

We discuss the effects which combinations of entangled and disentangled decoherence and transient response induce on the early decay of coherent electron-hole pairs in the intermediate states of two-photon photoemission (2PPE) from image potential bands. We find that their interplay gives rise to deviations of the pair decay probabilities from simple exponential laws governed by independent quasiparticle lifetimes obtained in the self-energes based on the adiabatic hypothesis. Assessment of these effects for paradigmatic Cu(111) surface shows that they are most pronounced in the interval of pump-probe photon pulse delay times typical of current 2PPE experiments.     

The influence of laser fluctuations on entanglement generation in a non-degenerate parametric amplifier

We consider the effects of the phase and the amplitude fluctuations of the pump field upon the entanglement generation in a non-degenerate optical parametric amplifier. We show that the entanglement between the signal and idler modes in a NOPA system are suppressed by these fluctuations. Our results also show that entanglement is more sensitive to phase fluctuations than to amplitude fluctuations.     

Tunable visible and near-infrared pulse generator in a 5 fs regime

Transform-limited (TL) visible pulses with as short a duration as 4.7 fs with a 5 7J pulse energy have been generated for the first time from a novel noncollinear optical parametric amplifier (NOPA). Both signal-idler group-velocity matching and pulse-front matching are essential to generate coherent down-converted pulses compressible to the TL pulse with more than a 150-THz bandwidth. Tunable operation with bandwidth-limited sub-10-fs pulses in the visible (550-700 nm) and near-infrared (900-1300 nm) ranges, achieved by increasing the seed chirp, is also a remarkable property. The NOPA is believed to be on useful light source for ultrafast spectroscopy on an extremely short time-scale.     

Cross-phase-modulation-induced instabilities and frequency shifts in a photonic-crystal fiber

Copropagating fundamental-wavelength and second-harmonic femtosecond pulses of Cr: forsterite laser radiation are used to study cross-phase-modulation-induced instabilities and frequency shifts in a photonic-crystal fiber. Parametric instability of the second-harmonic probe pulse induced through cross-phase modulation by the fundamental-wavelength pump pulse gives rise to distinct sidebands in the spectrum of the probe field transmitted through the fiber. The wavelength of these sidebands was tuned in our experiments within approximately 100 nm by varying the peak power and the delay time of the pump pulse, suggesting a convenient way of controlled parametric spectral transformation of ultrashort laser pulses.This revised version was published online in March 2005. In the previous version, the published online date was missing     

A Surface Femtosecond Two-Photon Photoemission Spectrometer for Excited Electron Dynamics and Time-Dependent Photochemical Kinetics

搭建了一套研究金属和金属氧化物表面的超快激发态电子动力学和光化学动力学的飞秒双光子光电子能谱仪. 该装置将半球形电子能量分析仪和成像技术相结合,同时测量光电子的能量和角度分布.通过Mach-Zehnder干涉仪测量时间分辨的双光子光电子能谱获得超快激发电子态的动力学信息. 这一功能在Cu(111)上得到了证实. 另外还发展了一个通过实时测量双光子光电子能谱来研究表面光化学的方法,并成功应用到CH₃CH₂OH/TiO₂(110)体系. 研究表明,只有将两种方法结合起来才能正确地研究光诱导的表面激发共振的动力学.     

Electron dynamics of silicon surface states: second-harmonic hole burning on Si(111)-(7 x 7)

We report the first all-optical study of homogeneous linewidths of surface excitations by the spectral-hole-burning technique with surface-specific second-harmonic generation as a probe. Measurement of transient spectral holes induced by a 100 fs pump pulse in excitations of the surface dangling-bond states of Si(111)-(7 x 7) led to a pump-fluence-dependent homogeneous linewidth as broad as approximately 100 meV or a dephasing time as short as 15 fs. The hole-burning spectra also revealed a strong coupling between the localized dangling-bond states and the associated surface phonon mode at 570 cm(-1). Carrier-carrier scattering was responsible for the linear dependence of the dephasing rate on pump fluence, and the carrier screening effect appeared to be weak.     

Origin of spectral interferences in femtosecond stimulated Raman microscopy

In femtosecond stimulated Raman microscopy (FSRM), a spectrally narrow (Raman pump) pulse and a broad (Raman probe) laser pulse are employed to generate the Raman spectra of microscopic objects. The resulting spectra exhibit, in addition to the Raman bands, spectral modulations of comparable amplitude. Here a model is devised that attributes these modulations to a four‐wave mixing (FWM) process. Two light fields of the probe pulse and one field of the pump pulse serve as input fields. The resulting FWM field experiences a heterodyne amplification by the probe field. Simulations based on this model reproduce the appearance of the spectral modulations. Furthermore, the amplitude of the modulations exhibits dependences on the energies of pump and probe pulses as well as on the nonlinear refractive index n₂, which are in line with the model. Copyright © 2011 John Wiley & Sons, Ltd.     

测量时间分辨拉曼谱实验装置的研制

测量时间分辨拉曼谱实验装置的研制王晶晶金榕周赫田邹英华（北京大学物理系人工微结构及介观物理国家重点实验室北京１００８７１）孙志勇（北京理工大学四系４３１教研室北京１０００８１）ＴｈｅＥｘｐｅｒｉｍｅｎｔａｌＳｅｔｕｐｆｏｒＴｉｍｅ－ＲｅｓｏｌｖｅｄＲ...     

A theoretical study of pump–probe experiment in single-layer,bilayer and multilayer graphene

The pump–probe experiment is typically used to study relaxation phenomena in nonlinear optical systems. Here we use it as a tool to study the phenomenon of anomalous Rabi oscillations in graphene that was predicted recently in single-layer graphene. Unlike conventional Rabi oscillations, anomalous Rabi oscillations are unique to graphene (and possibly to surface states of topological insulators (TIs)), attributable to the pseudospin (conventional spin for TI) degree of freedom and Dirac-fermion character of the graphene system. A pump pulse of a finite duration long enough to contain a large number of cycles induces a current density that oscillates with the frequency of the pump pulse. The amplitude associated with these fast oscillations is seen to exhibit much slower oscillations with a frequency given by \({ 2 \omega ^2_{\mathrm {R}} }/{ \omega } \) – the anomalous Rabi frequency, where ω _R is the conventional Rabi frequency and ω is the frequency of the external pump field. This effect is easily probed by a probe pulse subsequent to the pump, where it manifests itself as periodic oscillations of the probe susceptibility as a function of pump duration at each probe frequency. Alternatively, it is also seen as an oscillatory function of the pump–probe delay with other variables remaining fixed. This period corresponds to the anomalous Rabi frequency. An analysis of the previously reported experimental data confirms the presence of anomalous Rabi oscillations in graphene.     

Parameter-dependent photon echo induced in CdSe/ZnS quantum dot quantum well

The two pulse photon echo (2PPE) phenomena induced by the 1s-1s electronic transition in CdSe/ZnS quantum dot quantum well (QDQW) has been studied by employing semiconductor Bloch equations. The energy eigenvalues and eigenfunctions of electrons and holes have been obtained by solving the stationary Schrödinger equation under effective-mass approximation. The Coulomb interaction, which changes with the size variation of QDQW, has been calculated and analyzed as a perturbation. The variations of the electric transition dipole moment and the energy interval with the changing of the size and structure of the QDQW have also been obtained. It has been shown from the numerical calculation results that the efficiency of 2PPE can be controlled by the variation of the size and structure of the QDQW and the mechanism has been explained in terms of the quantum size confined effect (QSCE) theory.     

Optical pulse evolution in the presence of a probe light in CW-pumped nonlinear fiber

We investigate theoretically and numerically the evolutions of optical pulses in the time domain due to modulation instability(MI), where CW pump accompanied with a probe is used as the input of nonlinear fiber. As the fiber length increases, we show that it exhibits beat frequency between the pump and the probe first when the probe lies outside the MI resonance region, and then gradually transforms into a pulse train resulting from spontaneous MI rather than induced MI. However, the regular pulse train is easier to generate in the whole fiber if the probe exists in MI resonance region,and the period of the pulse train is inversely proportional to the frequency spacing between the pump and the probe. It is emphasized that the pulse period can be adjusted only when the probe is in MI resonance region. The numerical simulations are in agreement with the theoretical results. The obtained results are guidable for generating and manipulating the optical pulse train in the fiber.     

利用双光子吸收效应测量激光脉冲的相干时间

研究了利用反向泵浦－探测双光子吸收材料的方法来测量脉冲激光的相干时间，并测量量了２３ｐｓ，５３２ｎｍ Ｎｄ：ＹＡＧ激光脉冲的相干时间。实验和理论分析表明，光束相干作用增加了双光子吸收，利用此方法可测量其它脉宽激光的相干时间。     

A simple method for timing an XFEL source to high-power lasers

We propose a technique for timing an X-ray free-electron laser (XFEL) to a high-power conventional laser with femtosecond accuracy, yielding the relative jitter between pump and X-ray probe, and allowing sorting of experimental results over a certain time window. The same electron bunch is used to produce both an XFEL pulse and an ultrashort optical pulse by means of an optical radiator downstream of the X-ray undulator. Being produced by the same electron bunch, these pulses are perfectly synchronized. Cross-correlation techniques will allow to determine relative jitter between the optical pulse (and, thus, the XFEL pulse) and a pulse from an external pump-laser. Technical realization of the proposed timing scheme uses an optical-replica synthesizer setup to be installed after the final bunch-compression stage of the XFEL for electron bunch diagnostics purposes. A number of critical issues are quantitatively analyzed.     

Characterization of three-color CARS in a two-pulse broadband CARS spectrum

We demonstrate that a broadband coherent anti-Stokes Raman scattering (CARS) spectrum generated with a typical two-pulse scheme contains two distinct, significant signals: '2-color' CARS, where the pump and probe are provided by a narrowband pulse and the continuum pulse constitutes the Stokes light, and '3-color' CARS, where the pump and Stokes are provided by two different frequency components in the continuum pulse and the narrowband pulse serves as the probe. The CARS spectra from the two different mechanisms show distinct characteristics in Raman shift range, laser power dependence, and chirping dependence. We discuss the potential for a 3-color CARS signal to cover the fingerprint region with reduced photodamage of live cells. Official contribution of the National Institute of Standards and Technology; not subject to copyright in the United States.