Femtosecond laser photoelectron projection microscopy of organic nanocomplexes

Organic nanocomplexes obtained by applying an organic dye (Coumarin 153) solution onto the surface of a metal nanotip were studied by femtosecond laser photoelectron projection microscopy. The tip was a 100-nm-diameter capillary covered with a thin nickel layer. The image of a through hole of the capillary was a convenient reference for interpretation of the results. The spatial resolution achieved in these experiments was equal to about 5 nm.     

Visualization of the spatio-temporal structure of a pulsed photoelectron beam formed by femtosecond laser radiation

A method based on an original electron microscope created for investigating photoelectron beams is presented. It ensures a nanometer spatial resolution and picosecond time resolution. Electrons appearing when a metal needle is irradiated by femtosecond laser pulses are transmitted through a dielectric microcapillary and are subjected to a ponderomotive potential created by femtosecond laser radiation focused near the capillary tip. The position-sensitive detection scheme allows for the detection of the spatial profile of a photo-electron beam with a magnification of K ≅ 4 × 10⁴. The time structure of the photoelectron beam is visualized by scanning the delay time between laser pulses irradiating the needle and a laser pulse focused near the capillary tip.     

Generation of a directed nanolocalized photoelectron beam by means of femtosecond laser pulses

A directed photoelectron beam is obtained when photoelectrons from a sharp metal tip irradiated by femtosecond laser pulses are passed through a quartz nanocapillary. Such a nanolocalized photoelectron train makes it possible to conduct experiments with simultaneous femtosecond time and nanometer spatial resolutions.     

Ultrafast dynamics of o-fluorophenol studied with femtosecond time-resolved photoelectron and photoion spectroscopy

The ultrafast dynamics of o-fluorophenol via the excited states has been studied by femtosecond time-resolved photoelectron imaging. The photoion and photoelectron spectra taken with a time delay between 267 nm pump laser and 800 nm probe laser provide a longer-lived S1 electronic state of about ns timescale. In comparison,the spectra obtained by exciting the S2 state with femtosecond laser pulses at 400 nm and ionizing with pulses at 800 nm suggest that the S2 state has an ultrashort lifetime about 102 fs and reflects the internal conversion dynamics of the S2 state to the S1 state.     

Photoemission electron microscopy as a tool for the investigation of optical near fields

Photoemission electron microscopy was used to image the electrons photoemitted from specially tailored Ag nanoparticles deposited on a Si substrate (with its native oxide SiO(x)). Photoemission was induced by illumination with a Hg UV lamp (photon energy cutoff homega(UV) = 5.0 eV, wavelength lambda(UV) = 250 nm) and with a Ti:sapphire femtosecond laser (homega(l) = 3.1 eV, lambda(l) = 400 nm, pulse width below 200 fs), respectively. While homogeneous photoelectron emission from the metal is observed upon illumination at energies above the silver plasmon frequency, at lower photon energies the emission is localized at tips of the structure. This is interpreted as a signature of the local electrical field therefore providing a tool to map the optical near field with the resolution of emission electron microscopy.     

基于局域场加强的近场纳米加工技术

介绍了一种全新的基于飞秒激光局域场加强效应的纳米加工技术,通过使用了微焦级别脉宽为130 fs、波长800 nm的飞秒激光照射于原子力显微镜的探针针尖,利用其局域场加强效应在金薄膜表面加工出各种纳米图形。对加工参数对加工线宽的影响中,我们发现了随着加工能量的减小和加工速度的不断增大将导致加工线宽不断减小,最终达到了极限线宽(～10 nm)。这项技术可以广泛的应用与各种材料的加工中,尤其适合各种金属薄膜的加工,特别是结合了现有的自动化控制系统更是可以加工出任意复杂的二维纳米图形。     

Field emission tip as a nanometer source of free electron femtosecond pulses

We report a source of free electron pulses based on a field emission tip irradiated by a low-power femtosecond laser. The electron pulses are shorter than 70 fs and originate from a tip with an emission area diameter down to 2 nm. Depending on the operating regime we observe either photofield emission or optical field emission with up to 200 electrons per pulse at a repetition rate of 1 GHz. This pulsed electron emitter, triggered by a femtosecond oscillator, could serve as an efficient source for time-resolved electron interferometry, for time-resolved nanometric imaging and for synchrotrons.     

用质谱和光电子能谱研究飞秒强场中的电离动力学

报导了用自制飞秒激光器通过飞秒多光子电离质谱和光电子能谱对飞秒强激光场与分子（氨、苯）相互作用的研究,飞秒激光脉宽约１００ｆｓ,二倍频中心波长４０７．５ｎｍ,聚焦后脉冲功率密度达到１０＾１２Ｗ／ｃｍ＾２,氨的光电子能谱显示了（２＋２）ＲＥＭＰＩ和（２＋２）＋１ＡＴＩ、（２＋２）＋２ＡＴＩ三组电子峰,每组峰又包括伸缩振动ｖ１的带系,ＡＴＩ峰的振动布居出现反转,随着光强增加,谱峰加宽而且振动能级出现平     

Laser photoelectron projection microscopy of an organic conducting polymer

A conducting organic polymer is visualized on a laser photoelectron projection microscope, which is based on Letokhov’s concept and has a nanometer spatial resolution. Photoelectron images of polyaniline (which is the most promising representative of conducting polymers) with a magnification of ∼10⁵ have been obtained when a 100-nm quartz capillary coated with a film of this material was irradiated by femtosecond laser pulses. The projection photoelectron method using 400- and 800-nm laser radiation has made it possible to directly reveal the existence of the redox heterogeneity of the organic polymer, which is due to the contact of the sections of polyaniline with different oxidation degrees and strongly affects the electric conductivity of the sample.     

Measurement of the Gaponov-Miller force produced in vacuum by tightly focused intense femtosecond laser radiation

A method for measuring the Gaponov-Miller force (GMF) is demonstrated based on the deflection of a picosecond photoelectron beam exposed to tightly focused intense femtosecond laser radiation. It is shown experimentally that the action of this force produced by femtosecond laser pulses linearly depends on their intensity. The method can be used to verify the correctness of measuring the duration of an ultrashort electron bunch based on the GMF.     

用于宽带啁啾脉冲光参量放大系统精确同步的新方法

基于非共线光参量放大,以波长800 nm飞秒激光的倍频光为抽运光,以激光二极管640 nm连续波激光为信号光,产生了与800 nm飞秒激光精确同步的无直流本底的1064 nm脉冲光。实验结果显示该1064 nm的光脉冲在脉冲能量和空间光束质量上均可作为1064 nm波长脉冲放大器的种子光。仅通过一级简单紧凑的非共线参量放大就可实现光周期量级的光参量啁啾脉冲放大系统的抽运激光和种子光的精确同步。     

Microstructured polymer‐based substrates with broadband absorption for surface‐enhanced Raman scattering

Large area (3 × 3 cm²) substrates for surface‐enhanced Raman scattering were fabricated by combining femtosecond laser microstructuring and soft lithography techniques. The fabrication procedure is as follows: (i) femtosecond laser machining is used to create a silicon master copy, (ii) replicates from polydimethylsiloxane are made, and (iii) a 50‐nm‐thick gold film is deposited on the surface of the replicates. The resulting substrates exhibit strongly enhanced absorption in the spectral region of 350 ∼ 1000 nm and generate enhanced Raman signal with enhancement factor of the order of 10⁷ for 10^{‐ 6} M rhodamine 6G. The main advantages of our substrates are low cost, large active area, and possibility for mass replication. Copyright © 2013 John Wiley & Sons, Ltd.     

Evolution of femtosecond laser-induced damage in doped GaN thin films

Mg- and Si-doped GaN layers deposited by metalorganic chemical vapor deposition method were irradiated with femtosecond pulse duration laser of three different wavelengths 1,030, 515 and 343 nm. Both single and multiple shot laser induced damage thresholds of doped GaN layers were evaluated and discussed. The scanning electron microscopy employed with electron beam induced current and energy dispersive X-ray techniques were used to study laser damage morphology. It was observed that ablated area and laser-induced damage increased with irradiation fluence. The mechanism of damage generation by Gaussian beam profile laser was considered.     

Autler-Townes Splitting in Photoelectron Spectrum of Three-Level Li2 Molecule in Ultrashort Pulse Laser Fields

The Autler-Townes （AT） splitting in femtosecond photoelectron spectrum of three-level Li2 molecules is theoretically investigated using time-dependent quantum wave packet method. With proper femtosecond laser pulses, three peaks of the AT splitting can be observed in the photoelectron spectrum. The AT splitting stems from rapid Rabi oscillation caused by intense ultrashort laser pluses. The effects of laser parameters on the molecular ionization dynamics are also discussed.     

Nanosecond and femtosecond ablation of La0.6Ca0.4CoO3: a comparison between plume dynamics and composition of the films

Thin films of La_0.6Ca_0.4CoO₃ were grown by pulsed laser ablation with nanosecond and femtosecond pulses. The films deposited with femtosecond pulses (248 nm, 500 fs pulse duration) exhibit a higher surface roughness and deficiency in the cobalt content compared to the films deposited with nanosecond pulses (248 nm, 20 ns pulse duration). The origin of these pronounced differences between the films grown by ns and fs ablation has been studied in detail by time-resolved optical emission spectroscopy and imaging. The plumes generated by nanosecond and femtosecond ablation were analyzed in vacuum and in a background pressure of 60 Pa of oxygen. The ns-induced plume in vacuum exhibits a spherical shape, while for femtosecond ablation the plume is more elongated along the expansion direction, but with similar velocities for ns and fs laser ablation. In the case of ablation in the background gas similar velocities of the plume species are observed for fs and ns laser ablation. The different film compositions are therefore not related to different kinetic energies and different distributions of various species in the plasma plume which has been identified as the origin of the deficiency of species for other materials.     

Two-color cross-correlation of fs-laser pulses by two-photon induced photoconductivity for near and far field optical measurements

Two-color two-photon induced photoconductivity in a GaAsP diffusion type photodiode is demonstrated by measuring femtosecond cross-correlation functions for widely separated wavelength pairs of 775 and 1300 nm. Results are obtained for a range of tunable wavelengths and average powers of the incident lasers by measuring the two-photon induced photocurrent as a function of the optical delay between the pulses. The temporal autocorrelation of femtosecond laser pulses in the near-field of a small diameter aluminum coated optical fiber tip is also obtained with the same photodiode method for single colors.     

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.     

飞秒激光改性6H-碳化硅晶体表面光电导增益现象研究

半绝缘6H型碳化硅(6H-SiC)具有高电阻率性质,在可见光照射下进行光电导测量时,通常光生电流很小;然而经过飞秒激光辐照改性之后,发现在可见光波段的光电导有明显的增益.本文利用紫外-可见-近红外吸收谱、X射线光电子能谱和发光光谱测量分析了激光改性之后碳化硅样品的光谱吸收、发射和晶体元素比例变化情况.分析认为碳化硅光电导增益的原因是飞秒激光辐照过程改变了碳化硅表面的硅碳元素的原子浓度比,形成新的物质结构形式,从而导致了表面光电导性能的提高.     

Femtosecond pulsed laser ablation deposition of tantalum carbide

In this work a frequency-doubled Nd:glass laser with a pulse duration of 250 fs has been used to ablate a TaC target and to deposit thin films on silicon. The results have been compared with those previously obtained by nanosecond pulsed laser deposition and evidence of large differences in the plasma characteristics has been revealed. In particular, in the femtosecond and nanosecond plumes the energy and the velocity of neutral and ionized particles are very different. The features of femtosecond ablation include the delayed emission from the target of large and slow particles. The characteristics of the femtosecond plasma are clearly related to the morphology and composition of the deposited films and the results show a nanostructure consisting of a large number of spherical particles, with a mean diameter of about 50 nm, with a stoichiometry corresponding to Ta₂C. To explain these features, an ablation-deposition mechanism, related to the ejection of hot particles from the target, is proposed.