Attosecond probing of localized surface plasmon fields

局域表面等离子体(LSP)作为相干的集体电子激发，表现为纳米局域光场增强，由于LSP共振固有的大光谱带宽，它经历了超快的阿秒动力学。到目前为止，还没有在纳米空间和阿秒时间尺度上对这种光场动力学进行直接的实验观测。本项目的目标是通过应用几个周期的光抽运脉冲进行共振激发，并利用单个阿秒XUV脉冲诱导的阿秒定时光电子发射来探测阿秒光场纳米，从而为阿秒光场纳米观察设定实验前提条件。被解放的价带光电子与瞬时局域表面等离子体激元相互作用，导致光电子动能的变化，从而携带着超快LSP场的“快照指纹”。用飞行时间光电子发射显微镜作为探测器来记录发射光电子的空间位置和动能。测量是通过以下独特的实验设置来完成的：高重复频率(10 KHz)高次谐波产生源，提供*90 eV光子能量的单个阿秒XUV脉冲；具有800 nm/4 fs几个周期泵浦脉冲(可选400 nm倍频脉冲)的干涉泵浦探测延迟台；以及带有两个免费成像能量过滤器和检测器的UHV-ToF PEEM系统。首次对Au/Si纳米结构的阿秒XUV-PEEM图像和ToF谱的测量表明，对于快的Au-3D价带电子，通过提供低能量和高重复频率的XUV脉冲，可以有效地抑制空间电荷效应，而对于慢的二次电子，空间电荷效应占优势。因此，能量过滤的质子交换膜是成功实现阿秒纳米等离体场显微镜的必要前提。该项目未来要解决的局域表面等离子体的时间动力学的科学问题包括，但不限于以下问题：a)局域光场拍打在等离子体激元退相时间内的演化速度有多快，如何用理论模型来描述？B)纳米尺度上的电子相干性如何在阿秒时间尺度上的多光子激发中演化？C)等离子体纳米结构的尺寸、形状和材料性质如何影响场动力学，以及如何优化纳米结构以适应超快场动力学？D)几个周期的光脉冲的载波包络相位如何在阿秒时间尺度上控制等离子激元的场动力学？

Localized Surface Plasmons (LSP) as coherent collective electron excitations manifest itself as nanolocalized optical field enhancement, which undergoes ultrafast attosecond dynamics due to the inherently large spectral bandwidth of LSP resonances. To date, a direct experimental observation of this optical field dynamics on a nanometer spatial and attosecond temporal scale has not been accomplished. It is the goal of this project to set the experimental pre-requisites for attosecond optical field nanoscopy by applying fewcycle optical pump pulses for resonant excitation and attosecond-timed photoelectron emission induced by single attosecond XUV pulses for probing. The interaction of the liberated valence-band photoelectrons with the instantaneous localized surface plasmon field results in a change of the kinetic photoelectron energy, thus carrying a "snapshot fingerprint" of the ultrafast LSP field. Time-of-flight photoelectron emission microscopy is used as a detector to record the spatial location as well as kinetic energy of the emitted photoelectrons. The measurements are accomplished by a unique experimental setup of a high repetition rate (10 kHz) High Harmonic Generation source providing single attosecond XUV pulses at * 90 eV photon energy, an interferometrie pump probe delay stage with 800 nm/4 fsec few cycle pump pulses (400 nm SHG pulses optional), and a UHV-ToF PEEM system with two complimentary imaging energy filters and detectors. First measurements of attosecond XUV-PEEM images and ToF spectra on Au/Si nanostructures have indicated, that space charge effects, which occur when liberating more than one electron per pulse and that would spoil spatial and energy resolution, could be efficiently suppressed for the fast Au-3d valence band electrons by providing XUV pulses of low energy and at high repetition rate, while space charge effects prevail for slow secondary electrons. Thus, energy-filtered PEEM is an essential pre-requisite for the successful implementation of attosecond nanoplasmonic field microscopy. Future scientific topics on the temporal dynamics of localized surface plasmons to be addressed by the project include, but are not limited to the following questions: a) How fast does the localized optical field beating evolve during the plasmon dephasing time and how can it be described by theoretical models? b) How does electronic coherence on a nanoscale evolve in multiphoton excitation on an attosecond time scale ? c) How does size shape and material properties of plasmonic nanostructures affect the field dynamics and how can nanostructures be optimized for ultrafast field dynamics? d) How can plasmonic field dynamics be controlled on an attosecond time scale by the carrier envelope phase of few-cycle optical pulses?