Photon-induced near-field electron microscopy (PINEM): theoretical and experimental
Photon-induced near-field electron microscopy (PINEM): theoretical and experimental
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DOI：
10.1088/1367-2630/12/12/123028
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发表时间：
2010-12-17
影响因子：
3.3
通讯作者：
Zewail, Ahmed H.
中科院分区：
文献类型：
--
作者：
Park, Sang Tae;Lin, Milo M.;Zewail, Ahmed H.
利用皮米波长的定时（近相对论性）电子包与飞秒持续时间的光脉冲重合，在显微镜下实现了空间和时间的电子成像。光子（具有几个电子伏特的能量）用于脉冲加热或激发样品，以便实时跟踪结构从非平衡状态的演变。因此，在相对较低的影响下，电子和光子之间没有相互作用；在真空中当然是这样，因为能量动量守恒是不可能的。在纳米结构的存在和更高的影响下，能量动量守恒是可能的，电子包可以获得或失去光量子。最近，有报道称，当仅过滤获得能量的电子时，近场成像可以实现纳米级粒子和界面的可视化，并增强对比度（Barwick et al . 2009 Nature 462 902）。为了探索各种应用，重要的是通过解析公式来表达这种光子诱导近场电子显微镜（PINEM）所涉及的关键参数，并预测相关现象，例如电子包的四十光子吸收。本文给出了PINEM的理论和实验结果。特别地，在高动能极限下求解了纳米结构散射电磁场（近）场中超快电子包的时间依赖量子解，得到了入射电子包向离散动量小波叠加的演化过程。在瑞利散射和米氏散射的框架下讨论了电子-光子耦合光晕的特征长度和时间尺度，给出了PINEM效应与尺寸、极化、材料和时空局域化的依赖关系。我们还提供了一个基于等离子体特征的简单经典描述。本文的主要部分是将理论结果与最近获得的关于材料和生物系统成像的实验结果进行比较。
Electron imaging in space and time is achieved in microscopy with timed (near relativistic) electron packets of picometer wavelength coincident with light pulses of femtosecond duration. The photons (with an energy of a few electronvolts) are used to impulsively heat or excite the specimen so that the evolution of structures from their nonequilibrium state can be followed in real time. As such, and at relatively low fluences, there is no interaction between the electrons and the photons; certainly that is the case in vacuum because energy-momentum conservation is not possible. In the presence of nanostructures and at higher fluences, energy-momentum conservation is possible and the electron packet can either gain or lose light quanta. Recently, it was reported that, when only electrons with gained energy are filtered, near-field imaging enables the visualization of nanoscale particles and interfaces with enhanced contrast (Barwick et al 2009 Nature 462 902). To explore a variety of applications, it is important to express, through analytical formulation, the key parameters involved in this photon-induced near-field electron microscopy (PINEM) and to predict the associated phenomena of, e. g., forty-photon absorption by the electron packet. In this paper, we give an account of the theoretical and experimental results of PINEM. In particular, the time-dependent quantum solution for ultrafast electron packets in the nanostructure scattered electromagnetic (near) field is solved in the high kinetic energy limit to obtain the evolution of the incident electron packet into a superposition of discrete momentum wavelets. The characteristic length and time scales of the halo of electron-photon coupling are discussed in the framework of Rayleigh and Mie scatterings, providing the dependence of the PINEM effect on size, polarization, material and spatiotemporal localization. We also provide a simple classical description that is based on features of plasmonics. A major part of this paper is devoted to the comparisons between the theoretical results and the recently obtained experimental findings about the imaging of materials and biological systems.