Local orbital angular momentum revealed by spiral-phase-plate imaging in transmission-electron microscopy
Local orbital angular momentum revealed by spiral-phase-plate imaging in transmission-electron microscopy
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DOI：
10.1103/physreva.93.023811
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发表时间：
2015-12
影响因子：
2.9
通讯作者：
R. Juchtmans;J. Verbeeck
中科院分区：
文献类型：
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作者：
R. Juchtmans;J. Verbeeck
光波和物质波的轨道角动量(OAM)是近几年来越来越受到关注的一个参数。具有定义良好的OAM的光束，即所谓的涡旋光束，已经被应用于电信、天体物理、纳米操纵以及光学和电子显微镜中的手性测量。此外，与样品相互作用引起的波的OAM也显示出很大的潜力。在所有这些实验中，测量波的准确(局部)OAM含量是至关重要的，无论它是入射的涡旋光束还是与样品相互作用后的出射波。在这项工作中，我们研究了使用螺旋相位板来替代光学中用于测量OAM的可编程相位板。我们解析地推导出如何用这些方法来研究任意波函数的局域OAM分量。通过数值模拟，我们说明了如何测量纯涡光束的光学厚度。我们还观察了错位涡旋光束的总和，并展示了如何使用SPP来检测每个单独光束的位置和OAM。最后，我们研究了自由电子波上磁偶极子诱导的OAM，并展示了如何使用SPP来定位磁极并测量它们的“磁荷”。虽然我们的发现可以应用于研究任何波函数的OAM，但它们对电子显微镜特别感兴趣，因为多功能可编程相板还不存在。
The orbital angular momentum (OAM) of light and matter waves is a parameter that is getting increasingly more attention over the past couple of years. Beams with a well defined OAM, the so-called vortex beams, are applied already in e.g. telecommunication, astrophysics, nanomanipulation and chiral measurements in optics and electron microscopy. Also the OAM of a wave induced by the interaction with a sample, shows great potential of interest. In all these experiments it is crucial to measure the exact (local) OAM content of the wave, whether it is an incoming vortex beam or an exit wave after interacting with a sample. In this work we investigate the use of spiral phase plates as an alternative to the programmable phase plates used in optics to measure OAM. We derive analytically how these can be used to study the local OAM components of any wave function. By means of numerical simulations we illustrate how the OAM of a pure vortex beam can be measured. We also look at a sum of misaligned vortex beams and show how using SPPs the position and the OAM of each individual beam can be detected. Finally we look at the OAM induced by a magnetic dipole on a free electron wave and show how the SPP can be used to localize the magnetic poles and measure their "magnetic charge". Although our findings can be applied to study the OAM of any wave function, they are of particular interest for electron microscopy where versatile programmable phase plates do not yet exist.