In light optics, beams with orbital angular momentum (OAM) can be produced by employing a properly-tuned two-cylinder-lens arrangement, also called $\pi$/2 mode converter. It is not possible to convey this concept directly to the beam in an electron microscope due to the non-existence of cylinder lenses in commercial transmission electron microscopes (TEMs). A viable work-around are readily-available electron optical elements in the form of quadrupole lenses. In a proof-of-principle experiment in 2012, it has been shown that a single quadrupole in combination with a Hilbert phase-plate produces a spatially-confined, transient vortex mode.

Here, an analogue to an optical $\pi$/2 mode converter is realized by repurposing a CEOS DCOR probe corrector in an aberration corrected TEM in a way that it resembles a dual cylinder lens using two quadrupoles. In order to verify the presence of OAM in the output beam, a fork dislocation grating is used as an OAM analyser. The possibility to use magnetic quadrupole fields instead of, e.g., prefabricated fork dislocation gratings to produce electron beams carrying OAM enhances the beam brightness by almost an order of magnitude and delivers switchable high-mode purity vortex beams without unwanted side-bands.

在光光学中，可以通过采用适当调谐的双柱面透镜布置（也称为轨道角动量 (OAM)）来产生光束 $\pi$/2 模式转换器。由于商用透射电子显微镜 (TEM) 中不存在柱面透镜，因此无法将这一概念直接传达给电子显微镜中的光束。一种可行的解决方法是采用四极透镜形式的现成电子光学元件。在 2012 年的原理验证实验中，已经表明单个四极杆与希尔伯特相位板相结合会产生空间受限的瞬态涡旋模式。

在这里，一个类似于光学的 $\pi$/2 模式转换器是通过在像差校正 TEM 中重新利用CEOS DCOR探针校正器来实现的，其方式类似于使用两个四极杆的双柱面透镜。为了验证输出光束中 OAM 的存在，叉位错光栅被用作 OAM 分析仪。使用磁性四极场代替例如预制叉位错光栅来产生携带 OAM 的电子束的可能性将束亮度提高了几乎一个数量级，并提供可切换的高模式纯度涡旋束，而没有不需要的边带。