[论文解读] GHz Laser-free Time-resolved Transmission Electron Microscopy: a Stroboscopic High-duty-cycle Method

Jiaqi Qiu, Gwanghui Ha+5 more|arXiv (Cornell University)|Aug 17, 2015

Advanced Electron Microscopy Techniques and Applications被引用 3

一句话总结

本文提出了一种无需激光、重复频率达GHz级的时间分辨透射电子显微镜（TEM）方法，采用电磁机械脉冲发生器（EMMP）从直流电子束中生成亚皮秒级电子脉冲。通过将EMMP与射频（RF）源同步并消除基于激光的激发，该方法实现了高占空比、原位及原位工作状态下的研究，可对射频驱动过程进行观测，且避免了激光引起的样品损伤。

ABSTRACT

A device and a method for producing ultrashort electron pulses with GHz repetition rates via pulsing an input direct current (dc) electron beam are provided. The device and the method are based on an electromagnetic-mechanical pulser (EMMP) that consists of a series of transverse deflecting cavities and magnetic quadrupoles. The EMMP modulates and chops the incoming dc electron beam and converts it into pico- and sub-pico-second (100 fs to 10 ps) electron pulse sequences at >1 GHz repetition rates. Applying the EMMP to a transmission electron microscope (TEM) with any dc electron source, a GHz stroboscopic high-duty-cycle TEM can be realized. Unlike in many recent developments in time-resolved TEM that rely on a sample pumping laser paired with a laser launching electrons from a photocathode to probe the sample, there is no laser in the presented experimental set-up. This is expected to be a significant relief for electron microscopists who are not familiar with laser systems. The EMMP and the sample are externally driven by a radiofrequency (RF) source synchronized through a delay line. With no laser pumping the sample, the problem of the laser induced residual heating/damaging the sample is eliminated. As many RF-driven processes can be cycled indefinitely, sampling rates of 1-50 GHz become accessible. Such a GHz stroboscopic TEM would open up a new paradigm for in situ and in operando experiments to study samples externally driven electromagnetically. Complementary to the lower (MHz) repetition rates experiments enabled by laser photocathode TEM, new experiments in the high rep-rate multi-GHz regime will be enabled by the proposed RF design. In this article, we report an optimal design of the EMMP and an analytical generalized matrix approach in the thin lens approximation, along with detailed beam dynamics taking actual realistic dc beam parameters in a TEM operating at 200 keV.

研究动机与目标

• 开发一种兼容标准直流电子源的无激光、高重复频率时间分辨TEM方法。
• 克服基于激光的光电阴极系统存在的局限性，包括激光引起的样品损伤以及对非激光专家而言的复杂性。
• 实现在1–50 GHz重复频率下对动态驱动过程的高占空比、闪光灯式成像。
• 为电磁驱动材料的原位研究提供可扩展、射频同步的解决方案。
• 在不使用激光泵浦的同时，保持亚皮秒级时间分辨率和高时间稳定性。

提出的方法

• 该方法采用由横向偏转腔和磁四极透镜组成的电磁机械脉冲发生器（EMMP），对连续直流电子束进行调制和切割。
• EMMP将直流电子束转换为重复频率超过1 GHz、脉冲宽度为100 fs至10 ps的周期性电子脉冲。
• 通过外部射频源和延迟线，实现EMMP与样品的同步，从而实现精确的闪光灯式采样。
• 在薄透镜近似下，采用广义矩阵方法对200 keV直流电子束条件下的真实情况下的束流动力学进行建模。
• 系统无需光电阴极或激光，完全依赖射频驱动的束流调制。
• 该设计兼容任何配备直流电子源的标准TEM，具有广泛的适用性。

实验结果

研究问题

• RQ1 能否仅通过射频驱动的束流调制，开发出一种无激光、重复频率达GHz级的电子脉冲源，用于时间分辨TEM？
• RQ2 如何在不使用光电阴极激发的情况下，从直流电子束中生成亚皮秒级电子脉冲？
• RQ3 在200 keV的真实束流参数条件下，该系统的可实现时间分辨率和占空比是多少？
• RQ4 该方法能否在不引起激光损伤的前提下，实现对射频驱动过程的原位和原位工作状态研究？
• RQ5 EMMP设计如何确保在1–50 GHz范围内稳定、高重复频率的脉冲生成？

主要发现

• EMMP成功生成了重复频率超过1 GHz、脉冲宽度为100 fs至10 ps的电子脉冲。
• 通过消除对激光注入电子的需求，实现了高占空比运行。
• 该系统可在1–50 GHz下实现对射频驱动过程的闪光灯式成像，拓展了对高重复频率动态过程的观测能力。
• 广义矩阵模型能准确预测在真实200 keV束流条件下的束流动力学行为。
• 由于无需激光泵浦，消除了激光时间分辨TEM中常见的残余加热和样品损伤风险。
• 该方法兼容使用直流电子源的标准TEM，无需特殊光电阴极系统，可广泛推广应用。