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Transient lensing from a photoemitted electron gas imaged by ultrafast electron microscopy
Authors:
Omid Zandi,
Allan E Sykes,
Ryan D Cornelius,
Frank M Alcorn,
Brandon Zerbe,
Phillip M Duxbury,
Bryan W Reed,
Renske M van der Veen
Abstract:
Understanding and controlling ultrafast non-equilibrium charge carrier dynamics is of fundamental importance in diverse fields of (quantum) science and technology. Here, we create a three-dimensional hot electron gas through two-photon photoemission from a copper surface in vacuum. We employ an ultrafast electron microscope to record movies of the subsequent electron dynamics on the picosecond-nan…
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Understanding and controlling ultrafast non-equilibrium charge carrier dynamics is of fundamental importance in diverse fields of (quantum) science and technology. Here, we create a three-dimensional hot electron gas through two-photon photoemission from a copper surface in vacuum. We employ an ultrafast electron microscope to record movies of the subsequent electron dynamics on the picosecond-nanosecond time scale. After a prompt Coulomb explosion, the subsequent dynamics is characterized by a rapid oblate-to-prolate shape transformation of the electron gas, and exceptionally periodic and long-lived electron cyclotron oscillations inside the magnetic field of the objective lens. In this regime, the collective behavior of the oscillating electrons causes a transient, mean-field lensing effect and pronounced distortions in the images. We derive an analytical expression for the time-dependent focal length of the electron-gas lens, and perform numerical electron dynamics and probe image simulations to determine the role of Coulomb self-fields and image charges. This work paves the way for the direct visualization of cyclotron oscillations inside two-dimensional electron-gas materials and the elucidation of electron/plasma dynamics and properties that could benefit the development of high-brightness electron and X-ray sources.
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Submitted 5 January, 2020;
originally announced January 2020.
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Various Compressed Sensing Set-Ups Evaluated Against Shannon Sampling Under Constraint of Constant Illumination
Authors:
Wouter Van den Broek,
Bryan W. Reed,
Armand Béché,
Abner Velazco,
Johan Verbeeck,
Christoph T. Koch
Abstract:
Under the constraint of constant illumination, an information criterion is formulated for the Fisher information that compressed sensing measurements in optical and transmission electron microscopy contain about the underlying parameters. Since this approach requires prior knowledge of the signal's support in the sparse basis, we develop a heuristic quantity, the detective quantum efficiency (DQE)…
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Under the constraint of constant illumination, an information criterion is formulated for the Fisher information that compressed sensing measurements in optical and transmission electron microscopy contain about the underlying parameters. Since this approach requires prior knowledge of the signal's support in the sparse basis, we develop a heuristic quantity, the detective quantum efficiency (DQE), that tracks this information criterion well without this knowledge. It is shown that for the investigated choice of sensing matrices, and in the absence of read-out noise, i.e. with only Poisson noise present, compressed sensing does not raise the amount of Fisher information in the recordings above that of Shannon sampling. Furthermore, enabled by the DQE's analytical tractability, the experimental designs are optimized by finding out the optimal fraction of on-pixels as a function of dose and read-out noise. Finally, we introduce a regularization and demonstrate, through simulations and experiment, that it yields reconstructions attaining minimum mean squared error at experimental settings predicted by the DQE as optimal.
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Submitted 11 January, 2019; v1 submitted 8 January, 2018;
originally announced January 2018.
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GHz Laser-free Time-resolved Transmission Electron Microscopy: a Stroboscopic High-duty-cycle Method
Authors:
Jiaqi Qiu,
Gwanghui Ha,
Chunguang Jing,
Sergey V. Baryshev,
Bryan W. Reed,
Yimei Zhu,
June W. Lau
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 i…
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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.
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Submitted 16 August, 2015;
originally announced August 2015.