Ultrafast electron diffraction

Wang, Xuan; Li, Yutong

doi:10.1088/1674-1056/27/7/076102

Cited by 7 publications

(3 citation statements)

References 85 publications

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“…Attosecond electron pulses are the subject of recent pioneering research [6][7][8][9][10][11]. For example, ultrafast electron diffraction (UED) uses ultra-short electron pulses to resolve molecular dynamics on femtosecond time scales [12][13][14]. Attosecond-scale UED would allow sufficient resolution to resolve intra-molecular electronic dynamics.…”

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confidence: 99%

Net Acceleration and Direct Measurement of Attosecond Electron Pulses in a Silicon Dielectric Laser Accelerator

et al. 2019

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Net acceleration of attosecond-scale electron pulses is critical to the development of on-chip accelerators. We demonstrate a silicon-based, laser-driven, two-stage accelerator as an injector stage prototype for a Dielectric Laser Accelerator (DLA). The first stage converts a 57 keV, 500 ± 100 fs (FWHM) electron pulse into a pulse train of 700 ± 200 as (FWHM) microbunches. The second stage harnesses the tunability of dual-drive DLA to perform both a net acceleration and a streaking measurement. In the acceleration mode, the second stage increases the net energy of the electron pulse by 200 eV over 12.25 µm. In the deflection mode, the microbunch temporal profile is analyzed by a direct streaking measurement with 200 as resolution. This work provides a demonstration of a novel, on-chip method to access the attosecond regime, opening new paths towards attosecond science using DLA.

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confidence: 99%

Net Acceleration and Direct Measurement of Attosecond Electron Pulses in a Silicon Dielectric Laser Accelerator

et al. 2019

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“…For these applications, alkali-antimonide photocathodes are the photocathodes of choice because they provide high quantum efficiency and are less sensitive to ion bombardment than GaAs photocathodes. Alkali-antimonide photocathodes are prompt emitters with relatively low thermal emittance, thus capable of generating bright beams suitable for light source applications [4], and perhaps even for ultrafast electron diffraction applications if fabrication techniques can provide a sufficiently smooth surface [5,6].…”

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confidence: 99%

Thermal emittance and lifetime of alkali-antimonide photocathodes grown on GaAs and molybdenum substrates evaluated in a −300 kV dc photogun

Wang

Mamun

Adderley

et al. 2020

Phys. Rev. Accel. Beams

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Cs x K y Sb photocathodes grown on GaAs and molybdenum substrates were evaluated using a −300 kV dc high voltage photogun and diagnostic beam line. Photocathodes grown on GaAs substrates, with varying antimony layer thickness (estimated range from <20 nm to >1 um), yielded similar thermal emittance per rms laser spot size values (∼0.4 mm mrad=mm) but very different operating lifetime. Similar thermal emittance was obtained for a photocathode grown on a molybdenum substrate but with markedly improved lifetime. For this photocathode, no decay in quantum efficiency was measured at 4.5 mA average current and with peak current 0.55 A at the photocathode.

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“…Phase transitions change the symmetry of the unit cell of the material and, since crystals are periodic structures, the set of Bragg peaks arising from X-ray diffraction will change in each phase. Electron diffraction operates on a similar basis an has also been used to study dynamical phase transitions [25] This results in precise measurements of structural distortions and, because X-rays can be tuned to resonances of specific atoms, can even probe charge, orbital and spin distributions, which are proportional to the magnitude of the order parameter [13,26]. Here, the complications arise from the fact that in complex materials phase transitions tend to manifest with concomitant changes in different degrees of freedom, some of which might only be a side effect of the transition.…”

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confidence: 99%

Inhomogeneity and disorder in ultrafast phase transitions

Pérez Salinas

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In the recent decades, there has been a surge of interest in the wide array of emergent phenomena found in strongly correlated materials. Understanding the inner workings of this type of systems is a major challenge due to the complex way in which multiple degrees of freedom, such as electronic, structural and spin, interact with each other and themselves in non-trivial fashion. One of the most striking outcomes of these subtle interactions in correlated materials is the richness of their phase diagrams, which include exotic states that still elude a complete physical description. It is in the transition from one phase to another where the insights into the complex microscopic mechanisms may be most readily found, and so the study of phase transitions has become a staple in correlated materials science. The experimental techniques used to track phase transitions are steadily becoming more precise and, with the improvements, previously overlooked aspects of a phase transition become more apparent, such as inhomogeneity or disorder, which add another layer of complexity that may clash with our current understanding. This is particularly important in the relatively young field of ultrafast studies of correlated materials, which tackles these systems in a largely uncharted territory: non-equilibrium situations. In this thesis, we develop novel experimental techniques which push towards an assessment of disorder and/or inhomogeneity in non-equilibrium phase transitions, while still being able to accurately track the dynamics of the degrees of freedom involved. We then apply these techniques in two systems of current interest: La0.5Sr1.5MnO4, a prototypical layered manganite, and VO2, one of the most emblematic correlated materials. For La0.5Sr1.5MnO4, we introduce an all-optical tabletop pump-probe setup that is able to track the ultrafast melting of charge- and orbital-order parameter with high accuracy. We show how, in contrast with previous descriptions, the transition is incoherent and fits with the paradigm of an order-disorder process. A key factor in these dynamics, which is sometimes overlooked, is spatial phase separation into the depth of the material. With our setup, the role of initial inhomogeneity and its evolution can be readily tested. For VO2, we employ facility-scale X-ray sources to directly image phase inhomogeneity in the metal-to-insulator transition with coherent X-ray diffraction techniques. We show quantitative imaging of phase separation and domain growth statically, which in our experimental setups should be able to distinguish intermediate phases appart from the usual monoclinic insulator and rutile metal. We find no evidence of previously claimed intermediate phases such as monoclinic metal VO2. Finally, we show the first non-scanning spatially-resolved observation of the ultrafast phase transition in VO2 with nanometer resolution, where we identify a global, prompt change in the domain pattern in the femtosecond scale. En las últimas décadas hemos podido observar un gran aumento en el interés sobre la gran variedad de fenómenos emergentes que se pueden encontrar en los materiales fuertemente correlacionados. Comprender los mecanismos internos de este tipo de sistemas supone un gran desafío, debido a la complejidad con la que múltiples grados de libertad, como el electrónico, esctructural o spin, interactúan entre ellos y si mismos de forma no-trivial. Uno de los resultados más llamativos de éstas sutiles interacciones en materiales correlacionads es la riqueza de sus diagramas de fase, los cuales incluyen estados exóticos para los que todavía no se dispone de una descripción física completa. Es precisamente en la transición de una fase a otra donde el conocimiento sobre las complejas interacciones microscópicas estan a un mayor alcance y, por ello, el estudio de las transiciones de fase es fundamental en el campo de los materiales fuertemente correlacionados. Las técnicas experimentales que se usan para observar cambios de fase son cada vez más precisas y, gracias a estas mejoras, aspectos de las transiciones previamente pasados por alto se vuelven evidentes. Es el caso de la heterogeneidad o el desorden, que añaden otra capa de complejidad que puede entrar en conflicto con nuestro conocimiento actual. Esto es particularmente importante en el relativamente joven campo de los estudios ultrarrápidos en materiales correlacionados, que emprende el estudio de estos materiales en un territorio ampliamente inexplorado: el no equilibrio. En esta tesis hemos desarrollado novedosas técnicas experimentales con el objetivo de evaluar el efecto del desorden y/o la heterogeneidad en cambios de fase en el no equilibrio, siempre manteniendo una observación precisa de las dinámicas de los grados de libertad involucrados. Hemos puesto en práctica estas técnicas en dos sistemas de actualidad: La0.5Sr1.5MnO4, una manganita prototípica, y VO2, uno de los materiales correlacionados más emblemático. Para el estudio del La0.5Sr1.5MnO4, presentamos un montaje experimental óptico del tipo \textit{pump-probe}, que permite observar la destrucción ultrarrápida de la ordenación orbital y de carga con gran precisión. Mostramos cómo, contrastando con descripciones previas, la transición es incoherente y resulta compatible con el paradigma de un cambio de fase mediante un proceso de orden-desorden. Otro elemento clave en estas dinámicas, en ocasiones pasado por alto, es la presencia de separación de fase en la dirección en profundidad del material. Con nuestra técnica, el papel de la heterogeneidad inicial y su evolución pueden ser estudiados. Para el estudio del VO2 hemos empleado fuentes de rayos-X de gran escala para tomar imágenes directas de la heterogeneidad de fase en la transición metal-a-aislante por medio de ténicas difractivas coherentes. Presentamos imágenes con información cuantitativa sobre la separación de fase y crecimiento de dominios en función de la temperatura. Con nuestra puesta en práctica de estas técnicas podemos distinguir fases intermedias entre aislante monoclínico y conductor rutilo. No encontramos ninguna prueba de la presencia de fases intermedias previamente sugeridas, como el VO2 en presentación de metal monoclínico. Finalmente, mostramos la primera observación (con una técnica no basada en barridos) de la transición de fase ultrarrápida en VO2 con resolución espacial de nanómetros, en la que identificamos un cambio inmediato y global de la distribución de dominios en una escala de femtosegundos.

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Ultrafast electron diffraction

Cited by 7 publications

References 85 publications

Net Acceleration and Direct Measurement of Attosecond Electron Pulses in a Silicon Dielectric Laser Accelerator

Net Acceleration and Direct Measurement of Attosecond Electron Pulses in a Silicon Dielectric Laser Accelerator

Thermal emittance and lifetime of alkali-antimonide photocathodes grown on GaAs and molybdenum substrates evaluated in a −300 kV dc photogun

Inhomogeneity and disorder in ultrafast phase transitions

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