Pulse Compressor

Many atomic processes occur on timescales that are as short as tens to hundreds of femtoseconds. Pulsed lasers have the temporal resolution to investigate these processes, but they cannot provide the requisite spatial resolution. Pulsed electron techniques have been recently developed to examine the dynamics of these processes with adequate spatial resolution. In Ultrafast electron diffraction (UED) and Dynamic transmission electron microscopy (DTEM), the electron pulse is generated by illuminating a photocathode with a pulsed laser and by subsequently accelerating the photoemitted electrons towards the specimen. Unfortunately, Coulomb interactions between the electrons broaden the temporal and spatial extent of the pulse during the travel to the specimen. The Coulomb interactions mainly increase the beam energy spread (Boersch effect) from a fraction of an electron-Volt to hundreds or even thousands of electron- Volts. The Boersch effect has a two-fold impact on the electron optics: it spreads the arrival time window of the pulse from tens of femtoseconds to picoseconds; and it increases the objective lens chromatic aberration, which reduces the spatial resolution from ngstroms to nanometers. To date, there remains a strong demand for improving the temporal resolution of the probing pulse into the deep femtosecond range without sacrificing total pulse charge and spatial resolution. Electron Optica proposes to develop a novel pulse compressor that can both compress a linearly velocitychirped pulse at the sample into the sub-100 femtosecond range and correct the chromatic aberration of the objective lens. The pulse compressor will be designed to fit into both UED and DTEM instruments and to accommodate bunches with a range of electron densities and energy spreads. The compressor utilizes an electrostatic electron mirror combined with a magnetic beam separator that allows normal beam entrance to minimize mirror aberrations. Higher energy electrons penetrate more deeply into the retarding field of the electron mirror, leading to a longer path length. After reflection, the trailing edge of the pulse has a higher energy than the leading edge. This reversal results in time compression of the propagating pulse once the pulse is allowed to drift to the specimen. For DTEM, the negative chromatic aberration coefficient of a second electron mirror is adjusted to compensate the chromatic aberration of the objective lens. The static nature of the electron mirror simplifies the set-up and tuning of the pulse compressor and thus avoids the jitter problem associated with RF pulse compression techniques. In phase I, the proposed research will focus on establishing the feasibility of compressing the pulse while offsetting the chromatic aberration of the objective lens. A detailed electron-optical analysis of the key pulse compressor components, the beam separator and the mirror, will be performed using state-of-the-art simulation software. Particular attention will be paid to the Coulomb effects at the mirror, where the speed of the electron bunch goes to zero. The goal of the phase I research is to devise a design of an electron pulse compressor that can be prototyped in phase II. The observation of dynamic behavior at the femtosecond time scale with atomic level resolution would provide breakthroughs in the understanding of many phenomena across different fields including condensed matter physics, chemistry, and biology. Electron Optica aims to develop a modular pulse compressor that is flexible enough to be retrofitted between the photocathode gun and the remainder of the electron column of existing (and future) UED and DTEM instruments. This versatility would provide a path to a product targeted for the emerging applications of the ultrafast electron microscopy market. Key words: pulse compressor, ultrafast electron diffraction, dynamic transmission electron microscopy. Summary for Members of Congress: The attainable temporal resolution of currently available pulsed electron diffraction and electron microscopy techniques is limited by the Coulomb interactions between electrons, which broaden the pulse. Electron Optica proposes to develop a novel pulse compressor that can compress an electron pulse into the sub-100 femtosecond range and compensate the chromatic aberration of the objective lens.