SBIR-STTR Award

Many atomic processes occur on timescales that are as short as tens to hundreds of femtoseconds. While pulsed lasers have the temporal resolution to investigate these processes, they cannot provide the requisite spatial resolution. Ultrafast electron diffraction (UED) and Dynamic transmission electron microscopy (DTEM) are pulsed electron techniques that have been recently developed to examine the dynamics of these processes with adequate spatial resolution. Unfortunately, Coulomb interactions among the electrons broadens the temporal and spatial extent of the pulse during the travel of the beam 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 beyond; and it increases the objective lens chromatic aberration, which reduces the spatial resolution. Consequently, 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. High-energy, multi-MeV electron sources not only alleviate the impact of Coulomb interactions, they provide a path to single-shot real-space imaging and diffraction with spatio-temporal resolution in the range of Å - µs to ~ 10 nm - 10 ps. Electron Optica proposes to develop a novel electron beam column suitable for 4 MeV UEM and UED operating in single shot mode, with 10 million or more electrons per shot. The electron beam column is based on an electron source with a relative energy spread of 10-5 and utilizes state-of-the-art magnetic electron lenses and correction elements optimized for MeV operation. A detailed electron-optical analysis of the key column components, the objective lens, the illumination and projection optics, will be performed using state-of-the-art simulation software. Particular attention will be paid to the design of the objective lens, which defines the ultimate electron-optical performance of the column. The computation of the primary and higher order optical aberrations of the lens will be carried out in two steps: first, the magnetic flux distribution experienced by the beam is calculated with a proven high-accuracy, second-order finite element method (SOFEM); second, the ray-optical simulator calculates the aberration coefficients associated with the flux distribution along the column. For precision, the solver calculates the flux distribution in the magnetic circuit and coil windings, taking into account the effects of magnetic saturation in state-of-the-art magnetic materials. Furthermore, the requisite coil current for focusing the beam accounts for the relativistic mass increase of the electron, which is substantial at the beam energies of interest. The goal of the phase I research is to provide a detailed electron-optical design of a 4 MeV UEM/UED column that can be prototyped in phase II.

Many atomic processes occur on timescales that are as short as tens to hundreds of femtoseconds. While pulsed lasers have the temporal resolution to investigate these processes, they cannot provide the requisite spatial resolution. Ultrafast electron diffraction (UED) and Dynamic transmission electron microscopy are pulsed electron techniques that have been recently developed to probe the dynamics of these processes with adequate spatial resolution. Unfortunately, electron-electron interactions increase the beam energy spread from a fraction of an electron-Volt to hundreds or even thousands of electron-Volts, which broad- ens both the temporal and spatial extent of the electron pulse. Consequently, there remains a strong de- mand for improving the temporal resolution of the probing pulse without sacrificing total pulse charge and spatial resolution. In this SBIR project, EOI is developing a UEM with atomic resolution, on the order of 3?, and capable of reducing the pulse length into the sub-nanosecond range for single shot applications with up to 107 elec- trons/pulse, and femtosecond temporal resolution for stroboscopic applications with thousands of elec- trons/pulse. The key feature of this approach is the combination of a superconducting radio-frequency (SRF) gun and a novel electron column operating at 4 MeV. This approach has two major advantages: first, the acceleration to relativistic energies dramatically lowers the Coulomb effects due to electron-elec- tron interactions, which allows the use of significantly more electrons/pulse; and second, it provides high temporal resolution that is accompanied by atomic scale resolution owing to a lens system with low aber- rations and SRF gun with low energy spread. During Phase I, EOI successfully completed simulations demonstrating the capability of the UEM column to produce a single shot with 10 million or more electrons in a sub-nanosecond pulse while maintaining atomic resolution. EOI drafted a set of candidate UEM architectures, evaluated the trade-offs between temporal and spatial resolution, and designed a column optimized for ultrafast microscopy applications. In this initial Phase II, EOI will produce a detailed opto-mechanical design of a core UEM column con- taining the critical optical components, i.e. the illumination optics and objective lens. EOI will work with Dr. Xijie Wang’s group at SLAC to assemble the UEM column, integrate it with the SRF gun and verify experimentally the performance of the instrument. Without the magnifying power of the projection optics, the spatio-temporal resolution will be limited to approximately 50 nm - 100 ps in single shot mode. However, by inverting the orientation of the objective lens, the column can operate in selected area UED mode, probing regions as small as 10 nm with femtosecond temporal resolution. The full prototype with atomic resolution will be completed in a sequential phase II project. EOI’s novel UEM column provides a path to a product targeted for emerging ultrafast applications, thus making it suitable for enhancing the study of the structure, composition, and bonding states of new mate- rials at ultrafast time scales to advance material science research in the field of nanotechnology and in particular in biomedical research.