Visualizing lattice dynamic behavior by acquiring a single time-resolved MeV diffraction image

Abstract

We explore the possibility of visualizing the lattice dynamics behavior by acquiring a single time-resolved mega-electron-volt ultrafast electron diffraction (UED) image. Conventionally, multiple UED shots with varying time delays are needed to map out the entire dynamic process. The measurement precision is limited by the timing jitter between the pulses of the pump laser and the electron probe, the intensity fluctuation of probe pulses, and the premature sample damage. Inspired by the early transient spectroscopy studies via an ultrashort-pulse pump/long-pulse probe scheme, we show that, by converting the longitudinal time of an electron pulse to the transverse position of a Bragg peak on the detector, one can obtain the full lattice dynamic process in a single electron pulse. This time-to-position mapping can be achieved by the combination of longitudinally shaping the electron beam and introducing a time-dependent transverse kick after electrons are diffracted from the sample. We propose a novel design of time-resolved UED facility with the capability of capturing a wide range of dynamic features in a single diffraction image. To achieve the best possible temporal resolution, we implement a real-time tuning scheme for optimizing the match between the electron bunch length and the lattice dynamic timescale, varying in the sub-picosecond to tens of picosecond (ps) range depending on the specific process. This timescale match is in favor of the ultrafast phenomenon, which requires a 10 fs temporal resolution for resolving the sub-ps oscillation. A state-of-the-art photocathode gun being developed by Euclid could extend the timescale to hundreds of ps. To study the radiation damage and to mitigate such effect, longitudinally shaping the photocathode drive laser pulse (demonstrated in a previous study) can control and manipulate the electron beam current profile with a tunable periodical structure. Furthermore, we present numerical evidence illustrating the capability of acquiring a single time-resolved diffraction image based on the case-by-case studies of different lattice dynamics behaviors.