Next-generation electron photoinjector accelerators, such as the LCLS-II photoinjector, have increasingly tight requirements on the excitation lasers, often calling for tens of picosecond, temporally flat-top, ultraviolet (UV) pulse trains to be delivered at up to 1 MHz[1]. We present a numerical and experimental implementation of a non-colinear sum frequency generation scheme wherein the incident optical pulses have equal and opposite amounts of spectral dispersion resulting in a spectrally narrowband pulse with flat-top temporal profile[2]. In preliminary experiments we achieve upwards of 40% conversion efficiency with this process. We additionally demonstrate the ability for the narrowband pulses to be directly upconverted to the UV for driving a photocathode.

Active longitudinal beam optics can help FEL facilities achieve cutting edge performance by optimizing the beam to: produce multi-color pulses, suppress caustics, or support attosecond lasing. As the next generation of superconducting accelerators comes online, there is a need to find new elements which can both operate at high beam power and which offer multiplexing capabilities at Mhz repetition rate. Laser heater shaping promises to satisfy both criteria by imparting a programmable slice-energy spread on a shot-by-shot basis. We use a simple kinetic analysis to show how control of the slice energy spread translates into control of the bunch current profile, and then we present a collection of start-to-end simulations at LCLS-II in order to illustrate the technique.

Dark-adapted vision in mammals starts with the absorption of a photon and the activation of rhodopsin, a G protein-coupled receptor. The early stages of rhodopsin activation involve the cis-to-trans isomerization of the receptor's ligand (retinal) and a relaxation process that drives the receptor through several non-equilibrium intermediates. These ultra-fast phenomena have been previously characterized spectroscopically. However, the structural information available that describes the femtosecond-to-picosecond scale changes involved is limited. Time-resolved small- and wide-angle X-ray scattering with free electron lasers is an emerging technique that has been shown to provide insights into the functional protein dynamics that take place at these timescales. Still, extracting structural information from scattering data is challenging. All-atom molecular dynamics can aid the interpretation of these experimental signals. Starting from well-equilibrated dark-state simulations of bovine rhodopsin, we run and analyze thousands of 10 ps trajectories in two environments—bilayers and micelles—and two conditions—dark and light-excited—to model the process of energy dispersion across the receptor after light-excitation. We observe an increase in the radius of gyration of the receptor after light-excitation and a propagation of the light-induced perturbation across the protein that occurs roughly at the speed of sound.