Temporally Flattop Picosecond Pulses through Nonlinear Conversion and Phase Manipulation

Generating temporally flat-top picosecond UV laser pulses for reducing electron emittance from photocathodes driving high repetition rate x-ray free electron lasers (XFELs), such as LCLS-II, is challenged by shape distortions and material damage. One promising avenue towards this end is laser pulse shaping through non-colinear sum-frequency generation (SFG) between pulses with opposite spectral phase in a few dispersion order terms [1]. With this method, the SFG pulse is simultaneously generated, shaped, and spectrally compressed. Additionally, the narrowband pulses have near-flat phase such that they are resistant to shape distortions once generated. We present an experimental demonstration of this principle and generation of shaped narrowband picosecond shaped pulses from chirped femtosecond pulses. Additionally, we discuss how this principle may be applied to generate pulses with non-flattop shape.