Deep-ultraviolet picosecond flat-top pulses by chirp-matched sum frequency generation.
Picosecond, flat-top, deep-UV pulses are needed to generate high-brightness electron beams to efficiently drive x-ray free electron lasers. Current metal photocathodes have low efficiency and therefore require high-energy pulses, and the generation of high-energy, flat-top pulses in the deep UV is still challenging. The low efficiencies of both the harmonic generation and deep-UV pulse shapers restrict the accessible pulse energy. Moreover, the acceptance bandwidth of the harmonic generation limits the minimum rise time of the flat-top profile. We present the generation of few-hundred microjoule, picosecond, deep-UV pulses using chirp-matched sum frequency generation. This scheme combined with IR spectral manipulation is a novel approach for deep-UV pulse shaping. It permits flat-top pulses with high energy and fast rise time, highly suited for high-brightness photoelectron beam production.