Efficient chirped-matched third harmonic generation of long UV pulses

Given their work function of about 4.5eV, Free Electron Laser (FEL) e-gun cathodes require a strong UV laser illumination of several tens or hundreds of µJ to liberate bursts of electrons with a charge of a couple of nCs. Furthermore, the near optimum laser beam shape has to be flat-top in space and time with a sub-ps rise-time. Two main scenarii [1] have been considered so far to generate such laser pulses: Third Harmonic Generation (THG) of amplified 800 nm short pulses followed by direct UV pulse shaping and THG of long 800 nm shaped pulses. Despite the availability of commercial pulse shapers in the UV [2], the targeted level of energy (≫100 µJ) and pulse duration (5–20ps) are out of reach of the direct UV pulse shaping scheme without additional management of the pulse duration (pulse stretcher). On the other hand, IR pulse shaping before the third harmonic generation drastically lowers the UV efficiency and raises strong bandwidth issues.