Single-shot femtosecond EUV-pump/visible-probe cross-correlation on GaAs

The Free Electron Laser in Hamburg (FLASH) is currently the most intense femtosecond light source in the extreme-ultraviolet (EUV) range, which facilitates efficient pumping of inner shells in solid targets. For pump-probe experiments, a synchronized optical fs laser system is available at the facility. In our set-up, the EUV pulse (wavelength 28 nm,pulse length 20-30 fs) and the visible pulse (400 nm,130 fs) are non-collinearly overlapped in space and time on the surface of a GaAs crystal. Along its path, the EUV pump pulse changes the reflectivity of the GaAs sample for the visible probe pulse, the latter being imaged onto a CCD array. The spatial position of the reflectivity change captures the dynamics of the process in a single exposure. The technique can be utilized to determine individual EUV-visible delays with a precision of about 40 fs