Two-photon absorption as convenient method for 20 fs pulse length measurement in the deep UV

We show that two-photon absorption (TPA) in moderately thin crystals and glass disks is a convenient and easy to handle method for the characterization of ultrashort pulses in the UV. We create two replicas of the pulse by reflection off an uncoated fused silica cover slip and chirp compensation by a second cover slip and focus them in e.g., a BBO crystal chosen for its high TPA coefficient. At temporal overlap, the probe pulse is depleted by the strong pump pulse through TPA to create the autocorrelation signal. In contrast to the widely used inten sity autocorrelation by frequency doubling of visible pulses, TPA is not hampered by the need for phase match ing and can therefore be used down to the deep UV.