Shaped tunable pulses in the UV with 20 fs sub-structures

The visible sub-20 fs pulses of a noncollinear optical parametric amplifier (NOPA) are shaped with a LCD mask in a 4f setup and converted to the UV by sum frequency mixing with stretched NIR pulses from a Ti:sapphire amplifier system. To characterize the temporal shape of the resulting UV pulses, cross correlations are measured by difference frequency generation with 20 fs pulses at 700 nm generated by a second NOPA. The achieved pulse lengths are well below vibrational periods of molecular skeletal motions while the intervals in such a pulse train can be adopted to a vibrational frequency.