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Q: Quantenoptik

Q 5: Generation of Short Pulses I

Q 5.5: Talk

Monday, April 2, 2001, 16:45–17:00, H 2013

Beating the phase matching limit in the generation of ultrashort UV pulses — •J. Piel, M. Beutter, S. Lochbrunner, and E. Riedle — LS BioMolekulare Optik, Sektion Physik, Ludwig-Maximilians-Universitaet Muenchen

Widely tunable ultrashort UV pulses can be generated by frequency doubling of visible sub-20 fs pulses from a NOPA in a very thin BBO crystal. However, even for a 100 µm crystal the spectral width is limited by the phase matching bandwidth and the resulting UV pulses are lengthened instead of the temporal shortening predicted for a quadratic process. Fortunately, we observe that the spectral width suddenly increases by typically a factor of 2 if the intensity at the crystal is increased beyond a characteristic value. Simultaneously the UV beam blooms and its divergence is significantly increased. We believe that this behavior is due to cross phase modulation of the weak frequency doubled pulse by the strong fundamental pulse. The UV pulse of increased spectral width can be compressed very well in a fused silica prism compressor to pulse lengths below 25 fs. As an additional check for the postulated broadening mechanism we monitored the frequency doubled pulse when the intensity of the visible pulse was kept low and an additional strong pulse was overlapped in space and time in the BBO crystal. The results of this experiment will be discussed.