Sub-10-fs ultrabroad supercontinuum generated by self-focusing of few-cycle 800-nm pulses in argon

Focusing 10-fs pulses of 800 nm with moderate energy (0.35 mJ) into atmospheric-pressure argon gives rise to filamentation and a supercontinuum with a very broad pedestal, extending to 250 nm. According to the present understanding, the short wavelengths are produced by Kerr-effect self-phase modulation in the self-steepened trailing edge of the pulse. Pulses in this spectral range might therefore be intrinsically short. Indeed we demonstrate this by extracting the light near the end of the filament, terminating self-focusing by a pressure gradient at a pinhole, beyond which the argon is pumped away. We obtain pulses of 9.7 fs in the region of 290 nm without necessity of compression. This seems a simple and rugged source for tunable 10-fs pulses in the UV. A useful feature is that the spectrum is spatially homogeneous. The third harmonic only gives a negligible contribution to the supercontinuum. However, if the argon cell is substantially shortened, the third harmonic (270 nm) is produced with much higher yield (~10^-3), decreasing again at higher pressure, obviously due to beginning loss of phase matching. Pulse durations of 9.7 fs were also obtained, and the pulses were used for transient spectroscopy of cyclohexadiene and chromium hexacarbonyl with 10 times better time resolution than before. In the same way we got the fifth harmonic (162 nm, ~4 nJ, 10 fs). It was used to study dynamics of ultrafast isomerization of ethylene with 15 times better time resolution than before.