Abstract

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 [1]. 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 [2]. 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 (cut out from the spectrum by dielectric mirrors) without necessity of compression. This seems a simple and rugged source for tunable 10-fs pulses in the UV [3,4]. We applied them as a pump in time-resolved spectroscopy of metal carbonyls, and the improved time resolution revealed many more details [5]. (Hence for this application, the filamentation method is fully suitable and is simpler than using capillaries.) Further shortening is expected simply by more compression of the fundamental by standard techniques. This also extends the spectrum [2]. A useful feature is that the spectrum is spatially homogeneous (no conical emission).

© 2009 IEEE