Abstract

Supercontinuum radiation due to self-phase modulation (SPM) of an ultrashort pulse is widely used for pump-probe time-resolved spectroscopy. Spectra extending to the UV and VUV regions have remarkable interest in the investigation of wide-band-gap materials. With increasing frequency shift, however, the spectracl power density of the continuum drastically decreases. For example, the spectral intensity of the continuum around 600 nm is typically 4 orders of magnitude lower than the intensity of a primary laser pulse at 800 nm. Such a non-flat-top spectral distribution is mainly caused by a nonuniform intensity distribution in the beam cross section through a focal point because the modulation depth changes as a function of light intensity Efficient modulation is produced only in the central part of the focusing spot with limited interaction length. The maximum possible laser intensity is also limited by fatal nonlinear absorption or plasma formation in an SPM medium. It is difficult to generate a UV continuum directly from a Ti:Al₂O₃ laser without SHG or THG, because an extremely large light-intensity-interaction-length product is required.

© 1995 Optical Society of America