Phase relations, quasi-continuous spectra and sub-fs pulses in stimulated Raman scattering

High-order stimulated Raman scattering has been known for many years and provides an efficient transfer to a tremendously broad spectrum with many lines spreading from the far IR to the extreme UV. The aim to use this broadband spectrum for sub-fs pulse generation raises several fundamental questions: Are the Raman lines phase locked and is the generation of trains of sub-fs pulses or single sub-fs pulses possible? We explore this subject for realistic conditions with ps- and sub-ps excitation, with the account of linear dispersion and the Kerr effect and without the usually applied approximations. We find that SRS does not lead to phase locking. For ps-pulse excitation pulse trains with chirped pulses not shorter than /spl ap/2 fs are generated. With a broadband phase modulator these pulses can be shortened below 1 fs. For shorter sub-ps input pulses we show a novel feature in SRS as the generation of quasi-continuous spectra and the possibility to generate single sub-fs pulses by external phase control.