Subfemtosecond pulse generation by high-order cascaded stimulated Raman scattering

Summary form only given. Recent advances in ultrafast laser technology have made possible the generation of pulses with <5 fs in duration. At present, different concepts such as utilization of high-order harmonic generation, ultrawide spectral broadening by self-phase modulation or cascaded stimulated Raman scattering are in investigation to overcome this barrier and to achieve the sub-fs region. High-order cascaded stimulated Raman scattering (CSRS) with rather high conversion efficiency has been known for many years and provides a tremendously broad spectrum with many lines spreading from the far IR to the extreme UV. The aim to use this broad-band spectrum for sub-fs pulse generation raises several most challenging problems. The main aspects of this subjects are the investigation of the frequency dependence of the phase (chirp) of such broad-band spectrum and the question whether and in which way the spectral components can be locked to each other. We report results of a comprehensive theoretical study about high-order CSRS for different pulsed excitation regimes and parameter regions and with the account of linear dispersion and the Kerr effect, for which these problems are investigated. We show that sub-fs pulses can be generated by active phase control after the Raman medium by the application of a broadband phase modulator as, e.g., a liquid-crystal modulator. We investigated the conditions where the CSRS generated spectral components are partially self-locked to each other and significant pulse shortening appears during propagation without external phase control.