Anti-stokes self-shift and broadening of the femtosecond laser emission spectrum in a strongly absorbing medium

We have experimentally found a self-shift of the emission spectrum center of gravity for 35-fs output pulses of a Ti:sapphire laser system (λc = 800 nm) by 75 nm towards shorter wavelengths (more than two half-widths of the initial spectrum) upon focusing the pulses into a strongly absorbing an SZS-23 optical color filter (Kλ = 800 nm = 0.98 mm −1) for the power density in the focal spot ∼1013 W/cm 2. The effect was accompanied by a simultaneous homogeneous broadening of the spectrum up to 190 nm (FWHM) without the deep dips typical of a spectral supercontinuum, which shows that the phase self-modulation is smooth. The absence of phase distortions over the cross section of the beam with the broadened spectrum was confirmed by the beam divergence, which was close to that of the initial laser beam. These characteristics allow one to hope for the possibility of obtaining the time compression of the beam down to 8–10 fs in duration with the energy conversion factor of ∼10−4. Numerical simulation allowed us to explain the observed effect by nonlinear pulse reshaping and selective absorption in the medium.