Group-velocity control by quadratic nonlinear interactions

We give direct experimental evidence that the group velocity of ultrashort pulses can be controlled through x((2))-cascaded interactions, under the condition of large group-velocity mismatch. The group velocity can be finely tuned by acting on pulse intensity and phase mismatch. Group-delay shifts up to 50 fs are achieved by propagating 40 fs pulses around 1400 mn in a 25 mm long periodically poled stoichiometric lithium tantalate crystal.