  Three-wave mixing of ordinary and backward electromagnetic waves in pulsed
regime is investigated in the metamaterials, which enable co-existence and
phase matching of such waves. It is shown that opposite direction of phase
velocity and energy flux in backward waves gives rise to extraordinary
transient processes in greatly enhanced optical parametric amplification and in
frequency up or down shifting nonlinear reflectivity. The discovered transients
resemble slowed response of an oscillator on pulsed excitation in the vicinity
of its resonance
