  We predict several effects associated with the optical response of systems
prepared in a nonequilibrium state by impulsive optical excitations. The linear
response depends on the phase of the electric field even if the initial
nonequilirbium state has only populations, no coherences. Initial quantum
coherences induce additional phase dependence which also shows new resonances
in nonlinear wave mixing. In systems strongly driven by an external optical
field, the field frequency generates a phase dependent probe absorption. This
gives further control to manipulate the relative contribution to the linear
signal due to initial populations and coherences.
