Evaluation of optical probe signals from nonequilibrium systems

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 nonequilibrium 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.