(1) where t 0 is the time of interaction between the first laser and the system, where t 1 is the time when the system emits the response signal. Further, μ(t 1 ) is the dipole operator in the interaction picture, ρ(g) represents the initial density matrix of the system in the equilibrium ground state at t = -∞, (ρ (g) = ρ(-∞)).