ONSAGER RELATIONS AND TIME-REVERSAL SYMMETRY IN NONLINEAR OPTICS

Results following from time-reversal symmetry are developed for those nonlinear optical processes where a statistical average is required. T hese generalize previous analyses by removing the electric dipole appr oximation and allowing for intermediate resonances. For example, Onsag er relations for self-conjugate nonlinear optical processes (when inpu t and output photons form degenerate pairs) are derived and associated reversality relations generalized. In the nonresonant limit magnetic- dipole but not electric-quadrupole terms in coherent processes are sup pressed. For general processes purely electric dipole contributions to natural optical activity are possible when intermediate resonances ar e present.