Nonlinear optical efficiency of two-photon fields

An analysis is made of ω₁+ω₂=ω₃ sum-frequency generation in a nonlinear crystal and also of twophoton absorption under the action of "biphotons," i.e., correlated two-photon fields formed as a result of spontaneous parametric scattering of the initial pump field in another crystal. It is shown that at low pump powers, the biphoton efficiency in two-photon processes is considerably higher than the efficiency of classically correlated fields of the same intensity. It is found that in the case of coherent pumping, the regenerated sum-frequency field is also coherent.