Holographic nature of three-wave mixing

Three-wave interactions occurring in conditions of type I phase matching in a uniaxial crystal are considered in the general case of noncollinear wave propagation. It is demonstrated that, if one among the interacting fields at ωᵢ(i=1,2,3) is virtually unaffected in amplitude by the interaction, the remaining two fields at ωⱼ(j≠i) are holographic replicas of each other, the constant field acting as the reference field in holography. Experiments are presented in which a holographic image of an object consisting in a pointlike light source is obtained by either up- or down-conversion for any choice of the object- and reference-field frequencies among ω₁, ω₂, and ω₃.