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 omega(i) (i=1,2,3) is virtually unaffected in amplitude by the interaction, the remaining two fields at omega(j) (jnot equali) 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 omega(1), omega(2), and omega(3).