  The wave-front engineering for nonlinear optical interactions was discussed.
Using Huygens-Fresnel principle we developed a general theory and technique for
domain engineering with conventional quasi-phase-matching structures being the
special cases. By Fourier analysis we put forward the concept of local
quasi-phase matching, which suggests that the quasi-phase matching is fulfilled
only locally not globally. Experiments on focal effect of second-harmonic wave
agreed well with the theoretical prediction. The proposed scheme combines three
optical functions: generation, focusing and beam splitting of second-harmonic
wave, thus making the device more compact. Further the proposed scheme can be
used to perform the integration of multi-functional optical properties in
nonlinear photonics, as well as expand the use of nonlinear optical devices.
