摘要： Performance of many optical devices based on frequency conversion critically depends on spatial modulation of the nonlinear optical response of the materials. This modulation ensures, via the quasi phase matching (QPM), an efficient energy exchange between optical waves at different frequencies. The QPM structures, also known as the nonlinear photonic crystals, offer a variety of novel properties and functionalities that cannot be obtained in uniform nonlinear crystals. Typically, such nonlinear structures have been fabricated via electric-poling. However, this technique is restricted to particular crystallographic geometries, and does not allow to create isolated structures inside the nonlinear material. This means that one cannot realize fully 3-dimensional nonlinear structures via traditional electrical poling. In this work we discuss our novel technique capable of engineering quadratic nonlinearity, based on all-optical poling of ferroelectrics. This new approach, is similar to traditional laser writing in optical media. It involves a tight focusing of femtosecond infrared beam and subsequent its two photon absorption which heats locally the medium. The large temperature gradient induces thermoelectric field which, if sufficiently high, inverts locally direction of spontaneous polarization, changing the sign of nonlinearity. The method is extremely flexible, can be applied to different crystallographic orientations and enables formation of localised spatially modulated nonlinear response inside optical media. Here, we demonstrate its application to create structures for efficient frequency conversion in Lithium Niobate waveguides and transverse second harmonic generation. Moreover, we provide here the first ever experimental evidence of the 3D Nonlinear Photonic Crystal, fabricated in ferroelectric barium calcium titanate. Finally we show formation of multiple nonlinear photonic crystals in form of fork structures inside the bulk of ferroelectric calcium barium niobate, and demonstrate their application for wave-shaping in frequency doubling process, directly creating second harmonic optical vortices and conical beams.