Abstract

In 1998, Berger proposed the concept of NPC [1] (featuring a space-dependent second-order nonlinear coefficient χ⁽²⁾), which opens the door for nonlinear manipulation of light in high-dimensional space. The popular way to fabricate 1D and 2D NPCs is to use the electrically-poling method, in which the ferroelectric domains are selectively inverted by applying an external electric field. The experimental realization of 3D χ⁽²⁾ structures remains a great challenge until twenty years after its first proposal. Several types of femtosecond laser writing techniques [2,3] were proposed to erase and pole ferroelectric domains, which have been utilized to successfully fabricate 3D NPC structures in LiNbO₃ and BCT crystals, respectively. Recently, the feature size of 3D χ⁽²⁾ structure has been reduced below 100 nm [4].

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