Direct femtosecond laser writing or inscription is a useful technique, and it has been 1
employed to engineer various materials in many applications including nonlinear photonic crystals, 2
which are of periodically patterned second-order nonlinearity to get and control the coherent light at 3
new frequencies. By manipulation of second-order nonlinearity, either erased or poled, quasi-phase 4
matching has been achieved in several crystals, especially three-dimensional nonlinear photonic 5
crystals have been originally proposed and proved to be truly three-dimensional. Here we shortly 6
review on the recent advances in the research field of nonlinear photonic crystals inscribed by 7
femtosecond laser. We also discuss some phenomena that not understood yet and the future possible research topics in this field.