The present invention is related to a method to control the nucleation and to achieve designed domain inversion in single-domain ferroelectric substrates (e.g. MgO doped LiNbO3 substrates). It includes the first poling of the substrate with defined electrode patterns based on the corona discharge method to form shallow domain inversion (i.e. nucleation) under the electrode patterns, and is followed by the second crystal poling based on the electrostatic method to realize deep uniform domain inversion. Another objective of the present invention is to provide methods to achieve broadband light sources using a nonlinear crystal with a periodically domain inverted structure.