Already Armstrong et al. [Physical Review, 127(6):1918, 1962] pointed out that a periodic inversion of the sign of χ(2) can compensate for a phase mismatch between driving nonlinear polarization and the driven electromagnetic waves during three wave mixing, called quasi-phase matching. For our ferroelectric waveguide substrates the sign of χ(2) coefficients is related to the orientation of the spontaneous polarization. Therefore, a periodic change of the sign of χ(2) , a χ(2) grating, can be created by a periodic inversion of the spontaneous polarization, also called periodic poling.
Lithium niobate substrates can be periodically poled in our cleanroom facility using the standard field assisted poling technique with a monitor for the accumulated charge. Like structuring the waveguides, a photoresist is spin-coated to the sample’s surface, followed by the illumination through a specific mask. After development the properly baked resist structure acts as electrical insulation during the poling process whereas non-covered regions are contacted using a liquid electrode. We subsequently apply high voltage pulses to overcome the coercitive field strength of about 22 kV/mm. This technique allows for the realization of periodicities down to ΛG ~ 4 μm. The steps necessary for periodic poling are schematically shown in Fig 1.