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.
Because of the strong distortion of the crystal lattices during the poling procedure the samples must be annealed for several hours at temperatures in the range 250 − 400°C. This reduces stress-induced refractive index changes at the domain interfaces and the associated scattering losses in the waveguide.
For poling of the KTP-substrates control of the process by monitoring the accumulated charge does not work, due to the ionic conductivity of KTP. We therefore apply an optical monitoring technique as developed by Canalias et al. [Ferroelectrics, 340(1):27, 2006].