From disorder to order: Second harmonic generation in a multi-domain χ(2) nonlinearity

Nonlinear parametric processes are known to depend critically on phase matching between the phase velocities of the interacting waves. Phase matching is mostly achieved by using conventional methods like crystal birefringence or quasi-phase matching (QPM) techniques. Recently, it was pointed out that unpoled ferroelectric crystals can also be used for frequency conversion in nonlinear optics being considered as randomly structured domain media. This wide distribution, representing reciprocal wave vectors, allows e.g. for tunable phase-matched second-harmonic (SH) generation practically in the whole transparency range of the crystal. All experiments shown here rely on the propagation of the fundamental beam in the plane perpendicular to the ferroelectric domain walls. The SH emission in this configuration is distributed in a plane normal to the optical axis of the ferroelectric crystal. This distribution, referring the ferroelectric domain size variety, is quantitatively studied at different degrees of disorder. The ability to obtain different degrees of disorder, starting from disorder to order, is achieved, utilizing the possibility of switching the electrical alignment of the domains at room temperature. The resulting SH intensity distributions are monitored with directly scanning. As order parameters, the spontaneous polarization, corresponding to the poling status, the mean value, and the variance of the domain sizes are considered. Moreover, the wavelength dependency of this complex behavior for different poling status is also studied in detail.