Second harmonic generation in a strontium barium niobate crystal with a random domain structure

The phenomenon of the second-harmonic (SH) generation in a strontium barium niobate (SBN) crystal in directions not obeying the conditions of phase matching for spatially homogeneous media has been studied. The appearance of the SH signal is interpreted as resulting from the quasi-phase-matching nonlinear wave interaction due to the random domain structure of the SBN crystal containing acicular ferroelectric microdomains. A theoretical model describing the SH generation in such two-dimensional, spatially inhomogeneous nonlinear optical structures is proposed. An analysis of the second-harmonic emission directivity patterns shows that the main contribution to the noncollinear SH generation in SBN is due to near-surface domains extending to a depth of up to several dozen microns.