Detection of randomized nonlinear photonic crystal structure in a non-ferroelectric crystal

A spontaneously grown randomized nonlinear photonic crystal structure formed by 180 ◦ domains has been detected in strontium tetraborate via nonlinear diffraction. Domains have the form of sheets lying in the bc crystallographic plane with the domain walls oriented perpendicularly to the a crystallographic axis. The effective thickness of domains contributing to the nonlinear diffraction is determined from the Fourier spectrum of inverse superlattice vectors, and it falls within the range 180 nm to 6 μm. Nonlinear diffraction at the third harmonic frequency was observed in the same crystal sample.