Random quasi-phase-matching in a nonlinear photonic crystal structure of strontium tetraborate

Random quasi-phase-matching (RQPM) of the second-harmonic generation process is investigated experimentally and theoretically in a strongly randomized spontaneously grown nonlinear photonic crystal structure of strontium tetraborate. The thickness of domains in the structure is measured and employed in calculations of angular and spectral dependences of random quasi-phase-matching. Despite the great scale of randomness, the RQPM effect is preserved. The calculated enhancement factor due to RQPM is in excellent agreement with experiment, and good agreement between calculations and experiment is obtained for the shape of the angular dependence of RQPM. The calculated spectral dependence of RQPM consists of a number of narrow peaks, and a maximum enhancement factor is attained in the near ultraviolet. The band structure of randomized nonlinear photonic crystal is calculated.