Conversion of radiation in nonlinear photonic crystals of strontium tetraborate

Abstract

Strontium tetraborate (SBO) possesses transparency in the VUV that allows nonlinear generaton to the wavelengths as short as 125 nm [1]. Its nonlinearity is among the highest ones for the crystals transparent in the far UV region, however, angular phase matching is absent. SBO is not presently considered to be a ferroelectric, and the possibility to create regular domain structures for quasi phase matching is not obvious. Recently the existence of domain structures consisting of alternating oppositely poled domains was found in as grown crystals of SBO [2]. Domain walls are perpendicular to a crystallographic axis. Structures are well ordered in the directions of b and c crystallographic axes but highly randomized in the direction of a axis. Non-collinear generation (nonlinear diffraction) of the second and third harmonics of nanosecond Nd:YAG laser is used for the investigation of reciprocal superlattice vectors (RSV) spectrum of the domain structures. Dependence of the nonlinear diffraction angles on the rotation of the domain structure fairly agrees with calculations. Values of effective domain widths that contribute to the nonlinear diffraction spread from 8 microns to 180 nm. Analysis of RSV spectrum indicates that efficient nonlinear diffraction using typically obtainable samples of SBO domain structures must be expected at fundamental wavelengths down to 500 nm. However, possibilities exist to grow domain structures with narrower domains.