Nonlinear optics of randomly structured strontium tetraborate

Abstract

Strontium tetraborate (SBO) is attractive for nonlinear optics of short-wavelength range including VUV due to its transparency window [1]. Nonlinear coefficients of SBO are the largest among crystals transparent below 270 nm. Angular phase matching is, however, absent in SBO, and the only possibility of conversion enhancement is to employ self-organized random nonlinear photonic crystal (NPC) structures discovered in SBO. These NPC are capable of two limiting cases of phase matching, namely, nonlinear diffraction (NLD) and random quasi phase matching (RQPM). NLD allows detection of NPCs and testing their reciprocal superlattice vectors (RSV) spectra via variation of the incidence angle. NLD conversion of broadband fs pulses of modest power from Ti:S oscillator results in 1.9% efficiency, being, to our knowledge, the best result obtained for this type of phase matching. RSV spectrum is wide enough to ensure NLD tunability from 355 to 510 nm. This spectrum is not flat, but the narrowing of the generated radiation spectrum due to non-flatness is rather modest.