Nonlinear-optical susceptibility of hilgardite-like borates M2B5O9X(M=Pb,Ca,Sr,Ba; X=Cl,Br)

Usage of non-centrosymmetric boron oxide derivative crystals as nonlinear elements may be a key solution of pressing problem of obtaining powerful radiation at shorter wavelengths via frequency multiplication of solid state laser radiations. It is shown that Pb2B5O9Br and some other members of hilgardite-like borohalogenides demonstrate in powder form, high non-linear optical activity in the SHG experiments. An attempt is made using the Philips-Van Vechten-Levine-Xue bond theory to calculate second-order nonlinear coefficients for isostructural to mineral hilhardite noncentrosymmetric crystals M2B5O9X with M=Pb, Sr, Ba, Eu, Ca and X=Cl, Br. Theoretically obtained values of nonlinear optical coefficients dijk only partly correlate with the results of SHG measurements. In particular, the experiments distinctively show sharp increase of nonlinear activity along the series Ca < Sr < Ba < Pb and Cl < Br of metal boro-halogenides, while theoretical estimations give substantially more slack composition dependences. Analysis of contribution of different boron-oxygen bonds to optical nonlinearities of the compounds reveals important role of planar BO3 triangles, though this factor is yet unable to explain enormously high SHG output from Pb2B5O9Br.