Second-harmonic generating properties of polar noncentrosymmetric aluminoborate solid solutions, Al5−xGaxBO9 (0.0 ≤ x ≤ 0.5)

Abstract

A series of solid solutions of polar aluminoborate materials, Al_5−xGa_xBO₉ (0.0 ≤ x ≤ 0.5) have been synthesized by standard solid-state reactions using Al₂O₃, Ga₂O₅, and B(OH)₃ as reagents. The phase purities, crystal structures, and solid solution behavior of the reported materials have been investigated by powder X-ray diffraction. Solid solutions of Al_5−xGa_xBO₉ crystallize in the polar noncentrosymmetric space group, Cmc2₁, with a three-dimensional structure consisting of distorted MO₄, MO₅, MO₆, and BO₃ polyhedra (M = Al or Ga). Powder second-harmonic generating (SHG) measurements on the Al₅BO₉ using 1064 nm radiation, indicate the material has a SHG efficiency of approximately 2 times that of α-SiO₂ and is not phase-matchable (type 1). Further nonlinear optical (NLO) measurements on the Al_5−xGa_xBO₉ solid solutions indicate a sharp increase in SHG efficiency up to 10 times that of α-SiO₂ for x ≥ 0.4. Close structural examination suggests that the alignment of the asymmetric π-delocalization of BO₃ groups is responsible for the abrupt increase of SHG efficiency.