Patterning of c-axis-oriented Ba₂TiX₂O₈ (X = Si, Ge) crystal lines in glass by laser irradiation and their second-order optical nonlinearities

Crystal lines consisting of nonlinear optical fresnoite-type Ba₂TiX₂O₈ (X = Si, Ge) crystals are patterned on the surface of CuO (1 mol%)-doped 33.3BaO-16.7TiO₂-50SiO₂ or -50GeO₂ glasses by continuous-wave (cw) neodymium:yttrium aluminum garnet (Nd:YAG) laser (wavelength 1064 nm) irradiation. It is confirmed from polarized micro-Raman scattering spectra that the c axis of Ba₂TiX₂O₈ crystals in the lines are oriented along the laser-scanning direction (i.e., parallel to the lines). The azimuthal dependences of second-harmonic (SH) intensities for the crystal lines indicate unique fringe patterns, and the c-axis orientation of the crystals is supported from the analyses of fringe patterns. The value of the d₃₁/d₃₃ ratio is found to be ∼23 for Ba₂TiSi₂O₈ (BTS) crystal lines, and ∼13 for Ba₂TiGe₂O₈ (BTG) crystal lines, where d₃₁ and d₃₃ are the principal d tensors for the second-order optical nonlinearity of fresnoite-type crystals.