The technique for the writing of crystal lines in glass by Nd:YAG laser irradiation, selective atom heat processing, is applied to rare-earth (Sm3+, Dy3+) barium borate glasses, and nonlinear optical β-BaB2O4 (β-BBO) crystal lines with a width of ∼5 μm are fabricated. The crystal lines show uniform colors in polarized optical microscope and second-harmonic generation (SHG) measurements. The azimuthal dependence of SHG signals in the crystal lines is the same as that in commercially available β-BBO bulk single crystals. It is demonstrated that the crystal lines written by YAG laser irradiations are β-BBO single crystals with the c-axis orientation along the laser scanning direction.

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This paper reports recent progress in the patterning of non-linear optical crystals on the glass surface by laser irradiation. Two techniques for the writing of crystal lines have been developed, i.e., rare-earth (samarium) atom heat processing and transition metal atom heat processing, in which a continuous wave Nd:YAG laser (wavelength: k 5 1064 nm) is irradiated to the glasses containing rare-earth (RE: Sm 31 , Dy 31 ) ions or transition metal (TM: Ni 21 , Fe 21 , V 41 ) ions. The writing of crystal lines such as b-BaB 2 O 4 , Sm x Bi 1Àx BO 3 , and Ba 2 TiGe 2 O 8 showing second harmonic generations has been successful. It is clarified from the azimuthal dependence of second harmonic intensity and polarized micro-Raman scattering spectra that crystal lines consist of highly oriented crystals along the crystal line growth direction. It is also possible to write two-dimensional crystal bending or curved lines by just changing the laser scanning direction. The mechanism of the laser-induced crystallization has been proposed. II. REAH/TMAH ProcessingIn rare-earth atom heat processing, cw Nd:YAG lasers with l 5 1064 nm are irradiated onto glasses containing Sm 2 O 3 (or M. Davies-contributing editor

Second-order optical nonlinearities, d33 values, of fresnoite-type crystals, Ba2TiSi2O8 (BTS), Sr2TiSi2O8 (STS), and Ba2TiGe2O8 (BTG), have been evaluated by measurements on transparent surface-crystallized glasses consisting of fresnoite-type crystals with the Maker fringe technique. It is found that all these crystals show large d33 values, i.e., d33=13±2 pm/V for BTS, d33=7.2±0.4 pm/V for STS, and d33=22±3 pm/V for BTG. In particular, it is emphasized that BTG has an extremely large d33 value, comparable to LiNbO3 single crystal. It is clarified that the d33 value of fresnoite-type crystals increases with increasing c/a ratio in the lattice parameters of their structures. It is suggested that the increase in the c/a ratio results in the elongation of TiO5 pyramidal units along the c axis, consequently enhancing spontaneous polarizations in the fresnoite-type structure. Transparent surface-crystallized glasses consisting of fresnoite-type crystals are candidates for nonlinear optical materials.

A technique for the writing of crystal line in glass has been developed, in which a continuous-wave Nd:YAG laser (wavelength: λ=1064nm) is irradiated to the glasses containing transition metal (TM) ions such as Fe2+, Ni2+, and V4+. Laser energies are absorbed through d-d transitions of TM ions and dissipated to the lattice surrounding TM ions by nonradiative relaxation process, giving the increase in temperature in the laser irradiated region and inducing crystallization. This technique has been demonstrated for the writing of crystal lines consisting of nonlinear optical fresnoite-type Ba2TiGe2O8 and Ba2TiSi2O8 crystals in NiO-, Fe2O3-, and V2O5-doped (0.3–1mol%) BaO–TiO2–GeO2 and BaO–TiO2–SiO2 glasses. It is confirmed that crystals in the crystal lines are highly oriented along the laser scanning direction. The technique developed in this study is proposed to be called “transition metal atom heat processing.”