Semi-transparent barium borate surface crystallization for second harmonic generation

Glasses from the barium borate system, xB2O3–(100 − x)BaO (50 < x < 68 mol%), with a refractive index of 1.658–1.645 (λ = 435.8 nm) and densities ranging from 3.85 g cm−3 to 3.75 g cm−3, were synthesized by a conventional melting-quenching method. 75 × 75 × 5 mm3 samples were prepared from the barium borate glass, which was transparent in the visible region and very stable. The characteristic temperatures of the BBO60 (x = 60) glass, which were determined by differential thermal analysis at 10 °C min−1, were Tg = 570 °C, Tx = 671 °C, and Tp = 685 °C (±2 °C). The glass samples were then heat-treated for controlled crystallization. 57B2O3–43BaO planar samples, measuring 15 × 15 × 3 mm3, heat-treated at 610 °C for periods varying from 24 h to 60 h, exhibited a surface crystallization mechanism. The crystalline layers were analyzed by X-ray diffraction and identified as β-BaB2O4. These crystallized layers showed second harmonic generation (SHG) when irradiated by a 1064 nm laser beam (Nd:YAG) with 300 mW of power. The SHG intensity was compared with a Y-oriented single crystal α-quartz plate.