Fabrication of transparent tellurite-based glass-ceramics with graded optical nonlinearity

Transparent TeO₂-based glass-ceramics with graded optical nonlinearity such as second harmonic generation (SHG) have been fabricated for the first time through the two-step heat-treatment of 15K₂O-15Nb₂O₅-70TeO₂/ 7.5Li₂O-7.5Na₂O-15Nb₂O₅-70TeO ₂ joined glass sample. The gradual distribution of the species of alkali metal ions at the interface is confirmed from X-ray diffraction analyses. This result indicates the gradation of the second harmonic intensity at the interface, because the perfect solid solution between Na⁺ and K⁺ occurrs in the crystalline phase showing SHG and the intensity of second harmonicity decreases with increasing Na₂O/K₂O ratio. The fabrication of the transparent glass-ceramics having the gradient of the degree of the crystallization of the phase showing SHG has been also tried using the joined glass of 15K₂O-15Nb₂O₅-70TeO₂ (sample A) and 15K₂O-15W₂O₆-70TeO₂ (sample B). Sample B is very stable thermally against crystallization. The crack is observed at the interface of such glass-ceramics because a large difference in the thermal expansion coefficient between the crystallized sample A showing SHG and the sample B maintaining the glassy state. The transparent Er³⁺ and Eu³⁺-doped glass-ceramics have been successfully prepared, and it is found that the intensity of the frequency upconversion fluorescence at around 550 nm due to the ⁴S_3/2→⁴I_15/2 transition of Er³⁺ in the glass-ceramics is strong compared with that in their precursor glass. This gives a new possibility for the fabrication of optically functionally graded TeO₂-based glass-ceramics.