Sol–gel preparation and characterization of transparent KTiOPO4/SiO2 nanocomposite glass for second harmonic generation

A nanocomposite glass consisting of sol–gel derived nanocrystalline KTiOPO4 (KTP) and SiO2-based glass has been developed. The multicomponent glass in the K2O–TiO2–P2O5–SiO2 system has been prepared by the sol–gel method. After heat treatment at temperatures higher than 650°C, the glass transformed into a dense nanocomposite glass containing KTP nanocrystals. The transparent nanocomposite glasses, 10 mm in diameter and 2–3 mm thick, can be obtained by the heat treatment. The transmittance of the polished nanocomposite glasses 0.6 mm thick can be 70% at 0.532 μm wavelength. The structures of the nanocomposite glass in the 20KTiOPO4–80SiO2 systems were analyzed by XRD and TEM. During heat treatment in the temperature range 650–900°C, the KTP nanocrystals precipitate from the multicomponent parent glass. TEM analysis indicated that nanometer sized or larger crystallites of KTP are dispersed in the SiO2-based glass. The KTP nanocrystallite size is in the range 10–50(±8) nm. Using a Nd:YAG laser of 1.064 μm wavelength, second harmonic generation (SHG) was clearly observed in the transparent nanocomposite glasses and translucent glass–ceramics. SHG green light (0.532 μm) is so strong that it is observed by naked eye. In contrast, no second harmonic generation was detected in multicomponent K2O–TiO2–P2O5–SiO2 and other SiO2-based glass systems. These results suggest that second harmonic generation in the 20KTiOPO4–80SiO2 nanocomposite glass originate from KTP nanocrystals embedded in SiO2-based glass. Also, the nanocomposite glasses are chemically stable and can be easily polished. As a result, these transparent nanocomposite glasses have a potential for non-linear optical application.