Eu-doped ZnO–HfO₂ hybrid nanocrystal-embedded low-loss glass-ceramic waveguides

We report on the sol–gel fabrication, using a dip-coating technique, of low-loss Eu-doped 70SiO₂ –$(30-x)$ HfO₂–xZnO (x = 2, 5, 7 and 10 mol%) ternary glass-ceramic planar waveguides. Transmission electron microscopy and grazing incident x-ray diffraction experiments confirm the controlled growth of hybrid nanocrystals with an average size of 3 nm–25 nm, composed of ZnO encapsulated by a thin layer of nanocrystalline HfO₂, with an increase of ZnO concentration from x = 2 mol% to 10 mol%  in the SiO₂–HfO₂ composite matrix. The effect of crystallization on the local environment of Eu ions, doped in the ZnO–HfO₂ hybrid nanocrystal-embedded glass-ceramic matrix, is studied using photoluminescence spectra, wherein an intense mixed-valence state (divalent as well as trivalent) emission of Eu ions is observed. The existence of Eu²⁺ and Eu³⁺ in the SiO₂–HfO₂–ZnO ternary matrix is confirmed by x-ray photoelectron spectroscopy. Importantly, the Eu${}^{2+,3+}$-doped ternary waveguides exhibit low propagation losses (0.3 ± 0.2 dB cm^–1 at 632.8 nm) and optical transparency in the visible region of the electromagnetic spectrum, which makes ZnO–HfO₂ nanocrystal-embedded SiO₂–HfO₂–ZnO waveguides a viable candidate for the development of on-chip, active, integrated optical devices. (paper)