We previously showed silica-hafnia (SiO₂-HfO₂) thin-film systems to be suitable for fabricating amorphous planar waveguides, glass-ceramic waveguides, spherical microresonators, and tapered rib waveguide lasers.¹Using appropriate top-down and bottom-up techniques, we can make erbium (Er³⁺)-activated glass-ceramic planar waveguides. Top-down approaches seek to create nanocomposite devices by using larger externally controlled zones to construct a new system, while bottom-up methods focus on smaller components and arrange them into a more complex system. These materials exhibit attenuation coefficients as low as 0.3dB/cm at 1542nm, and the HfO₂crystalline phase greatly enhances the spectroscopic properties of embedded erbium ions. X-ray diffraction and high-resolution transmission electron microscopy analyses have shown the formation of tetragonal HfO₂nanocrystals with dimensions of about 3–5nm (see Figure 1), depending on the HfO₂ content.¹

While silica remains one of the best low-cost materials, fluoride glasses are attractive because of their ability to solubilize rare earth ions (> 5×10²¹ ions cm⁻³) and their inherently low phonon cut-off energy. Mortier et al. showed that transparent glass ceramics can be obtained by heating zirconium fluoride-based glass with high erbium content (8mol%) at 70°C above the glass-transition temperature (390°C) for 40min.³ This causes a so-called spinodal decomposition, whereby the chemical composition of the glass fluctuates continuously until it decomposes into two separate and distinguishable phases, producing a glass ceramic with absorbance cross section increased by 20%. The morphology of the crystallites is dendritic with high connectivity. We have obtained fluoride glass waveguides with composition close to bulk glass by physical vapor deposition,² which opens up the possibility of using fluoride glass-ceramic materials for photonic applications.

Fluoride glasses provide an interesting system for fabricating both amorphous and glass-ceramic waveguides activated by rare earth ions. They take advantage of high Er³⁺ solubility and low phonon energy compared with oxide glasses. Moreover, they enable incorporation of rare earth ions into the crystal phase after thermal annealing (see Figure 2). Bulk fluoride glasses and glass ceramics activated by Er³⁺ and ytterbium ions (Yb³⁺) were investigated with the aim of quantifying the influence of Yb³⁺ on the spectral characteristics of Er³⁺ in these systems. Glassy samples co-doped with Yb³⁺with a ratio Yb:Er of 5:1 present an absorption coefficient and emission intensity four times higher at 1532nm than samples activated only with Er³⁺.²

In summary, glass-ceramic waveguides activated by rare earth ions are nanocomposite systems that exhibit specific morphological and spectroscopic properties. They allow exploration of interesting new physical concepts and enable us to construct novel photonic devices based on luminescence enhancement. Fabrication techniques based on both bottom-up and top-down approaches have been shown to be viable, though it remains true that precise adherence to protocol is required to achieve the reliability and reproducibility necessary for such devices. Our work now focuses on fabrication protocols that preferentially embed rare earth ions inside the crystal phase, and on patterning techniques that minimize degradation of optical properties.