Water-soluble polymers as chelating agents for the deposition of Er³⁺/Yb³⁺:LiNbO₃ waveguiding films

Compared to other oxide materials, the sol-gel deposition of an optically transparent LiNbO₃ waveguiding film of sufficient thickness (approx. 1 μm) is complicated by the presence of a highly hydrolyzing Nb(V) in the starting solution. Thicker films require more concentrated solutions that are not easily achieved for such ions. This problem may be solved using strong chelating agents such as water-soluble polymers. To prepare a stable Er(III)/Yb(III)/Li(I)/Nb(V)/2-methoxyethanol solution with high metal concentration, we tested three such polymers: polyethylene glycol (PEG), polyacrylic acid (PAA) and polyvinyl alcohol (PVA), and compared them with already used polyvinylpyrrolidone (PVP). The solutions were spin-coated on crystalline sapphire substrates under a multi-step heating-deposition regime. Apart from Er³⁺/Yb³⁺ photoluminescence properties, we evaluated the influence of the film microstructure (SEM, AFM) on optical transparency and waveguiding ability in the UV/Vis/NIR region (transmission and m-line spectroscopy). Among the newly tested polymers, only PEG was able to prevent Nb(V) hydrolysis up to a maximum metal concentration of 0.6 mol/L. For PEG and PVP, the crystallization temperature of the deposited films (between 700 °C and 1000 °C) was compared. After further optimization of the heating-deposition process, we were able to prepare a transparent Er³⁺/Yb³⁺:LiNbO₃ film thick enough to guide an optical signal in the NIR region. Thus, the use of PEG results is one of the very few non-hydrolytic sol-gel methods suitable for the preparation of not only luminescent, but also waveguiding Er³⁺/Yb³⁺:LiNbO₃ structures.