Water-soluble polymers as chelating agents for the deposition of Er3+/Yb3+:LiNbO3 waveguiding films
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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.
KeywordsLithium niobate Polyethylene glycol Polyvinylpyrrolidone Sol-gel Waveguide Erbium
This work was financially supported by specific university research (MSMT No. 20-SVV/2017) and by the GA CR, project No. P108/12/G108.
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Conflict of interest
The authors declare that they have no conflict of interest.
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