Ageing of PVP/LiNbO3 solutions and its impact on the optical properties of Er3+/Yb3+:LiNbO3 waveguiding films
Introduction
Among oxide materials, lithium niobate (LiNbO3) is one of the most often used because of its electro-optical, non-linear and piezoelectric properties [1]. After doping with rare earth metals, lithium niobate can be used in optical applications over a very wide range of wavelengths. Many of these applications require LiNbO3 in the form of films with a sufficient microstructure quality. Besides other deposition techniques (pulsed laser deposition, ion implantation, liquid phase epitaxy) [2], [3], [4], [5], LiNbO3 films can be prepared by a sol-gel method as a non-vacuum and inexpensive process. Such films are usually deposited on a monocrystalline substrate by the use of dip- or spin-coating. Thin films of LiNbO3 intended for waveguiding applications have been prepared by the sol-gel technique based on the hydrolysis of metal alkoxides as the starting compounds [6]. In the case of the alkoxide methods, the conventional parts of the process are alkoxide hydrolysis, condensation reaction, the ageing of sols (within hours), and gelation — steps that together determine the crystallinity and microstructure of the final oxide. However, to prepare thicker films for multi-mode waveguiding is rather complicated when using the alkoxide method.
Apart from the hydrolytic alkoxide sol-gel methods, there are several non-hydrolytic sol-gel methods. In these non-hydrolytic methods, no active hydrolysis is present and gelation is induced by an increase in the abundance of a non-bonding interactions in a solution, for example by the addition of a polymer with chelating groups [polyvinylalcohol (PVA) or polyvinylpyrrolidone (PVP)] [7], [8]. Such a polymer-assisted sol-gel method can also have other advantages, such as an increase in solution viscosity (thus enabling the preparation of thicker layers) or stress relaxation during the final thermal treatment of a film. Furthermore, non-bonding interactions can also be induced by simple ageing or by a solution reflux. However, given the high price of starting compounds and the need to prepare a fresh solution, it is desirable to find solutions that can stand for a long time without any deteriorative influence on the microstructure of the final film. Despite this, the influence of solution ageing in non-hydrolytic methods has been described only sporadically and for a short period of days only [9], [10]. Although methods not exploiting active hydrolysis have been described for LiNbO3 films [8], the waveguiding properties of these films are not reported.
In our previous study [11], however, we described the successful non-hydrolytic preparation of Er3+/Yb3+ doped LiNbO3 in the form of waveguiding films. We used a polyvinylpyrrolidone (PVP) solution in 2-methoxyethanol and its spin-coating on sapphire substrates. The microstructural and optical properties were strongly dependent on the thermal treatment of the deposited LiNbO3 films; some films were of high quality required for the guiding of an optical signal. After this study was published, we noticed that with prolonged ageing, the thermal behaviour of the solutions changed, but without harming the high-quality microstructure of the films deposited. After modifying the thermal treatment, it was possible to use the PVP solutions for several months. In this paper, we describe this potentially useful phenomenon in detail.
We present the ageing of Er:Yb:Li:Nb:PVP solutions for up to 10 months and its influence on the optical properties of Er3+/Yb3+:LiNbO3 waveguiding films. We use the mid-infrared (MIR) spectroscopy to illustrate the changes that take place in the non-bonding interactions in the solution during the ageing. In the subsequent deposition-heating process of the aged solutions, two thermal treatment regimes (one-step and two-step heating) are tested. The change in the thermal behaviour of deposited films is demonstrated by TG/DTA analysis. The influence on microstructure and, consequently, on the luminescence and waveguiding properties of the LiNbO3 films is established. We show that a PVP solution can be successfully used for an extended time after its preparation, but also that the thermal treatment of the layers must be modified in order to prepare a waveguiding structure.
Section snippets
Thin films preparation
The preparation of a LiI/NbV/ErIII/YbIII starting solution with polyvinylpyrrolidone (PVP) in 2-methoxyethanol (2-ME) was described in our previous paper [11]. PVP of two molecular masses was used−29,000 g/mol and 360,000 g/mol (further denoted as PVP29 and PVP360, respectively). The molar ratio of starting compounds was kept according to the ratio Li:Nb:PVP:2-ME = 1:1:1:45. Er3+ and Yb3+ ions were added in a concentration of 0.5 at % with respect to the LiNbO3 stoichiometry. All deposited
Results and discussion
In our previous study [11], we demonstrated a successful sol-gel preparation of Er3+/Yb3+:LiNbO3 thin films on sapphire substrates. The starting solution was prepared as a 2-methoxyethanol solution containing polyvinylpyrrolidone (PVP) of two different molar masses: 29,000 g/mol or 360,000 g/mol (denoted as PVP-29 or PVP-360, respectively). The two-step decomposition of deposited layers (350 °C and 700 °C, both for 10 min) produced films with a microstructure sufficient to guide an optical
Conclusions
We have demonstrated that PVP/2-methoxyethanol solutions of the ErYbLiNb system can be used for the successful preparation of Er3+/Yb3+:LiNbO3 active waveguides. Moreover, we describe the behaviour of the solutions after ageing for up to 10 months. It is proved that the solution ageing may not have deteriorative influence on the microstructure and waveguiding ability of the films deposited by spin-coating.
Despite these promising results, attention must be paid to the heat treatment of deposited
Acknowledgements
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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