Laser processing of Yb3 +/Er3 + co-doped LiYF4 thin films with up-conversion properties
Introduction
The study of up-conversion (UC) properties (i.e. near-infrared (NIR) conversion into the visible spectral range) of rare-earth (RE) doped materials [1] has been undertaken mostly in view of their favorable properties for applications in photovoltaic cells [2], [3], but also for bio-applications [4], [5] and waveguides for optical components [6]. Among the RE-doped fluoride crystals, LiYF4 has been widely studied mainly for laser applications, due to having the largest number of laser lines (from the UV to mid-IR range), very good processing characteristics and high capacity for isomorphous replacement of yttrium ions by trivalent RE ions (due to their similar size), without strongly affecting the lattice structure. While solid-state reaction has been one of the most successfully applied techniques for the obtaining of RE-doped fluoride powders, these materials need to be synthesized as thin films in order to make them suitable for the envisioned applications. Their obtaining as thin films has been achieved by growth techniques such as molecular beam epitaxy [7], sol-gel [8], pulsed laser deposition (PLD) and matrix-assisted pulsed laser evaporation (MAPLE) [9]. While in the case of PLD thin film growth is usually achieved by ablating a solid target with a pulsed laser source, MAPLE employs a frozen target consisting of the material of interest and a diluting agent that is evaporated and pumped out following laser irradiation. It was interesting to see how a technique usually used for the transfer of polymers and biomaterials would fare in the case of RE-doped fluoride glasses. The control of the thin films morphological characteristics is also a key point for their subsequent use for laser induced forward transfer (LIFT) related applications, a technique that enables the achievement of complex 3D structures and micropatterns having a wider range of functionalities [10].
The present work comes as a continuation of our previous study on Eu3 +-doped LiYF4 thin films and capitalizes on that experience [9]. Therefore, we will not insist on aspects relating to the techniques that were used or to the properties and advantages of the LiYF4 system, in order to avoid overlapping discussion. In our previous study only the photoluminescence (PL) properties of the transferred films were investigated, due to the fact that they did not possess UC capabilities. In the current study we focused on up-conversion luminescence properties of the Yb3 +/Er3 + co-doped LiYF4 thin films grown by pulsed laser deposition and matrix-assisted pulsed laser evaporation. This is a major advancement with respect to our previous work, as the results presented in this study are more relevant from an applicative point of view. Moreover, the investigation of the functional properties of the obtained RE-doped fluoride glasses helps us achieve a more complete picture about the potential of MAPLE as a growth technique used for the obtaining of such thin films, and therefore about its relevance to the field.
Section snippets
Target preparation
LiYF4 co-doped Yb3 +(4%), Er3 + (1%) powder was prepared using conventional solid state route, from a stoichiometric mixture of LiF, YF3, YbF3 and ErF3 that was thoroughly ground (down to tens on microns) and dried at 60 °C. To obtain the PLD target, the powder was pressed at 8 MPa (as polycrystalline pellet) and annealed at 650 °C for 2 h in dry nitrogen atmosphere [9]. For MAPLE experiments, frozen solutions of 20% (Yb3 +/Er3 +):LiYF4 powder and 80% dimethyl sulfoxide (DMSO) weight percentages were
Structural properties
Fig. 1 presents the X-ray diffraction patterns of the (Yb3 +/Er3 +) co-doped LiYF4 thin films grown by PLD and MAPLE, as compared to the target (polycrystalline pellet). Diffraction peaks of the LiYF4 crystalline phase are assigned according to JCPDS file no. 081–2254. It can be seen that the LiYF4 phase of the target material is found in the thin films without any other peaks that might be assigned to YF3, indicating that no Li migration occurred during growth.
Samples morphology and ellipsometry
The interpretation of ellipsometry
Conclusions
We investigated the up-conversion properties of Yb3 +, Er3 + co-doped crystalline LiYF4 thin films grown by MAPLE and PLD techniques. The samples exhibit a surface morphology that is typical for laser processed thin films of RE-doped fluoride glasses, with roughness values of the order of few hundreds of nanometers. The determined refractive index and extinction coefficient values differ for the two samples, likely due to an increased density of defects in the sample obtained by MAPLE.
Under 980 nm
Acknowledgements
We acknowledge the financial support of the Romanian Education and Research Ministry (IDEI Project no. 290/05.11.2011) and the National Authority for Research and Innovation in the frame of Nucleus programme - contract 4N/2016.
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