ORIGINAL PAPER: SOL-GEL AND HYBRID MATERIALS FOR OPTICAL, PHOTONIC AND OPTOELECTRONIC APPLICATIONS Sol–gel-derived planar waveguides of Er 3+ :Yb 3 Al 5 O 12 prepared by a polyvinylpyrrolidone-based method T. Hla´sek 1 • V. Pola´k 1 • K. Rubesˇova´ 1 • V. Jakesˇ 1 • P. Nekvindova´ 1 • O. Jankovsky´ 1 • D. Mikola´sˇova´ 1 • J. Oswald 2 Received: 16 February 2016 / Accepted: 30 May 2016 / Published online: 4 June 2016 Ó Springer Science+Business Media New York 2016 Abstract Erbium (Er 3? )-doped ytterbium garnet [(Er x Yb 1-x ) 3 Al 5 O 12 ; x = 0.005, 0.02 and 0.1, YbAG] planar waveguides have been prepared by an aqueous sol–gel method using polyvinylpyrrolidone as a gelling agent. The thermal behavior of gel was studied by thermogravimetry and differential ther- mal analysis. The phase composition of the films was deter- mined using X-ray diffraction. Surface analysis was carried out by atomic force microscopy. A pure infrared emission of Er 3? ions was observed in all prepared films. Light propagation was studied by m-line spectroscopy using several wavelengths (633, 964, 1311 and 1552 nm). The minimum coating cycles to observe one propagated mode at 1552 nm were determined to be 7. The sol–gel-derived Er:Yb 3 Al 5 O 12 seems to be a promising material for light amplifiers in the IR region. Graphical Abstract Keywords Sol–gel Á Polyvinylpyrrolidone Á Planar waveguide Á Ytterbium-aluminum garnet Á Erbium 1 Introduction Nowadays, Er 3? ion and its infrared emission are used worldwide for optical signal transmission through silica fibers. Even though the loss is low at 1530 nm, the signal needs to be amplified every 70–100 km and in devices such as splitters [1]. The amplification can be achieved in planar waveguide structures doped with erbium. Garnets are promising host materials for many optical applications such as solid-state lasers [2], sensors [3] and also optical signal amplifiers [4]. The garnets with a general formula A 3 B 5 O 12 (where A is Y or a rare earth ion and B is a trivalent metal such as Al, Fe or Ga) are non-toxic, ther- mally and chemically stable, and their properties, such as refractive index and unit cell parameter, can be adjusted by changing the composition [5]. Besides the material parameters adjustment, the variable composition of garnets can be used to overcome the problem of a low absorption cross section of erbium ions. This drawback is commonly solved by co-doping with sensitizer ions. Ytterbium is a well-known sensitizer for erbium with a pumping wavelength of 980 nm. This co- doping was successfully used in materials such as LiNbO 3 [6, 7], NaYF 4 [8], KY(WO 4 ) 2 [9] or YAG [10, 11]. The energy transfer from ytterbium ( 2 F 7/2 ) to erbium ( 4 I 11/2 ) is highly effective when ytterbium is used as a part of the host matrix and not just as a co-dopant (e.g., YbAG) [12]. To produce the planar waveguide, it is necessary to use a thin film preparation technique that leads to a homoge- neous film with a low surface roughness. Commonly the techniques like liquid-phase epitaxy [13], chemical vapor & T. Hla´sek hlasekt@vscht.cz 1 Department of Inorganic Chemistry, University of Chemistry and Technology Prague, Technicka´ 5, 166 28 Prague 6, Czech Republic 2 Institute of Physics v.v.i., Academy of Sciences of the Czech Republic, Cukrovarnicka´ 10, 162 00 Prague 6, Czech Republic 123 J Sol-Gel Sci Technol (2016) 80:531–537 DOI 10.1007/s10971-016-4098-9