Upconversion Luminescence from Sol-Gel-Derived Erbium- and Ytterbium-Doped BaTiO[sub.3] Film Structures and the Target Form

Sol-gel technology has attracted attention in the fabrication of diverse luminescent materials and thin film structures, with forms that range from powders to microcavities. The optical properties of sol-gel-derived structures depend on the sol composition, deposition, and heat treatment conditions, as well as on the film thicknesses and other factors. Investigations on the upconversion luminescence of lanthanides in film structures and materials are also ongoing. In this study, we synthesized three different types of materials and film structures using the same sol, which corresponded to a Ba[sub.0.76]Er[sub.0.04]Yb[sub.0.20]TiO[sub.3] xerogel, as follows: (a) the target form, which used the explosive compaction method for sol-gel-derived powder; (b) single-layer spin-on xerogel films annealed at 450 and 800 °C; and (c) microcavities with an undoped SiO[sub.2]/BaTiO[sub.3] Bragg reflector surrounding a Ba[sub.0.76]Er[sub.0.04]Yb[sub.0.20]TiO[sub.3] active layer. The BaTiO[sub.3]:(Er,Yb)/SiO[sub.2] microcavity exhibited an enhancement of the upconversion luminescence when compared to the BaTiO[sub.3]:(Er,Yb) double-layer film fabricated directly on a crystalline silicon substrate. The reflection spectra of the BaTiO[sub.3]:(Er, Yb)/SiO[sub.2] microcavity annealed at 800 °C demonstrated a deviation of the maxima of the reflection within 15% for temperature measurements ranging from 26 to 120 °C. From the analyses of the transmission and reflection spectra, the optical band gap for the indirect optical transition in the single layer of the BaTiO[sub.3]:(Er,Yb) spin-on film annealed at 450 °C was estimated to be 3.82 eV, while that for the film annealed at 800 °C was approximately 3.87 eV. The optical properties, upconversion luminescence, and potential applications of the BaTiO[sub.3]:(Er,Yb) sol-gel-derived materials and structures are discussed in this paper.