Optical Properties and Upconversion Luminescence of BaTiO₃ Xerogel Structures Doped with Erbium and Ytterbium

Erbium upconversion (UC) photoluminescence (PL) from sol-gel derived barium titanate (BaTiO₃:Er) xerogel structures fabricated on silicon, glass or fused silica substrates has been studied. Under continuous-wave excitation at 980 nm and nanosecond pulsed excitation at 980 and 1540 nm, the fabricated structures demonstrate room temperature PL with several bands at 410, 523, 546, 658, 800 and 830 nm, corresponding to the ²H_9/2 → ⁴I_15/2, ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2, ⁴F_9/2→ ⁴I_15/2 and ⁴I_9/2→ ⁴I_15/2 transitions of Er³⁺ ions. The intensity of erbium UC PL increases when an additional macroporous layer of strontium titanate is used beneath the BaTiO₃ xerogel layer. It is also enhanced in BaTiO₃ xerogel films codoped with erbium and ytterbium (BaTiO₃:(Er,Yb)). For the latter, a redistribution of the intensity of the PL bands is observed depending on the excitation conditions. A multilayer BaTiO₃:(Er,Yb)/SiO₂ microcavity structure was formed on a fused silica substrate with a cavity mode in the range of 650-680 nm corresponding to one of the UC PL bands of Er³⁺ ions. The obtained cavity structure annealed at 450 °C provides tuning of the cavity mode by 10 nm in the temperature range from 20 °C to 130 °C. Photonic application of BaTiO₃ xerogel structures doped with lanthanides is discussed.