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

Abstract

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.

Keywords: barium titanate; erbium; luminescence; microcavity; sol-gel; strontium titanate; upconversion; xerogel; ytterbium.

Figure 1

SEM-images of BaTiO₃:(Er,Yb) xerogel layer deposited directly on a monocrystalline Si wafer (Sample C) (a) and on the same wafer using an intermediate macroporous SrTiO₃ xerogel sublayer (Sample D) (b).

Figure 2

Upconversion PL spectra of BaTiO₃:Er (sample B) and BaTiO₃:(Er,Yb) (samples C and D) xerogel film structures on a Si substrate, taken under CW excitation at the wavelength of 980 nm. Samples B and D were obtained using an additional macroporous SrTiO₃ xerogel sublayer before the deposition of the active BaTiO₃ layer. Inset: comparison of the PL spectra of the samples B (multiplied by 28) and D.

Figure 3

Time-resolved spectra of upconversion PL obtained from BaTiO₃:Er (sample B) (a,b) and BaTiO₃:(Er,Yb) (sample D) (c) xerogel layers deposited on a silicon wafer using a porous SrTiO₃ xerogel sublayer. The spectra were measured under pulsed excitation with the wavelengths of λ_ex = 980 nm (a,c) and λ_ex = 1540 nm (b). The intensity of the upconversion PL signal is plotted as a function of the wavelength and the time delay after the laser pulse.

Figure 4

Time dependencies of the upconversion PL intensity (at 550 nm) observed in the BaTiO₃:Er (sample B) and BaTiO₃:(Er,Yb) (sample D) xerogel layers deposited on the SrTiO₃/Si substrate. The PL kinetics were obtained under pulsed excitation at 980 nm with the pulse duration of 10 ns (blue curve). Each curve was normalized to the maximum value of the PL intensity.

Figure 5

Energy level diagram showing mechanisms of upconversion PL excitation under 980 nm optical pumping: (a) “excited state absorption” (ESA) and (b) “energy transfer upconversion” (ETU) in Er-doped material; (c) “energy transfer upconversion” in Er/Yb-codoped material. “ET”—energy transfer.

Figure 6

SEM image (a) and 3D SIMS images (b–d) of the BaTiO₃:(Er,Yb)/SiO₂ Bragg reflector on a glass substrate (sample E) after annealing at 450 °C for 30 min in air.

Figure 7

SEM images of the BaTiO₃/SiO₂ microcavity (sample F) after heat treatment at 450 °C (a) and 600 °C (b).

Figure 8

Room-temperature optical spectra of BaTiO₃:(Er,Yb)/SiO₂ microcavity (sample F): (a) transmission (solid lines) and reflection (dashed lines) spectra of BaTiO₃/SiO₂ microcavity on a fused silica after final annealing at 450 °C (black lines), 500 °C (red lines) and 600 °C (green lines); (b) upconversion PL spectra of the microcavity obtained after final annealing at 450 °C (black line), 500 °C (red line) and 600 °C (green line). Each barium titanate layer is doped with Er and Yb.

Figure 9

Fragments of the reflection spectra of the BaTiO₃:(Er,Yb)/SiO₂ microcavity (sample F, annealed at 450 °C) in the vicinity of the cavity mode, measured in the temperature range 20–130 °C. Inset: central wavelength of the cavity mode as a function of temperature.