Thermal sensitivity of frequency upconversion in Al4B2O9:Yb3+/Nd3+ nanoparticles

Abstract

We report measurements of frequency upconversion (UC) processes in ytterbium (Yb³⁺) and neodymium (Nd³⁺) doped Al₄B₂O₉ nanoparticles – labeled as Al₄B₂O₉:Yb³⁺/Nd³⁺ – prepared by a polymeric precursor method. The nanoparticles investigated contained 1 mol%Yb³⁺ and 2 mol%Nd³⁺. The samples were characterized by X-ray diffraction, high-resolution transmission electron microscopy, optical diffuse reflectance and photoluminescence. UC emissions in the range of 770 to 940 nm were studied by exciting the samples with a continuous-wave diode laser operating at 977.7 nm, on resonance with the Yb³⁺ transition ²F_7/2 → ²F_5/2. The linear dependence of the UC signals versus the laser intensity was investigated and the thermal sensitivity of the UC process was measured by changing the sample temperature from 26 to 60 °C. The UC photoluminescence behavior was understood to be due to the energy transfer from the excited Yb³⁺ to Nd³⁺ initially in the ground state manifold, in a process involving phonon-annihilation. The potential of the Al₄B₂O₉:Yb³⁺/Nd³⁺ nanoparticles to be used as optical thermometers was favorably evaluated in comparison with other materials. A large average thermal sensitivity of 2.6 × 10⁻² K⁻¹ was achieved.