The Fluorescent Properties Of Quantum Cutting And Upconversion In Tb³⁺-Yb³⁺ System

Trivalent rare earth ions exhibit rich energy levels and the ability of photonfrequency conversion, which has significant application values in many fields. Thedownconversion quantum cutting is widely used in lighting and display system, andespecially, the near infrared quantum cutting is promising to improve the efficiencyof Si solar cells. The upconversion luminescence plays an important role in shortwavelength solid-state lasers,3D display, biofluorescence imaging and so on. Theabsolute photoluminescence efficiency is a vital parameter to evaluate theluminescent capability and application prospects of materials. Based on Tb³⁺-Yb³⁺co-doped system, the luminescent mechanism and absolute photoluminescenceefficiency in different host materials were investigated detailed.The linear quantum cutting properties of Tb³⁺-Yb³⁺co-doped system werestudied. The near infrared quantum cutting was realized under the excitation of a473nm continuous wave laser in Tb³⁺-Yb³⁺co-doped oxyfluoride glass through theenergy transfer from Tb³⁺to Yb³⁺. The cooperative quantum cutting rate equationmodel was created, which incorporated the interactions between donor Tb³⁺andacceptor Yb³⁺ions. Two judgment criteria were developed to verify the proposedmechanism: the slopes of visible and near infrared luminescence pump power curvesare1; the decay curve of acceptor ion Yb³⁺includes the lifetime values of Tb³⁺andYb³⁺. The experimental results meet the two judgment criteria, which confirmed theproposed linear cooperative quantum cutting mechanism in Tb³⁺-Yb³⁺co-dopedoxyfluoride glass. A method to study the quantum cutting mechanism by rateequation was established. In addition, the theoretical quantum efficiency wascalculated.The nonlinear quantum cutting properties of Tb³⁺-Yb³⁺co-doped system wereinvestigated. The near infrared quantum cutting was realized in NaYF₄: Tb³⁺,Yb³⁺nanocrystal. The slopes of near infrared luminescence power dependence curveswere fitted to be between0.5and1, which indicated the quantum cutting processwas nonlinear. A virtual energy level of Tb³⁺was introduced to participate thequantum cutting process. Based on this, the corresponding quantum cutting rateequation model was developed to explain the nonlinear process. Analysis resultsshowed that the linear downconversion process combined with second-orderquantum cutting process induced the nonlinear quantum cutting process. A specificgravity factor was defined to evaluate the proportions of the two processes. Itshowed the second-order quantum cutting process to be the dominant process. The investigation will contribute to a deeper understanding of the physical mechanism inTb³⁺-Yb³⁺co-doped system.The absolute quantum efficiency was investigated. The integrating spheredetection system was set up to quantitatively measure the absolute quantumefficiency of the near infrared quantum cutting process in Tb³⁺-Yb³⁺co-dopedoxyfluoride glass. The absolute quantum efficiency did not change with the pumppower. The visible and near infrared quantum efficiencies were on the order of10-2,and the total quantum efficiency was less than100%. As the Yb³⁺concentrationraised, the near infrared quantum efficiency exhibited exponential growth with alimit of13.5%, and the visible and total quantum efficiencies monotonicallydecreased. The drawbacks in the estimation method of theoretical quantumefficiency were discussed. The low radiative efficiency （<61%） of Tb³⁺andsignificant concentration quenching from Yb³⁺make the theoretical quantumefficiency give an upper limit.The upconversion luminescence mechanism and absolute upconversionefficiency of Tb³⁺-Yb³⁺co-doped oxyfluoride glass were investigated. Therelationship between the time-resolved spectra of Tb³⁺and Yb³⁺were contradictoryto the generally accepted cooperative sensitization upconversion rate equationmodel. Since the host environmental effect was excluded by comparativeexperiments, we believe that there should be another energy transfer mechanism inTb³⁺-Yb³⁺co-doped oxyfluoride glass besides the cooperative sensitizationupconversion process. The upconversion quantum and power conversion efficiencieswere investigated both theoretically and experimentally. The relative lower Yb³⁺doping concentration and higher pump power density are beneficial for theimprovement of the upconversion efficiency. The ratio of the quantum efficiency topower conversion efficiency is only related to photon frequency and fluorescencebranching ratio, thus, it is a constant and independent of the pump power densityand Yb³⁺concentration. The absolute upconversion emission efficiency ofTb³⁺-Yb³⁺codoped glass was determined to be in the order of10-4in ourmeasurement range （40-96W/cm2）.