The Investigation On The Upconversion Excitation Processes With A Square Wave Laser

The study on upconversion luminescence of rare earth ions is important for the applications in the areas such as high-density optical storage,infrared detection, laser anti-forgery,biomarkers,short-wavelength solid-state lasers,three-dimensional display,and the medical diagnostics.Over the past four decades,up conversion luminescence of various crystals and glasses doped with rare-earth ions was widely studied.Research works recently still focus on looking for the best upconversion materials and on understanding the upconversion excitation mechanism in depth.There are usually many possible transition routes that might be listed by the analysis based only on the energy match and the excitation intensity dependence of the upconversion emission,because of rich levels in the 4f^N configuration of the rare earth ions.The purpose of this thesis is to show how the measurements of the upconversion dynamics can help to clarify the upconversion excitation mechanism.The thesis is organized in the following 4 sections:1.A square wave light source was built.Square wave light with tunable repetition rate was obtained from a laser diode driven by a square wave power supply,which consisted from four parts:multi-harmonic oscillator,frequency divider,preamplifier and driver.2.Temporal behaviors of the green and red upconversion emissions in Er³⁺ doped oxyfluoride tellurite glass were measured with the square wave lights excitation from 808 nm and 980 nm laser diodes.Rate equations were established to describe the dynamical processes of the upconversion by excited state absorption.Through analyzing the rise and decay curves of the up conversion emissions,the intermediate states,consequently the excitation routs of the up conversion have been determined.The results clearly indicated that the green upconvertions under 980 nm and 808 nm excitations were with different intermediate states.Saturation of the green up conversion emission under the excitation of 808 nm was studied.By fitting the power dependence deduced from the steady state rate equations with the experimental results, it was concluded that the saturation of the upconversion emission was originated from the saturation of excited state and ground state absorptions.3.Rise and decay curves of the upconversion emissions from ²H_11/2+⁴S_3/2 and ⁴F_9/2 states of Er³⁺ in Er³⁺ doped and Er³⁺,Yb³⁺ codoped oxyfluoride tellurite glass excited by 980 nm square wave laser were measured and compared.Effects of the energy transfer by sensitizer,Yb³⁺,on the rise and decay behaviors were analyzed.Rate equations were established for the upconversions originated from both the stepwise energy transfer and the excited state absorption processes.Contributions from both processes were determined by analyzing the rise curves of the upconversions in Er³⁺-doped and Er³⁺-Yb³⁺-codoped samples.Upconversion excitation routs in the studied samples were then clarified.4.Rate equations of the upconversions in Er-doped and Er,Yb-codoped systems without the limitations of weak excitation and weak energy transfer were solved numerically.Excitation state absorption,energy transfer from Yb and cross relaxation between two excited Er³⁺ ions were considered.Fitting with the experimental results, the conclusions obtained from weak excitation approximation in the previous chapter were further confirmed.By varying the parameters,conditions for each upconversion excitation mechanism being dominant and the its reflections in decay and rise curves were determined.