Pulsed periodic laser excitation of upconversion luminescence for deep biotissue visualization

Emission spectral properties and quantum efficiency of upconversion particles NaYF 4 , SrF 2 , LaF 3 , BaF 2 и CaF 2 , doped with rare earth ions pair Yb 3+ –Er 3+ were studied using continuous wave (CW) and pulsed periodic excitation modes in the near infrared (NIR) spectral range. Analysis of the obtained results showed that the intensity ratio of upconversion luminescence in green and red spectral ranges depends on excitation pulse duration. Thus, by changing the pulse duration the spectral properties of upconversion luminescence can be controlled. Crystals with higher phonon energy are more sensitive to the change of pumping mode. Interpretation of results was performed on the rate equation model basis. Using numerical methods for all energy levels involved in the upconversion process the population and depopulation dynamics were obtained with respect to the duration of the excitation pulses. It was shown that about 30 ms was required for the complete population of 4 F 9/2 state, from which the luminescence in the red spectral range occurs. When the pulse duration was less than 30 ms, the 4 F 9/2 population did not reach a steady state and the intensity of the luminescence in the red part of the spectrum was reduced. The theoretical dependence of the upconversion luminescence intensity in the green and red ranges of the excitation pulse duration for NaYF 4 :Yb 0.2 –Er 0.02 composition was obtained and demonstrates good agreement with the experimental results.