| Popis: |
The investigation of the production of visible luminescence from infrared radiation in the systems LnF 3 : Yb 3+ , R.E. 3+ , where Ln is La, Y, Gd, or Lu, and R.E. is Er, Ho or Tm, revealed that the excitation process involves absorption by Yb 3+ ions followed by two or more consecutive energy transfers to R.E. 3+ , and is not achieved by cooperative sensitization. The dependence of luminescence intensity on excitation intensity and the dependence of luminescence on sensitizer concentration are given as evidence. The steady state solutions for resonant and nonresonant two photon excitation are compared and it is shown that highest efficiency will result for activator absorption at slightly less energy than sensitizer emission, so that energy transfer from activator to sensitizer does not occur. The energy efficiency for LaF 3 : Yb 0.12 Er 0.02 , which has two resonant transfers, and GdF 3 : Yb 0.4 Tm 0.0015 which has at least one phonon assisted transfer, were measured. The quantum efficiency for the Tm 3+ material was found to be 1.8% at 0.36 mW cm -2 nm -1 , 80 times that of the Er 3+ material. The intensity dependence of the green Er 3+ emission was quadratic in agreement with the theoretical prediction, while that of the 0.8 μ Tm 3+ emission was subquadratic, indicating ground state depletion. Yb 3+ lifetimes were measured and used to obtain the energy transfer coefficients for the nonresonant transfer. Estimates of lifetimes from the literature and measured parameters were used to calculate the emission rate ratio of the Tm 3+ material to the Er 3+ material and agreement was satisfactory. |
