M. Pollnau, D. R. Gamelin,
S. R. Lüthi, H. U. Güdel and M. P. Hehlen
Power dependence of upconversion luminescence in lanthanide
and transition-metal-ion systems
Phys. Rev. B 61, 3337-3346 (2000)
Abstract:
We show theoretically with the simplest possible model that the
intensity of an upconversion luminescence that is excited by the
sequential absorption of n photons has a dependence on
absorbed pump power P, which may range from the limit of
Pn down to the limit of P1
for the upper state and less than P1 for the
intermediate states. The two limits are identified as the cases of
infinitely small and infinitely large upconversion rates,
respectively. In the latter case, the dependence of luminescence
intensities from intermediate excited states on pump power changes
with the underlying upconversion and decay mechanisms. In certain
situations, energy-transfer upconversion and excited-state absorption can be
distinguished by the measured slopes. The competition between linear
decay and upconversion in the individual excitation steps of
sequential upconversion can be analyzed. The influence of nonuniform
distributions of absorbed pump power or of a subset of ions
participating in energy-transfer upconversion is investigated. These
results are of importance for the interpretation of excitation
mechanisms of luminescent and laser materials. We verify our
theoretical results by experimental examples of multiphoton-excited
luminescence in Cs3Lu2Cl9:Er3+,
Ba2YCl7:Er3+,
LiYF4:Nd3+, and Cs2ZrCl6:Re4+.