Population of higher-energy levels in LiY_(1-x)Er_xF_4 (x=O.003 ÷1) crystals under CW IR laser-diode pumping

Steady-state population of 7 lowest excited erbium. levels in LiY¬¬_(1-x)Er_xF_4 (YLF:Er^(3+) (x=0.003-1) crystals was studied under upconversion. CW InGaAs laser-diode pumping with varied power density. Theoretical and experimental concentration and power dependencies of population of higher-energy radiative levels were obtained. Relative changes in populations of studied levels in YLF:Er^(3+) crystals were experimentally controlled by visible spectra of steady-state luminescence in the wavelength ranges corresponding to transitions ^4S_(3/2) --> ^4I(15/2) (0.52--0.57) mum and ^4 F_(9/2) --> ^I_(15/2) (0.64--0.68) µm. IR-pumped luminescence kinetic curves of higher-energy transitions ^4S_(3/2) --> ^4I_(15/2) (0.55 µm) and ^2H_(9/2) --> ^4I_(15/2) (0.41 µm) were recorded. The energy-transfer mechanisms were determined, and the predominant mechanisms responsible for upconversion excitation were elucidated. Microparameters of energy transfer and concentration dependencies of the selfquenching rates and non-linear coupling were obtained on the basis of theoretical and experimental estimates of the rates of intra- and intercenter relaxation processes (migration, selfquenching, and upconversion) allowing for statistics of coupling between the impurity centers in the system. The steady-state dependencies of population on the erbium concentration and pumping power density were calculated within the framework of rate balance equations. Good agreement between the theory and experimental data was obtained