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Soviet Journal of Quantum Electronics

Determination of the probability of relaxation of the population of the4I11/2level of neodymium ions from luminescence dumping in a small external sample

V V Grigor'yants, M. E. Zhabotinskiǐ, Valery M. Markushev

Published Aug 31, 1982• 6 citations

Abstract

An improved method was developed for determining the probability W21 of relaxation of the population of the 4I11/2 level of neodymium ions. The relationship between W21 degree of dumping, and parameters of the dumping pulses was obtained by solving the rate equations for the populations subject to an allowance for the Stark structure of the energy levels 4F3/2 and 4I11/2. At T=300°K the probability values were W21=108 sec−1 for Y3Al5O12:Nd3+ and W21=1.5×108 sec−1 for La2O2S:Nd3+. In the case of a BaGd2-xNdx(MoO4)4 crystal at 77°K the probability was W21=2.5×108 sec−1. The influence of Wn on the efficiency of utilization of the stored energy was found to depend on the energy gap between the lower Stark sublevel of the 4I11/2 level and the final sublevel in the stimulated transitions.

DOI: 10.1070/qe1982v012n08abeh005765

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An improved method was developed for determining the probability W21 of relaxation of the population of the 4I11/2 level of neodymium ions. The relationship between W21 degree of dumping, and parameters of the dumping pulses was obtained by solving the rate equations for the populations subject to an allowance for the Stark structure of the energy levels 4F3/2 and 4I11/2. At T=300°K the probability values were W21=108 sec−1 for Y3Al5O12:Nd3+ and W21=1.5×108 sec−1 for La2O2S:Nd3+. In the case of a BaGd2-xNdx(MoO4)4 crystal at 77°K the probability was W21=2.5×108 sec−1. The influence of Wn on the efficiency of utilization of the stored energy was found to depend on the energy gap between the lower Stark sublevel of the 4I11/2 level and the final sublevel in the stimulated transitions.

An experimental method was developed for the determination of the rate of relaxation of the 4I11/2 state and of the absorption cross section of the 4I11/2–4F3/2 transition. These parameters were deduced from the dependences of the intensity of the λ = 0.88 μ luminescence, excited as a result of the 4I11/2–4F3/2 transition by laser pulses of λ = 1.06 μ wavelength, on the energy density and duration of the laser pulses. Estimates of the relaxation time obtained for the investigated silicate (GLS-1) and phosphate (LGS-40, LGS-I) glasses, and also for a liquid based on POCl3-SnCl4, gave τ21 < 2 nsec. A considerable increase in the population of the 4I11/2 level occurred as a result of the Raman scattering of high-power coherent radiation by vibrations of the PO4 group.

A study was made of various cases of a population inversion between the 4I11/2 and 4I13/2 levels in Y3Al5O12:Er3+ crystals. It is found that when nonradiative transitions from the upper levels to the active levels by the mechanism of multiphonon energy transfer are more probable, laser action occurs due to transitions between the Stark components of the 4I11/2 and 4I13/2 levels with the highest oscillator strengths. If nonradiative transitions due to ion-ion interaction are more probable, a red shift is observed in the lasing spectra.

Studies of the spectra of stimulated emission from Y 3 AI 5 O 12 :Er 3+ crystals ( 4 I 11/2 – 4 I 13/2 ) containing 20 wt. % activator ions, showed that, as the temperature decreased from 300 to 77°K, a change was observed in the emission wavelength (at T = 300°K it was found that λ 1 = 2.9364 μ λ 2 = 2.8310 μ and at T = 77°K it was found that λ 1 = 2.6930 μ , λ 2 = 2.8970 μ ). This could not be attributed to a thermal redistribution of the populations of the Stark components of the 4 I 11/2 and 4 I 13/2 upper and lower active levels. An analysis of the results showed that the temperature shift of the stimulated emission spectrum was due to changes in the processes of relaxation of excitation from the upper levels of the Er 3+ ion which caused substantial changes in the populations of the 4 I 11/2 and 4 I 13/2 multiplets.

The population kinetics of laser levels of neodymium ions in the uranium-containing POCl 3 —SnCl 4 — 235 UO 2 2+ —Nd 3+ laser liquid irradiated in a BARS-6 pulsed reactor is studied experimentally and theoretically. The relations between the populations of the upper and lower laser levels are studied at different time moments with respect to the pump pulse. An inverse population of laser levels is observed and found to be directly proportional to the pump power. The efficiency of pumping of the upper laser level of neodymium ions in the laser medium by uranium fission fragments is determined, and quantitative information is obtained on the inverse population, gain, and their dependences on the pump power.