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Journal of Physics C Solid State Physics

Quantum counter action in Er3+ : Yb3+ doped LiYF4

M R Brown, W. A. Shand

Published Nov 1, 1969• 3 citations
Publisher

Abstract

This paper considers the mechanisms whereby the addition of Yb 3+ enhances the efficiency of quantum counter action in Er 3+ doped LiYF 4 . The particular scheme chosen for this investigation is the doubling scheme in which two photons of wavelength close to 9800 Å excite green fluorescence from the Er 3+ ion. The important factors which have been identified are that the influence of an increase of Yb 3+ concentration on the quantum efficiency of the 4 S 3/2 to 4 I 15/2 fluorescence is very much less than an equivalent increase in the Er 3+ concentration, the absorption coefficient of the Yb 3+ in the 9800 Å region is over an order of magnitude larger than that for Er 3+ and the efficiency of the transfer of energy from Yb 3+ to Er 3+ is sufficiently high to ensure the energy absorbed by the Yb 3+ is transferred to the Er 3+ . The optimum concentration has been found to be in the region of 2% Er 3+ : 18% Yb 3+ doped LiYF 4 .

DOI
10.1088/0022-3719/2/11/303

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Physical Review B•Apr 21, 2006•72 citations
This paper considers the mechanisms whereby the addition of Yb 3+ enhances the efficiency of quantum counter action in Er 3+ doped LiYF 4 . The particular scheme chosen for this investigation is the doubling scheme in which two photons of wavelength close to 9800 Å excite green fluorescence from the Er 3+ ion. The important factors which have been identified are that the influence of an increase of Yb 3+ concentration on the quantum efficiency of the 4 S 3/2 to 4 I 15/2 fluorescence is very much less than an equivalent increase in the Er 3+ concentration, the absorption coefficient of the Yb 3+ in the 9800 Å region is over an order of magnitude larger than that for Er 3+ and the efficiency of the transfer of energy from Yb 3+ to Er 3+ is sufficiently high to ensure the energy absorbed by the Yb 3+ is transferred to the Er 3+ . The optimum concentration has been found to be in the region of 2% Er 3+ : 18% Yb 3+ doped LiYF 4 .
Abstract : Previous work on the infrared quantum counter action in Er3+ doped SrF2 showed that it was possible to increase the efficiency of the action by codoping with a second rare earth ion. This report covers the work done on the Er3+ doped LiYF4 and Er3+: Yb3+ doped LiYF4 system to evaluate the energy levels of the Er3+ in a site that does not require charge compensation and the increase of efficiency that comes with codoping with Yb3+. The results show that the positions of the energy levels are substantially unchanged in moving from one lattice to another and an increase in efficiency of the action, of the order, forty times can be achieved with a composition in the region of 2% Er3+: 18% Yb3+ doped LiYF4. (Author)
The pumping intensity and concentration dependences of the fluorescence output from three infrared quantum-counter schemes that involve coupled ions in Er3+-doped SrF2 have been measured at 300 and 77°K. The pumping intensity depended linearly on the numbers of photons absorbed in the schemes. The power of the concentration dependence was found to be noninteger in the range 1%–10%. This has been interpreted as indicating the importance of energy migration between the Er3+ ions as mechanism for populating the energy levels of the coupled ions.
Here we describe a class of cw upconversion lasers in which inversion is due exclusively to cooperative interactions of rare earth ions in highly doped solids, resulting in unique opportunities for output wavelength agility, large upconversion factors, and high quantum efficiencies through nonlinear dynamics. We have isolated capabilities of the cooperative mechanism by performing experiments in Er:CaF2 and Er:LiYF4 with infrared pumping of the first Er3+ resonance at 1.53 μm. At liquid helium temperatures, laser action at 544 nm was achieved in an astigmatically compensated, three-mirror cavity with 5% efficiency in LiYF4. The upconversion factor (ratio of pump to laser wavelength) was 2.81, to our knowledge the highest ever reported for cw upconversion lasers. At 77 K, laser action at 551 nm was observed. Surprisingly, output at both 544 and 551 nm was truly cw with cooperative pumping, unlike earlier green erbium lasers, which exhibited self-pulsing. Also at 77 K, near infrared laser action was obtained at 0.855 pm with exceptionally high efficiency in CaF2(28%) and LiYF4 (20%) on self-quenching transitions. Successful demonstration of cw laser action on such transitions also suggests a means whereby quantum efficiencies may exceed unity. The lower laser level in such cases is the origin of cooperative pumping to the upper laser level, but also receives population as the result of stimulated emission. Hence population can be recycled for enhanced quantum efficiency. Additionally, population pulsations can occur and sustained oscillations in laser output have been observed in a cooperative erbium upconversion laser at 2.8 μm.
This paper reviews the results of a program of experimental work on infrared quantum counter action in trivalent rare earth doped CaF2, SrF2, BaF2, and LaF3lattices. The action has been found in eight of the nine possible ions that could give an output in the visible region. While the strengths of the various schemes at a 1 percent doping level are consistant with a low concentration theory, the strengths of the schemes at a 10 percent doping level indicate that there are ion-ion interactions influencing the systems. These ion-ion couplings are so effective that three photon addition schemes have been identified.
The enhanced laser cooling performance of rare-earth-ion-doped nanocrystalline powders is predicted, compared to the bulk material, using Yb3+:Y2O3{\mathrm{Yb}}^{3+}:{\mathrm{Y}}_{2}{\mathrm{O}}_{3}Yb3+:Y2​O3​ as the model material. This is achieved by enhancing the off-resonance, phonon-assisted absorption, which is proportional to the three factors considered in this paper: the dopant concentration, the pumping field energy, and the excitation coefficient. Using the energy transfer theory for concentration quenching, the optimum concentration corresponding to the maximum cooling power is found to be considerably larger than the currently used value, suggesting noticeable enhancement effects for laser cooling. The pumping field energy is enhanced in random nanopowders compared with bulk crystals under the same irradiation, due to the multiple scattering of photons. Photons are thus localized in the medium and do not propagate through, increasing the photon absorption of the pumping beam (and it is shown that the reabsorption of the fluorescence is negligible). Using molecular dynamics simulations, the phonon density of states (DOS) of the nanopowder is calculated, and found to have broadened modes, and extended small tails at low and high frequencies. The second-order electronic transition rate for the anti-Stokes luminescence is calculated using the Fermi golden rule, which includes the influence of this phonon DOS, and is shown to have enhancement effects on the laser cooling efficiency using nanopowders. It is finally concluded that these three enhancement mechanisms are essentially to increase the population of the three participating carriers (electron, photon, and phonon) in the interacting volume, and this also points out directions for enhancing laser cooling performance in bulk materials.