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British Journal of Applied Physics

Some aspects of infra-red quantum counter action in Er-doped SrF 2

M. R. Brown, W. A. Shand, J.S.S. Whiting

Published May 1, 1965• 22 citations
Publisher

Abstract

Measurements are presented of the optical properties and strength of quantum counter action in Er3+-doped SrF2 which demonstrate the validity of a simple rate equation theory of quantum counter action. This analysis is used to assess the quantum counter as a potential narrow band infra-red detector. An extension of the theory to the case of modulated input radiation is shown to provide a method for measuring lifetimes of excited states which may not be accessible by conventional techniques, e.g. non-fluorescent levels. Some results obtained by this method are reported.

DOI
10.1088/0508-3443/16/5/304

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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 .

Journal of Physics C Solid State Physics•Nov 1, 1969•3 citations
Measurements are presented of the optical properties and strength of quantum counter action in Er3+-doped SrF2 which demonstrate the validity of a simple rate equation theory of quantum counter action. This analysis is used to assess the quantum counter as a potential narrow band infra-red detector. An extension of the theory to the case of modulated input radiation is shown to provide a method for measuring lifetimes of excited states which may not be accessible by conventional techniques, e.g. non-fluorescent levels. Some results obtained by this method are reported.
The intensities of optical absorption spectra of Er 3+ in single crystals of yttrium gallium garnet at 700 °K have been shown to fit Judd's theory of 4f-4f transitions with the following parameters: T 2 = 1·44 × 10 -21 s, T 4 = 1·12 × 10 -21 s, T 6 = 1·42 × 10 -21 s. These have been used to estimate Einstein coefficients useful in a discussion of the quantum efficiency of the fluorescence from the 4 S 3/2 state, and the efficiency of infra-red quantum counter action in the material. For pure spontaneous radiation an efficiency of 71% for transitions to the 4 I 15/2 ground state is expected. The quantum efficiency has also been directly measured and a value of 12% was obtained. This is lower owing to competing phonon processes. Using the experimental value for the quantum efficiency, and the above values of T , a theoretical photon gain for quantum counter action of 0·55 × 10 -8 has been calculated.
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)
We report on a scheme for particle detection based on the infrared quantum counter concept. Its operation consists of a two-step excitation process of a four level system, which can be realized in rare earth-doped crystals when a cw pump laser is tuned to the transition from the second to the fourth level. The incident particle raises the atoms of the active material into a low lying, metastable energy state, triggering the absorption of the pump laser to a higher level. Following a rapid non-radiative decay to a fluorescent level, an optical signal is observed with a conventional detector. In order to demonstrate the feasibility of such a scheme, we have investigated the emission from the fluorescent level 4S3∕2 (540 nm band) in an Er3+-doped YAG crystal pumped by a tunable titanium sapphire laser when it is irradiated with 60 keV electrons delivered by an electron gun. We have obtained a clear signature that this excitation increases the 4I13∕2 metastable level population that can efficiently be exploited to generate a detectable optical signal.
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.
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.
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 .