New Yb-doped fluoride phosphate laser glass-structural investigations using probe ions

JOURNAL OF

LUMINESCENC Journal

ELSEVIER

of Luminescence

72-74

(1997) 449-450

New Yb-doped fluoride phosphate laser glass-structural investigations using probe ions W. Seeber*, St. Barth, F. Seifert, H. Ebendorff-Heidepriem, Universitiit Jena, Otto-Schott-Institut,

Fraunhojerstr.

D. Ehrt

6. D-07743 Jena, Germany

Abstract

Various probe ions have been employed to improve the performance of Yb-doped laser glasses. Lanthanide ions similar to the Yb3’, e.g. Nd3+, Eu3+ and Gd3+, are sensitive to their site surroundings and that is why they are useful for obtaining the structural information needed to tailor the laser host matrix. A new promising laser glass based on these investigations has been developed. We assume that the favorable laser properties of this glass can be explained by formation of [Yb(PO&]-clusters which are nearly isolated from the main part of the network. The electronphonon coupling in optimized fluoride phosphate glasses with tetrahedral anions may efficiently depopulate the terminal laser level. Keywords:

Local structure; Yb(PO,),-cluster; Yb-laser glass

1. Introduction

2. Concept of optimized Yb local structure

Recently, rare-earth-doped fluoride phosphate (FP)-glasses are again under investigation because of their advantageous combination of properties: e.g. less-stringent manufacturing requirements compared to fluoride glasses, low refractive indices, low dispersion together with low non-linear refractive indices. Additionally, it has been shown that Yb-doped FP-glasses have the potential for cheap diode pumped tunable room temperature cw-lasers acting at wavelengths around 1.04 pm [l]. The tailoring of the Yb host matrix to improve the laser performance needs information about the Yb local structure itself.

Similar to investigations by DeLoach et al. using apatite crystals [2], we trace the surprisingly good laser parameters of Yb:FP-glasses to the special energy level structure of the Yb:2F,,2 ground manifold in these fluoride phosphate materials. The laser transition terminates at an electronic Yb-level (about 600 cm- ’ above the ground state) interacting with the local vibrational modes of localized Yb-0 bonds. The huge variety of FP-glass compositions provides the basis to tailor the local structure at the sites of the Yb3+ ions and in this way the electron-phonon interactions and the laser performance itself. Details of the preparation of the Yb-doped FPglasses have been published elsewhere [3]. We used Nd3 ‘, Eu3 + and Gd3+ as probe ions incorporated into the FP-glasses to investigate relevant rare earth ion sites of the amorphous matrix [4]. The

*Corresponding author. Fax: (+ +49-3641)

636172;

e-mail: [email protected].

0022-2313/97/$17.00 c 1997 Elsevier Science B.V. All rights reserved PII SOO22-23 13(96)00389-4

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W. Seeber et al. /Journal

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72-74 (1997) 449-450

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Fig. 1. Phonon sideband spectroscopy using Eu 3+-doped FP-glass with 20 mol% phosphate content. Centre and right: Eu3+ energy level diagram indicating relevant transitions. Left: absorption and excitation spectra.

Nd3+:41 9,2 + 2P1j2 absorption transition (at about 435 nm) has been employed to sense the strength of the crystal field in the vicinity of the laser ion. The maximum Stark splitting AE(41g,2)can be linked to the scalar crystal field strength parameter IV,,and gives information about the emission quenching behavior [S]. The Eu3+ : 5Do + 7F2 and Gd3+: 6P,,2 -+ 8S7,z emission transitions at 612 and 315 nm, respectively, are useful for the determination of electron-phonon interactions between rare earth ions and surrounding network components. Additional bands in the measured spectra indicate coupling mechanisms and provide insights into the dynamical properties of the local rare earth site. Fig. 1 shows the excitation spectrum of a Eu3+doped FP-glass (80mol% Z(A1F3, alkaline earth fluorides)-20 mol% phosphates) together with the ground-state absorption curve at 300 K and a schematic energy level diagram to symbolize the origin of the detected phonon sidebands (PSB’s). The PSB at about lOOOcm_’ scan be associated with PO vibrations (v3) of tetrahedral PO, groups in the vicinity of the active rare earth ion. Similar results

have been derived from Raman spectra of these FP-glasses and support the [Yb(PO,),]-cluster model.

Acknowledgements

The work has been funded by the DFG, contract number Se 698/1-l/3.

References

Cl1 E. Mix, E. Heumann, G. Huber, D. Ehrt and W. Seeber, Adv. Solid-State Lasers, Memphis, TN, Tech. Dig., WB5-1 (1995) p. 230. VI L.D. DeLoach, S.A. Payne, W.L. Kway, J.B. Tassano, S.N. Dixit and W.F. Krupke, J. Lumin. 62 (1994) 85. Habilitationsschrift, Universittit Jena, Seeber, c31 W. Chemisch-Geowissenschaftliche FakultPt (1995). [41 W. Seeber and D. Ehrt, Ber. Bunsenges. Phys. Chem. 100 (1996), to be published. c51 F. Auzel, J. Chavignon, D. Meichenin and M. Polain, J. Non-Cryst. Solids 161 (1993) 109 and references therein.