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MATERIALS RESEARCH

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Czochralski growth and spectroscopic investigations of Yb3+, La3+:Na2SO4(I) and Nd3+:Na2SO4(I) Patric Mikhail, Reto Basler, and Ju¨rg Hulligera) Department of Chemistry and Biochemistry, University of Berne, Freiestrasse 3, CH-3012 Berne, Switzerland (Received 18 March 1999; accepted 27 June 1999)

Ln3+-stabilized Na2SO4 (phase I) single crystals were grown by the Czochralski method. Differential thermal analysis revealed the influence of the ionic radius of Ln3+ on the stabilization of Na2SO4(I). Distribution coefficients (∼0.8–1.1) were measured by the inductively coupled plasma optical emission spectroscopy method and x-ray fluorescence spectroscopy. Spectroscopic investigations yielded absorption cross sections of 0.6 × 10−20 cm2 (␲-polarized, 928.5 nm) and 1.5 × 10−20 cm2 (␲-polarized, 797.3 nm) for Yb3+, La3+:Na2SO4 and Nd3+:Na2SO4, respectively. Crystal growth of Gd3+-stabilized Na2SO4(I) provides an interesting new material for stimulated Raman scattering experiments.

I. INTRODUCTION

The first Yb3+-based solid-state lasers were demonstrated a long time ago. 1 In Yb 3+ there are only two manifolds, the ground 2F7/2 state and an excited 2 F 5/2 state, which are separated by approximately 10,000 cm−1 (1 ␮m), which is also the typical laser wavelength of Yb3+ lasers. A narrow absorption band has made it inefficient to pump Yb3+-doped crystals with broad band flashlamps, which were the only pump sources available in the 1960s and 1970s. The recent development of InGaAs laser diodes, showing narrow emission bands that almost match the absorption band of Yb3+, led to an accessible pump source. The simple electronic structure of Yb3+ may therefore provide an advantage over other widely used activator ions such as Nd3+, because two accessible electronic states preclude both upconversion and excited state absorption. Additionally, a small quantum defect can lead to high slope efficiencies2 and low heat generation.3 The emission lifetime of Yb3+ is larger than for Nd3+ by a factor of 3–4, if comparison is made for the same host.4,5 Yb3+ lasers are expected to be quasi-four-level systems (300 K). The energy difference ⌬E (around 200–600 cm−1) is given by the terminal laser level above the ground state. Such an energy difference is comparable to the thermal energy present at room temperature (kT ∼200 cm−1): hence, the smaller the crystal field splitting, the more the terminal laser level is populated and therefore inversion population is more difficult to obtain.

a)

Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 14, No. 10, Oct 1999

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With respect to new host lattices bearing a potential for laser applications, some new materials—e.g., M-fluorapatite [M5(PO4)3F; M ⳱ Ca, Sr; fluorapatite (FAP), strontium fluorapatite (S-FAP), respectively]6–9 and Y– or Gd–calcium oxyborate [MCa4O(BO3)3; M ⳱ Y, Gd]10–12 have recently attracted interest. Taking into account the potenti