Method for producing large, stable concentrations of Sc 2+ in optically clear CaF 2 crystals
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Method for producing large, stable concentrations of Sc2+ in optically clear CaF2 crystals C. L. Marquardt, J. F. Pinto, R. E. Allen, and L. Esterowitz Naval Research Laboratory, Washington, DC 20375
A. Yu Dergachev, S. Ke, and S. B. Mirov University of Alabama at Birmingham, Birmingham, Alabama 35294 (Received 9 May 1997; accepted 16 July 1997)
This communication describes a new method for producing stable, high concentrations of Sc21 in optically clear CaF2 crystals. We have achieved Sc21 concentrations as high as 3 3 1018 cm23 without degradation of optical quality. We have converted as much as 5% of the scandium dopant to the divalent state. The concentration of divalent scandium is stable during room temperature storage for periods of at least one year.
Divalent scandium (Sc21 ) in crystals such as CaF2 has recently generated interest as a potential solid state laser material with broad tunability in the near ultraviolet spectral region.1,2 This expectation is based primarily on the analogy with Ti31 which has the same electronic configuration (3d1 ) as Sc21 . Ti : sapphire is the most important new solid state laser material since Nd : YAG. It is broadly tunable in the near infrared. Spectral features of divalent scandium are expected to be similar to those of Ti : sapphire, but substantially blue shifted. This should permit tunable laser operation in the 350–450 nm spectral region. Various optical properties have been attributed to Sc21 : CaF2 in previous reports.3–5 However, many of those properties have subsequently been shown1,2 to arise from species other than Sc21 . At the present time, unambiguous identification of Sc21 in CaF2 can be obtained only from cryogenic Electron Paramagnetic Resonance (EPR) measurements. Three previous studies have used EPR to confirm the presence of Sc21 in CaF2 crystals which had been doped with scandium and then exposed to ionizing radiation.6–9 In the early work7–9 the scandium doping was accomplished by evaporating a film of scandium metal onto a pure CaF2 crystal surface, and then diffusing it into the bulk at an elevated temperature prior to irradiation. This process is not suitable for production of laserquality optical material. It severely limits the ultimate concentration of Sc21 and introduces permanent metal colloids which degrade the optical transmission throughout the spectrum. Recent investigators1,2,6 have doped the pure CaF2 starting material with ScF3 prior to crystal growth. Trivalent doping levels of 1–2% have been achieved without degrading the optical quality of the crystals. After a subsequent gamma irradiation of 2 3 105 rad, Sc21 concentrations as high as 8 3 1016 cm23 have been detected using EPR, and the crystals have still retained their high optical quality. Nonetheless, all J. Mater. Res., Vol. 13, No. 2, Feb 1998
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methods which employ irradiation suffer from unavoidable drawbacks which make them less than optimal for production of laser mater
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