Advances in the Development of Quantum Splitting Phosphors
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ADVANCES IN THE DEVELOPMENT OF QUANTUM SPLITTING PHOSPHORS A.A. Setlur1, H.A. Comanzo1, A.M. Srivastava1, W.W. Beers2, W. Jia3, S. Huang4, L. Lu4, X. Wang4, and W.M. Yen4 1 GE Corporate Research and Development, Niskayuna, NY 12309 2 GE Lighting, Cleveland, OH 44110 3 Department of Physics and Astronomy, University of Puerto Rico, Mayaguez, PR 00681 4 Department of Physics and Astronomy, University of Georgia, Athens, GA 30602 ABSTRACT Quantum splitting phosphors (QSPs) are phosphors that could convert VUV radiation into more than one visible photon. These phosphors have the potential to improve the efficacy of current Hg fluorescent lamps and/or Xe lamps by reducing the Stokes shift energy loss after VUV excitation (λexc=185 nm for Hg lamps or 147 nm and 172 nm for Xe lamps provided the emission color of the phosphor matches the eye sensitivity. The current technology in QSPs and their potential limitations will be discussed in this paper. At GE-CRD, we have discovered and developed QSPs that meet the requirements for use in current Hg based fluorescent lamps. The steady state and time resolved optical properties of one of these phosphors, SrAl12O19:Pr3+,Mg2+, has been measured to estimate the maximum quantum efficiency and onset of concentration quenching in this phosphor. The maximum quantum efficiency for SrAl12O19:Pr3+,Mg2+ has been calculated to be ~125-135% for 185 nm excitation with an upper bound on the Pr3+ doping level of ~1%. INTRODUCTION Phosphors which have quantum efficiencies for visible light greater than unity are well known for cathode ray or x-ray excitation. However, the efficiency of phosphors that convert ultraviolet (UV) or vacuum ultraviolet (VUV) light into visible light is typically less than unity. If phosphors are developed which emit more than one photon for each incident UV or VUV photon absorbed, a quantum splitting phosphor (QSP), this could significantly improve the efficacy of current Hg or Xe based fluorescent lamps. The current focus of QSP research is develop visible quantum splitting phosphors to minimize energy loss upon VUV excitation, either from the 185 nm line from Hg lamps or the 172 nm/147 nm lines from Xe lamps. One potential route to developing QSPs is to use energy transfer and cross-relaxation between various rare earth ions for the emission of two visible photons after VUV excitation. This has been clearly demonstrated in LiGdF4:Eu3+ which potentially has a quantum efficiency of 190% upon excitation into the 6GJ levels of Gd3+ at ~200 nm (Figure 1) [1]. After VUV excitation, the 6GJ→6PJ transition is resonant with the 7F1→5D0 transition, leading to crossrelaxation and energy transfer to a Eu3+ ion. The Eu3+ ion then emits a orange-red photon, the first photon in this scheme. After the cross-relaxation between Gd3+ and Eu3+, the 6PJ level of Gd3+ is populated and this energy can then migrate along the Gd3+ sub-lattice until it reaches a Eu3+ ion, giving the second visible photon. Other quantum splitting schemes involving Er3+Gd3+-Tb3+ in LiGdF4 also have been demonst
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