Photonic Applications of Rare-Earth-Doped Materials
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nd industry are equally fascinating and have remained important to this day. Commercial applications of RE elements began after World War II, when their available quantity and purity were greatly enhanced by improved Separation techniques developed as a part of the Manhattan Project. Until fairly recently, the main industrial application of RE ele ments has been in permanent magnets. The unpaired 4/electrons result in some RE elements having the highest magnetic moments of any element. The development and applications of RE magnets are reviewed in a very interesting article by Livingston 3 in a previous MRS Bulletin issue. In this issue of MRS Bulletin, we have taken as our aim to review some of the properties and applications of RE ele ments relevant to photonics. A simple but useful definition of pho tonics is the technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The ränge of photonics appli cations extends from energy generation and detection to Communications and information-storage processing. The basic mechanisms through which RE ele ments play a role in photonics involve excitation and luminescence between the energy levels of RE 3+ ions with partially filled 4/Shells from Ce 3+ to Yb 3+ . Photonic applications of RE elements discussed in this issue include solid-state lasers for ultraviolet (UV) and visible wavelengths, the prospects for lasers in semiconducting materials (primarily Si), visible and infrared (1R) light-emitting devices in the wide-bandgap semiconductor GaN, RE-doped glass fibers for telecommunications, optical data storage using RE-
doped crystals, and the incorporation of RE elements into a variety of host mate rials for achieving visible displays. As Moncorge, Merkle, and Zandi State in the introduction to their article: "An issue on novel applications of materials doped with rare-earth ions can scarcely fail to address l a s e r s . . . . " Their article concentrates on the possibilities for REbased solid-state lasers for UV and vis ible wavelengths rather than the near-IR region where significant successes have already been established with Nd 3 + as well as Er and Tm lasers. They discuss a variety of approaches that are being explored: up-conversion or frequencymultiplication (doubling, tripling, etc.) phenomena, optical parametric oscillator t u n i n g , or a combination of these mechanisms. They also discuss related applications, such as e n v i r o n m e n t a l monitoring of pollutants, that would greatly benefit from UV lasers at very specific wavelengths. The extension from RE-based solidstate lasers to semiconductor-based lasers is treated by Gregorkiewicz and Langer. They review the issues and conditions relevant to efficient light emission in semiconductors in general and then apply them to the Si:Er material System. They also discuss the use of a freeelectron laser at mid-IR wavelengths to probe the energy-transfer mechanisms limiting the excitation process. This is a critical issue, in that the energy "ba
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