Cr4+ doped Yttrium aluminum garnet (YAG) has a strong spontaneous emission that can generate near-infrared emission from 1.2 to 1.6 £gm. This broadband emission have aroused great interest in developing tunable wavelength lasers and amplified spontaneous emitter (ASE).In this dissertation, The valence states of Cr ions in Ca or Ca/Mg co-doped Cr:YAG single-crystal fibers are studied. The fibers were grown using the laser-heated pedestal growth (LHPG) method, followed by annealing treatments up to 1500 oC. The concentrations of the Cr3+ and Cr4+ ions in octahedral and tetrahedral sites in oxygen or nitrogen environments were characterized for the first time to our knowledge.
Above 700 oC, migration of Cr4+ between octahedral and tetrahedral sites takes place, the ratio is about 4%; its relative stabilization energy was estimated. For Ca,Cr:YAG annealed in an oxygen or nitrogen environment, it was 0.25 and 0.3 eV, respectively. For Mg,Ca,Cr:YAG annealed in oxygen or nitrogen, it was 0.47 and 0.49 eV, respectively. For the Ca,Cr:YAG crystal fiber (Ca/Cr=113.1%) with oxygen annealing, about 35% and 2.5% of Ca ions took part in charge compensation for Cr4+ in the octahedral and tetrahedral sites, respectively. The density of oxygen vacancies depends on the concentration of Ca ions. The estimated ratios of the unreacted oxygen vacancies to total oxygen vacancies were about 63% and 88% for oxygen and nitrogen annealing, respectively. The main limitation on the concentration of Cr4+ in the tetrahedral site of YAG is the presence of unreacted oxygen vacancies.
Furthermore, chromium ions tend to diffuse outward during the LHPG of YAG crystal fiber, in which the average Cr4+ ion concentration decreases significantly after each diameter-reduction step. The Cr4+ ions are replenished using an electron gun to deposit Cr2O3 and divalent-ion oxide (CaO or MgO) on the source rod circumference before growth. It was observed that Ca2+ has better efficiency to diffuse into the source rod more efficiently than Mg2+generating fewer defects and stacking faults. By CaO deposition and post growth annealing at 1350 oC under an oxygen environment, a 110% increase in Cr4+ concentration was obtained. The achieved Cr4+ concentration and the ratio of Cr4+ to total Cr were 1.76 10^18 cm^-3 and 5.5, respectively.
| Identifer | oai:union.ndltd.org:NSYSU/oai:NSYSU:etd-0723108-162918 |
| Date | 23 July 2008 |
| Creators | Tsai, Cheng-Nan |
| Contributors | Yi-Jen Chiu, Ying-Chien Tsai, Chia-Chen Hsu, Tsong-Sheng Lay, Wood-Hi Cheng, Sheng-Lung Huang, Chao-Kuei Lee, Pochi Yeh |
| Publisher | NSYSU |
| Source Sets | NSYSU Electronic Thesis and Dissertation Archive |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.lib.nsysu.edu.tw/ETD-db/ETD-search/view_etd?URN=etd-0723108-162918 |
| Rights | unrestricted, Copyright information available at source archive |
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