Synthesis, Consolidation, and Processing of Bulk Polycrystalline Transparent YAG, Ruby, and Over-Equilibrium Rare-Earth Doped Alumina for Photonic Applications

Doctor of Philosophy, Graduate Program in Mechanical Engineering
University of California, Riverside, August 2016
Dr. Javier Garay Chairperson

The  past  decade  has  seen  significant  advances  in  the  development  and  improvements  to high-energy laser technologies, with improvements coming from all directions, i.e. pumping technology, cavity  design,  cooling  methods,  and  improved  gain  media  quality,  etc.  Regardless,  the  continued development  of  high-energy  lasers  and  the  supporting  technologies  remains intense.  From  a  materials development  perspective,  the  need  for  gain  media  with  superior  optical,  thermal,  and  mechanical properties  is  alluring  because  improvements  in  the  materials  properties often  translate  directly  to increases   in   device   performance.   Advances   in   powder   processing   and   sintering/consolidation techniques, in the past two decades have produced polycrystalline ceramics with the requisite densities, transparencies,  and  photoluminescence  properties  to  be  viable  laser  gain  materials.  In  fact,  the performance of some cubic (optically isotropic) ceramics now rival and even surpass their single-crystal counterparts. In the first portion of this dissertation Current Activated Pressure Assisted Densification (CAPAD)  is  implemented  to  process  and  consolidate  transparent  bulk  polycrystalline  YAG  and Ce:YAG  ceramics  via  a  simultaneous  solid-state  synthesis  and  densification  route.  The  simultaneous
reaction/densification  during  CAPAD  processing  results  in  improved  densification  rates  and  with reaction  kinetics that are  about 2  orders  of  magnitude  higher  when  compared  to  traditional  solid-state reaction pressureless sintering and that the higher reaction kinetics occurring during CAPAD result at much lower temperatures, (~600°C) compared to conventional reaction sintering. In the second portion of  this  dissertation,  the  increased  consolidation  and  reaction  kinetics  are  leveraged  to  develop transparent bulk polycrystalline Cr:Al2O3 (ruby) and rare-earth (RE), RE:Al2O3 into viable laser gain materials. The advantages of Al2O3 as an optical gain media over state of the art gain materials such as YAG and laser glasses are significant; it has higher thermal conductivity, chemical inertness and higher mechanical toughness, all attributes that could lead to more stable, more powerful lasers. Despite these promising  attributes,  producing  RE:Al2O3  ceramics  with  suitable  functional  properties  for  laser applications  has  steep  processing  challenges.  RE:Al2O3  cannot  be  made  using  traditional  equilibrium methods because the equilibrium solubility limits of REs in the Al2O3 is on the order of 100-3 to 10-4 %, not high enough to produce lasing (~10-1 %, required).  In addition, Al2O3 is birefringent which can cause severe scattering in ceramics. These obstacles are mitigated through careful powder and CAPAD processing`in order to produce ceramics with fine grain sizes that mitigate scattering and with rare-earth dopant concentrations as high as 0.5 at.% (RE:Al), orders of magnitude higher than previously reported. Results are presented that prove for the first time that bulk polycrystalline RE:Al2O3 is a viable laser gain  media.  A  common  theme  of  this  work  will  be  the  interplay  between  material  processing,  the resultant  material  properties,  and  the  development  of  optical  devices  using  bulk  polycrystalline transparent ceramics.