Improved Crystals Transition for Laser Technology Demonstration Published Nov. 8, 2007 By Materials & Manufacturing Directorate AFRL/RX WRIGHT-PATTERSON AIR FORCE BASE, Ohio -- AFRL researchers managed an innovative research and development (R&D) effort conducted with BAE Systems, Inc., that achieved a tenfold increase in the energy-per-pulse performance of zinc germanium diphosphide (ZnGeP2) crystals--important components in the construction of high-power (and now, high-energy) laser systems. Due to the significant performance increase, the technology transitioned to AFRL's Directed Energy Directorate prior to the conclusion of the R&D effort; there, it will see immediate application as an enabling technology in a planned infrared (IR) laser technology demonstration. The development brings the Air Force a key capability for achieving high-energy IR pulses and thus marks an important step in establishing IR laser sources for military applications. The AFRL-industry R&D effort improved the damage hardness of the nonlinear optical ZnGeP2 crystal by a factor of two and increased the surface area of the aperture by a factor of nine. The researchers increased the crystal's level of damage hardness by polishing it using an innovative technique. Whereas the process has typically involved moving a crystal back and forth atop an abrasive surface and using a stream of water to wash away waste, the new technique submerses the entire crystal in a water bath and positions the abrasive surface above--rather than beneath--the crystal. This method allows waste to float away in the bath rather than being dragged across the crystal, eliminating the scratch defects (known as sleeks) inherent to the former method. Researchers successfully widened the aperture by growing the crystal boule to larger dimensions--a difficult task requiring that three problems be overcome. First, the larger quantity of phosphor needed for creating a larger boule resulted in increased pressures and greater danger of ruptures. Designing the ampoule (sealed quartz tube) with thicker glass solved this problem. Secondly, the weight increase stemming from both the thicker glass and the greater amount of material caused sagging and furnace failure. The researchers resolved this issue by redesigning the ampoule support rods. Lastly, the larger container, or boat, used to grow the larger crystals produced crystals exhibiting spurious nucleation and grain boundaries. To address this final obstacle, the team incorporated a graphite support to stabilize the thin stem of the boat, which contains the oriented starting material from which the growth of the entire boule is initiated.