We have focused our fabrication efforts on garnets, rare-earth oxides and spinel materials. We also design and prepare engineered transparent ceramic structures that will eventually be required to generate laser outputs exceeding a few hundred watts with high-beam quality in an all-solid-state laser gain medium.
There are several challenges on the road to transparent ceramic lasers. First, the grain boundaries present a discontinuity in the index of refraction. These boundaries must be kept smaller than ~1/100th of a wavelength to insure that they do not scatter light. The second challenge is to eliminate the porosity of the material. The size distribution of the voids strongly affects the attenuation coefficient. For instance, a 0.5 cm thick sample with a porosity of 10 ppm has an attenuation coefficient on the order of 0.2 cm-1. Another issue relates to index variations caused by local variations in stoichiometry, which can occur with some powder production procedures. These continuous variations in refractive index along the path of light reduce the brightness of an optical beam by distorting the wavefront.
Different techniques are currently being developed to produce transparent ceramics of cubic materials such as YAG and rare-earth oxides. These techniques include:
- pressureless sintering
- high pressure sintering
- sintering aids
- transient liquid- or solid-phase agents
All these techniques have their pros and cons with regards to the optical quality of the material they can produce (not only their transmittance but also their spectroscopic performance), the control of the microstructure they offer, and their scalability in size.