Rare earth (RE)-doped solid state lasers have helped push the boundaries of high power lasers. Since lasing was first demonstrated in polycrystalline ceramic lasers, the possibility to dope higher concentrations of RE elements using non-equilibrium processes has enabled larger laser slope efficiencies. The limits in power densities in high power lasers are often due to thermal rollover caused by losses due to thermal lensing and depolarization effects, while catastrophic failures can be caused by thermal stress fracture. In such systems, a higher thermal conductivity of the lasing media lowers the thermal gradients in the lasing media, thus allowing higher lasing powers. Ceramics with twice the thermal conductivity of state of the art Nd:YAG lasers have been developed to work towards this goal, while recently visible photoluminescence was demonstrated in a Tb:AlN ceramic.
In the above polycrystalline ceramics, additional scattering caused by reflection of the heat carrying phonons at the grain boundaries can lead to a thermal conductivity which is smaller than that for single crystals by more than an order of magnitude. The mean free path of phonons in this boundary scattering regime is typically directly proportional to the grain sizes. On the other hand, the slight anisotropy of the unit cell of AlN leads to the birefringence effect. Light propagating through grains with misaligned orientation see a slightly different refractive index (Δη_max ~0.05 for red light). For small birefringence and grain sizes, when the Rayleigh-Gans-Debye approximation holds, the extinction coefficient of light due to scattering decreases with decreasing grain sizes. Thus, light transmission and thermal conductivity of these sintered ceramics scale oppositely with increasing grain sizes.
This thesis is divided into three parts. First, we investigate the effect of anisotropic grains on light transmission and thermal conductivity for active ion doped AlN and Al¬2O3 ceramics. Models to predict the transport properties are developed and verified, following which we use material properties from literature to predict the expected properties as a function of grain sizes and anisotropy. A figure of merit is proposed which can be used to select the microstructure that helps maximize the lasing power.
The anisotropic microstructure of the materials is expected to result in anisotropic transport properties of the bulk material. To measure the anisotropic thermal conductivity tensor of such materials, we developed a technique based on the electrothermal 3ω method. This method had been previously been used to measure the thermal conductivity of materials with the principal thermal conductivity directions aligned along the obvious surfaces of the sample. In this work, we found the solution for the general case where the principle axes may be aligned in any arbitrary direction. This method was verified against numerical FEM simulations and were demonstrated with experiments on a naturally occurring anisotropic mineral, mica.
Finally, we developed easily accessible and cheap methods to measure the light scattering properties of thin scatterers. Three different methods were developed. Two of the approaches require the use of an USAF 1951 target to measure the modulation transfer function, MTF. Here, the MTF of the image is obtained with the sample of interest and the USAF target placed in between the image plane and the light source. The third method uses the edge spread function following very similar ideas as the MTF method. Two of the methods use a collimated light source while one of the MTF methods uses a diffuse light source. We show experimental demonstration of the methods involving collimated light sources while numerical ray tracing is used to verify the method involving a diffuse light source.
